Food Processing Handbook E-Book
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- Herausgeber: Wiley-VCH
- Kategorie: Fachliteratur
- Sprache: Englisch
The second edition of the Food Processing Handbook presents a comprehensive review of technologies, procedures and innovations in food processing, stressing topics vital to the food industry today and pinpointing the trends in future research and development. Focusing on the technology involved, this handbook describes the principles and the equipment used as well as the changes - physical, chemical, microbiological and organoleptic - that occur during food preservation. In so doing, the text covers in detail such techniques as post-harvest handling, thermal processing, evaporation and dehydration, freezing, irradiation, high-pressure processing, emerging technologies and packaging. Separation and conversion operations widely used in the food industry are also covered as are the processes of baking, extrusion and frying. In addition, it addresses current concerns about the safety of processed foods (including HACCP systems, traceability and hygienic design of plant) and control of food processes, as well as the impact of processing on the environment, water and waste treatment, lean manufacturing and the roles of nanotechnology and fermentation in food processing. This two-volume set is a must-have for scientists and engineers involved in food manufacture, research and development in both industry and academia, as well as students of food-related topics at undergraduate and postgraduate levels. From Reviews on the First Edition: "This work should become a standard text for students of food technology, and is worthy of a place on the bookshelf of anybody involved in the production of foods." Journal of Dairy Technology, August 2008 "This work will serve well as an excellent course resource or reference as it has well-written explanations for those new to the field and detailed equations for those needing greater depth." CHOICE, September 2006
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Seitenzahl: 1466
Veröffentlichungsjahr: 2012
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Table of Contents
Related Titles
Title Page
Copyright
Preface to the Second Edition
Preface to the First Edition
List of Contributors
Content of Volume 1
Chapter 1: Postharvest Handling and Preparation of Foods for Processing
1.1 Introduction
1.2 Properties of Raw Food Materials and their Susceptibility to Deterioration and Damage
1.3 Storage and Transportation of Raw Materials
1.4 Raw Material Cleaning
1.5 Sorting and Grading
1.6 Blanching
1.7 Sulfiting of Fruits and Vegetables
References
Chapter 2: Thermal Processing
2.1 Introduction
2.2 Reaction Kinetics
2.3 Temperature Dependence
2.4 Heat Processing Methods
2.5 Special Problems with Viscous and Particulate Products
2.6 Ohmic Heating
2.7 Filling Procedures
2.8 Storage
References
Chapter 3: Evaporation and Dehydration
3.1 Evaporation (Concentration, Condensing)
3.2 Dehydration (Drying)
References
Chapter 4: Freezing
4.1 Introduction
4.2 Refrigeration Methods and Equipment
4.3 Low Temperature Production
4.4 Freezing Kinetics
4.5 Effects of Refrigeration on Food Quality
References
Chapter 5: Irradiation
5.1 Introduction
5.2 Principles of Irradiation
5.3 Equipment
5.4 Safety Aspects
5.5 Effects on the Properties of Food
5.6 Detection Methods for Irradiated Foods
5.7 Applications and Potential Applications
References
Chapter 6: High Pressure Processing
6.1 Introduction
6.2 Effect of High Pressure on Microorganisms
6.3 Ingredient Functionality
6.4 Enzyme Activity
6.5 Foaming and Emulsification
6.6 Gelation
6.7 Organoleptic Considerations
6.8 Equipment for HPP
6.9 Pressure Vessel Considerations
6.10 Current and Potential Applications of HPP for Foods
References
Chapter 7: Emerging Technologies for Food Processing
7.1 Introduction
7.2 Pulsed Electric Field Processing
7.3 Ultrasound Power
7.4 Other Technologies
7.5 Conclusions
References
Chapter 8: Packaging
8.1 Introduction
8.2 Factors Affecting the Choice of a Packaging Material and/or Container for a Particular Duty
8.3 Materials and Containers Used for Packaging Foods
8.4 Modified Atmosphere Packaging
8.5 Aseptic Packaging
8.6 Active Packaging
8.7 Intelligent Packaging
8.8 The Role of Nanotechnology in Food Packaging
References
Content of Volume 2
Chapter 9: Separations in Food Processing Part 1
9.1 Introduction
9.2 Solid–Liquid Filtration
9.3 Centrifugation
9.4 Solid–Liquid Extraction (Leaching)
9.5 Distillation
9.6 Crystallization
References
Chapter 10: Separations in Food Processing: Part 2 – Membrane Processing, Ion Exchange, and Electrodialysis
10.1 Membrane Processes
10.2 Ion Exchange
10.3 Electrodialysis
References
Chapter 11: Mixing, Emulsification, and Size Reduction
11.1 Mixing (Agitation, Blending)
11.2 Emulsification
11.3 Size Reduction (Crushing, Comminution, Grinding, Milling) of Solids
References
Chapter 12: Baking
12.1 Introduction
12.2 The Key Characteristics of Existing Bakery Product Groups
12.3 Bread Making
12.4 The Manufacture of Cakes
12.5 Biscuit and Cookie Making
12.6 The Manufacture of Pastry Products
References
Chapter 13: Extrusion
13.1 General Principles
13.2 Extrusion Equipment
13.3 Effects of Extrusion on the Properties of Foods
References
Chapter 14: Food Deep-Fat Frying
14.1 General Principles
14.2 Effect of Deep-Fat Fried Food on Human Health
14.3 Oil Absorption in Deep-Fat Fried Food
14.4 Deep-Fat Frying Equipment
14.5 French Fries, Potato Chips, and Fabricated Chips Production
References
Chapter 15: Safety in Food Processing
15.1 Introduction
15.2 Safe Design
15.3 Prerequisite Programs
15.4 HACCP, the Hazard Analysis and Critical Control Point System
15.5 Ongoing Control of Food Safety in Processing
References
Chapter 16: Traceability in Food Processing and Distribution
16.1 What Is Traceability?
16.2 Traceability and Legislation
16.3 Traceability and International/Private Standards
16.4 Traceability and Private Standards
16.5 Traceability in the Food Supply Chain
16.6 Product Identification
16.7 Management of Traceability Information
16.8 The Traceability System
16.9 Examples of Traceability Systems
References
Chapter 17: The Hygienic Design of Food Processing Plant
17.1 Introduction
17.2 Engineering Factors Influencing Hygiene
17.3 Hygienic Equipment Design
17.4 Process Design
17.5 Process Operation and Control
17.6 Future Trends
17.7 Conclusions
References
Chapter 18: Process Control in Food Processing
18.1 Introduction
18.2 Measurement of Process Parameters
18.3 Control Systems
18.4 Process Control in Modern Food Processing
18.5 Concluding Remarks
References
Chapter 19: Environmental Aspects of Food Processing
19.1 Introduction
19.2 Waste Characteristics
19.3 Wastewater Processing Technology
19.4 Resource Recovery from Food Processing Wastes
19.5 Environmental Impact of Packaging Wastes
19.6 Refrigerants
19.7 Energy Issues Related to the Environment
19.8 Life Cycle Assessment
19.9 Calculating Greenhouse Gas Emissions
References
Chapter 20: Water and Waste Treatment
20.1 Introduction
20.2 Fresh Water
20.3 Wastewater
20.4 Sludge Disposal
20.5 Final Disposal of Wastewater
References
Chapter 21: Process Realisation
21.1 Synopsis
21.2 Manufacturing Design
21.3 Process and Plant Design
21.4 Process Economics – Investment Criteria
21.5 Determining and Improving Process Performance
21.6 Variation
21.7 Brief Introduction to Lean and Waste
21.8 Tools for Continuous Improvement
References
Chapter 22: Microscopy Techniques and Image Analysis for the Quantitative Evaluation of Food Microstructure
22.1 Introduction
22.2 Microstructure, Nanostructure, and Levels of Structure
22.3 Microscopy Techniques
22.4 Image Analysis
22.5 Applications of Microscopy and Image Analysis Techniques
22.6 Concluding Remarks
References
Chapter 23: Nanotechnology in the Food Sector
23.1 Introduction
23.2 The Driving Force for Nanotechnology Development
23.3 Manufacture of Nanosystems: General Principles
23.4 Nanotechnology and Food
23.5 Delivery Systems for Functional Food Ingredients
23.6 Application of Nanotechnology in Food Packaging and Other Contact Surfaces
23.7 Other Areas of Application
23.8 Potential Health and Safety Concerns Involved with Ingestion of Nanoparticulates
23.9 Regulatory Aspects
23.10 Recent Initiatives
References
Chapter 24: Fermentation and the Use of Enzymes
24.1 Introduction
24.2 Fermentation Theory
24.3 Fermented Foods
24.4 Enzyme Technology
References
Index
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The Editors
James G. Brennan, MSc FIFST
16 Benning Way
Wokingham, Berks RG40 1XX
United Kingdom
Dr. Alistair S. Grandison
Department of Food and Nutritional
Sciences
University of Reading
Whiteknights
Reading RG6 6AP
United Kingdom
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Preface to the Second Edition
In this second edition of Food Processing Handbook the chapters in the first edition have been retained and revised by including information on recent developments in each field and updating the reference lists. Some of the most notable changes are: the inclusion of a new section on ohmic heating in the Chapter on thermal processing (Chapter 2); extending the packaging chapter to cover intelligent packaging (Chapter 8); explaining the calculation of greenhouse gas emissions (carbon footprints) and providing a case study in the chapter on environmental aspects of food processing (Chapter 19). The original chapter entitled Baking, Extrusion and Frying has been split into three individual chapters providing extended coverage of these three important processes (Chapters 12, 13, and 14). Several new topics have been added to reflect recent trends and concerns in the food industry. These include chapters on: traceability in food processing and distribution (Chapter 16); hygienic design of food processing plant (Chapter 17); process realisation (Chapter 21); microscopy techniques and image analysis for the quantitative evaluation of food microstructure (Chapter 22); nanotechnology in the food sector (Chapter 23) and fermentation and the use of enzymes (Chapter 24). These changes have necessitated dividing the book into two volumes, the first consisting of the more basic food preservation processes and packaging, while volume 2 includes other manufacturing processes and other considerations relating to safety and sustainable manufacturing.
It is hoped that this much extended edition will be of interest to scientists and engineers involved in food manufacture and research and development in industry, and to staff and students participating in food related courses at undergraduate and postgraduate levels.
James G. Brennan,
Alistair S. Grandison
Preface to the First Edition
There are many excellent texts available which cover the fundamentals of food engineering, equipment design, modelling of food processing operations etc. There are also several very good works in food science and technology dealing with the chemical composition, physical properties, nutritional and microbiological status of fresh and processed foods. This work is an attempt to cover the middle ground between these two extremes. The objective is to discuss the technology behind the main methods of food preservation used in today's food industry in terms of the principles involved, the equipment used and the changes in physical, chemical, microbiological and organoleptic properties that occur during processing. In addition to the conventional preservation techniques, new and emerging technologies, such as high pressure processing and the use of pulsed electric field and power ultrasound are discussed. The materials and methods used in the packaging of food, including the relatively new field of active packaging, are covered. Concerns about the safety of processed foods and the impact of processing on the environment are addressed. Process control methods employed in food processing are outlined. Treatments applied to water to be used in food processing and the disposal of wastes from processing operations are described.
Chapter 1 covers the postharvest handling and transport of fresh foods and preparatory operations, such as cleaning, sorting, grading and blanching, applied prior to processing. Chapters 2, 3 and 4 contain up-to-date accounts of heat processing, evaporation, dehydration and freezing techniques used for food preservation. In Chapter 5, the potentially useful, but so far little used process of irradiation is discussed. The relatively new technology of high pressure processing is covered in Chapter 6, while Chapter 7 explains the current status of pulsed electric field, power ultrasound, and other new technologies. Recent developments in baking, extrusion cooking and frying are outlined in Chapter 8. Chapter 9 deals with the materials and methods used for food packaging and active packaging technology, including the use of oxygen, carbon dioxide and ethylene scavengers, preservative releasers and moisture absorbers. In Chapter 10, safety in food processing is discussed and the development, implementation and maintenance of HACCP systems outlined. Chapter 11 covers the various types of control systems applied in food processing. Chapter 12 deals with environmental issues including the impact of packaging wastes and the disposal of refrigerants. In Chapter 13, the various treatments applied to water to be used in food processing are described and the physical, chemical and biological treatments applied to food processing wastes are outlined. To complete the picture, the various separation techniques used in food processing are discussed in Chapter 14 and Chapter 15 covers the conversion operations of mixing, emulsification and size reduction of solids.
The editor wishes to acknowledge the considerable advice and help he received from former colleagues in the School of Food Biosciences, The University of Reading, when working on this project. He also wishes to thank his wife, Anne, for her support and patience.
Reading, August 2005
James G. Brennan
List of Contributors
Volume 1
1
Postharvest Handling and Preparation of Foods for Processing
Alistair S. Grandison
1.1 Introduction
Food processing is seasonal in nature, both in terms of demand for products and availability of raw materials. Most crops have well-established harvest times – for example, the sugar beet season lasts for only a few months of the year in the United Kingdom, so beet sugar production is confined to the autumn and winter, yet demand for sugar is continuous throughout the year. Even in the case of raw materials that are available throughout the year, such as milk, there are established peaks and troughs in volume of production, as well as variations in chemical composition. Availability may also be determined by less predictable factors, such as weather conditions, which may affect yields or limit harvesting. In other cases demand is seasonal, for example, ice cream or salads are in greater demand in the summer, whereas other foods are traditionally eaten in the winter months, or even at more specific times, such as Christmas or Easter.
In an ideal world, food processors would like a continuous supply of raw materials, whose composition and quality are constant and whose prices are predictable. Of course this is usually impossible to achieve. In practice, processors contract ahead with growers to synchronize their needs with raw material production.
The aim of this chapter is to consider the properties of raw materials in relation to food processing, and to summarize important aspects of handling, transport, storage, and preparation of raw materials prior to the range of processing operations described in the remainder of this book. The bulk of the chapter will deal with solid agricultural products including fruits, vegetables, cereals, and legumes, although many considerations can also be applied to animal-based materials such as meat, eggs, and milk.
1.2 Properties of Raw Food Materials and their Susceptibility to Deterioration and Damage
The selection of raw materials is a vital consideration to the quality of processed products. The quality of raw materials can rarely be improved during processing, and while sorting and grading operations can aid by removing oversize, undersize, or poor-quality units, it is vital to procure materials whose properties most closely match the requirements of the process. Quality is a wide-ranging concept and is determined by many factors. It is a composite of those physical and chemical properties of the material which govern its acceptability to the “user.” The latter may be the final consumer, or more likely in this case, the food processor. Geometric properties, color, flavor, texture, nutritive value, and freedom from defects are the major properties likely to determine quality.
An initial consideration is selection of the most suitable cultivars in the case of plant foods (or breeds in the case of animal products). Other preharvest factors (such as soil conditions, climate, and agricultural practices), harvesting methods and postharvest conditions, maturity, storage, and postharvest handling also determine quality. These considerations, including seed supply and many aspects of crop production, are frequently controlled by the processor or even the retailer.
The timing and method of harvesting are determinants of product quality. Manual labor is expensive, therefore mechanized harvesting is introduced where possible. Cultivars most suitable for mechanized harvesting should mature evenly, producing units of nearly equal size that are resistant to mechanical damage. In some instances, the growth habits of plants (e.g., pea vines, fruit trees) have been developed to meet the needs of mechanical harvesting equipment. Uniform maturity is desirable as the presence of over-mature units is associated with high waste, product damage, and high microbial loads, while under-maturity is associated with poor yield, lack of flavor and color, and hard texture. For economic reasons, harvesting is almost always a “once over” exercise, hence it is important that all units reach maturity at the same time. The prediction of maturity is necessary to coordinate harvesting with processors' needs, as well as to extend the harvest season. It can be achieved primarily from knowledge of the growth properties of the crop combined with records and experience of local climatic conditions.
The “heat unit system,” first described by Seaton [1] for peas and beans, can be applied to give a more accurate estimate of harvest date from sowing date in any year. This system is based on the premise that growth temperature is the overriding determinant of crop growth. A base temperature, below which no growth occurs, is assumed, and the mean temperature of each day through the growing period is recorded. By summing the daily mean temperatures minus base temperatures on days where mean temperature exceeds base temperature, the number of “accumulated heat units” can be calculated. By comparing this with the known growth data for the particular cultivar, an accurate prediction of harvest date can be computed. In addition, by allowing fixed numbers of accumulated heat units between sowings, the harvest season can be spread, so that individual fields may be harvested at peak maturity. Sowing plans and harvest date are determined by negotiation between the growers and the processors, and the latter may even provide the equipment and labor for harvesting and transport to the factory.
An important consideration for processed foods is that it is the quality of the processed product, rather than the raw material, that is important. For minimally processed foods, such as those subjected to modified atmospheres, low dose irradiation, mild heat treatment, or some chemical preservatives, the characteristics of the raw material are a good guide to the quality of the product. For more severe processing, including heat preservation, drying, or freezing, the quality characteristics may change markedly during processing. Hence, those raw materials which are preferred for fresh consumption may not be most appropriate for processing. For example, succulent peaches with delicate flavor may be less suitable for canning than harder, less flavorsome cultivars, which can withstand rigorous processing conditions. Similarly, ripe, healthy, well-colored fruit may be perfect for fresh sale, but may not be suitable for freezing due to excessive drip loss while thawing. For example, Maestrelli [2] reported that different strawberry cultivars with similar excellent characteristics for fresh consumption, exhibited a wide range of drip loss (between 8 and 38%), and hence would be of widely different value for the frozen food industry.
1.2.1 Raw Material Properties
The main raw material properties of importance to the processor are geometry, color, texture, functional properties, and flavor.
1.2.1.1 Geometric Properties
Food units of regular geometry are much easier to handle and are better suited to high-speed mechanized operations. In addition, the more uniform the geometry of raw materials, the less rejection and waste will be produced during preparation operations such as peeling, trimming, and slicing. For example, potatoes of smooth shape with few and shallow eyes are much easier to peel and wash mechanically than irregular units. Smooth-skinned fruits and vegetables are much easier to clean and are less likely to harbor insects or fungi than ribbed or irregular units.
Agricultural products do not come in regular shapes and exact sizes. Size and shape are inseparable, but are very difficult to define mathematically in solid food materials. Geometry is, however, vital to packaging and controlling fill-in weights. It may, for example, be important to determine how much mass or how many units may be filled into a square box or cylindrical can. This would require a vast number of measurements to perform exactly, and thus approximations must be made. Size and shape are also important to heat processing and freezing, as they will determine the rate and extent of heat transfer within food units. Mohsenin [3] describes numerous approaches by which the size and shape of irregular food units may be defined. These include the development of statistical techniques based on a limited number of measurements and more subjective approaches involving visual comparison of units to charted standards. Uniformity of size and shape is also important to most operations and processes. Process control to give accurately and uniformly treated products is always simpler with more uniform materials. For example, it is essential that wheat kernel size is uniform for flour milling.
Specific surface (area/mass) may be an important expression of geometry, especially when considering surface phenomena, such as the economics of fruit peeling, or surface processes such as smoking and brining.
The presence of geometric defects, such as projections and depressions, complicate any attempt to quantify the geometry of raw materials, as well as presenting processors with cleaning and handling problems, and yield loss. Selection of cultivars with the minimum defect level is advisable.
There are two approaches to securing optimum geometric characteristics: first, the selection of appropriate varieties, and second, sorting and grading operations.
1.2.1.2 Color
Color and color uniformity are vital components of the visual quality of fresh foods, and play a major role in consumer choice. However, it may be less important in raw materials for processing. For low-temperature processes, such as chilling, freezing, or freeze drying, the color changes little during processing, and thus the color of the raw material is a good guide to suitability for processing. For more severe processing, the color may change markedly during the process. Green vegetables such as peas, spinach, or green beans change color on heating from bright green to a dull olive green. This is due to the conversion of chlorophyll to pheophytin. It is possible to protect against this by addition of sodium bicarbonate to the cooking water, which raises the pH. However, this may cause softening of texture, and the use of added colorants may be a more practical solution. Some fruits may lose their color during canning, while pears develop a pink tinge. Potatoes are subject to browning during heat processing due to the Maillard reaction. Therefore, some varieties are more suitable for fried products, where browning is desirable, than for canned products, in which browning would be a major problem.
Again there are two approaches: procuring raw materials of the appropriate variety and stage of maturity, and sorting by color to remove unwanted units.
1.2.1.3 Texture
The texture of raw materials is frequently changed during processing. Textural changes are caused by a wide variety of effects, including water loss, protein denaturation which may result in loss of water-holding capacity or coagulation, hydrolysis, and solubilization of proteins. In plant tissues, cell disruption leads to loss of turgor pressure and softening of the tissue, while gelatinization of starch, hydrolysis of pectin, and solubilization of hemicelluloses also cause softening of the tissues.
The raw material must be robust enough to withstand the mechanical stresses during preparation, for example, abrasion during cleaning of fruit and vegetables. Peas and beans must be able to withstand mechanical podding. Raw materials must be chosen so that the texture of the processed product is correct, such as canned fruits and vegetables in which raw materials must be able to withstand heat processing without being too hard or coarse for consumption.
Texture is dependent on the variety as well as the maturity of the raw material, and may be assessed by sensory panels or commercial instruments. One widely recognized instrument is the tenderometer used to assess the firmness of peas. The crop would be tested daily and harvested at the optimum tenderometer reading. In common with other raw materials, peas at different maturities can be used for different purposes, so that peas for freezing would be harvested at a lower tenderometer reading than peas for canning.
1.2.1.4 Flavor
Flavor is a rather subjective property which is difficult to quantify. Flavor quality of horticultural products is influenced by genotype and a range of pre- and postharvest factors [4]. Optimizing maturity/ripeness stage in relation to flavor at the time of processing is a key issue. Again, flavors are altered during processing, and following severe processing, the main flavors may be derived from additives. Hence, the lack of strong flavors may be the most important requirement. In fact, raw material flavor is often not a major determinant as long as the material imparts only those flavors which are characteristic of the food. Other properties may predominate. Flavor is normally assessed by human tasters, although sometimes flavor can be linked to some analytical test, such as sugar/acid levels in fruits.
1.2.1.5 Functional Properties
The functionality of a raw material is the combination of properties which determine product quality and process effectiveness. These properties differ greatly for different raw materials and processes, and may be measured by chemical analysis or process testing.
For example, a number of possible parameters may be monitored in wheat. Wheat for different purposes may be selected according to protein content. Hard wheat with 11.5–14% protein is desirable for white bread, and some whole wheat breads require even higher protein levels (14–16%) [5]. On the other hand, soft or weak flours with lower protein contents are suited to chemically leavened products with a lighter or more tender structure. Hence protein levels of 8–11% are adequate for biscuits, cakes, pastry, noodles, and similar products. Varieties of wheat for processing are selected on this basis, and measurement of protein content would be a good guide to process suitability. Furthermore, physical testing of dough using a variety of rheological testing instruments may be useful in predicting the breadmaking performance of individual batches of wheat flours [6]. A further test is the Hagberg Falling Number which measures the amount of α-amylase in flour or wheat [7]. This enzyme assists in the breakdown of starch to sugars, and high levels give rise to a weak bread structure. Hence, the test is a key indicator of wheat baking quality and is routinely used for bread wheat, and often determines the price paid to the farmer.
Similar considerations apply to other raw materials. Chemical analysis of fat and protein in milk may be carried out to determine its suitability for manufacturing cheese, yoghurt, or cream.
1.2.2 Raw Material Specifications
In practice, processors define their requirements in terms of raw material specifications for any process on arrival at the factory gate. Acceptance of, or price paid for, the raw material depends on the results of specific tests. Milk deliveries would be routinely tested for hygienic quality, somatic cells, antibiotic residues, extraneous water, as well as possibly fat and protein content. A random core sample is taken from all sugar beet deliveries and payment is dependent on the sugar content. For fruits, vegetables, and cereals, processors may issue specifications and tolerances to cover the size of units, the presence of extraneous vegetable matter, foreign bodies, levels of specific defects (e.g., surface blemishes, insect damage), and so on, as well as specific functional tests. Guidelines for sampling and testing many raw materials for processing in the United Kingdom are available from Campden BRI (www.campden.co.uk).
Increasingly, food processors and retailers may impose demands on raw material production which go beyond the properties described above. These may include “environmentally friendly” crop management schemes in which only specified fertilizers and insecticides are permitted, or humanitarian concerns, especially for food produced in developing countries. Similarly animal welfare issues may be specified in the production of meat or eggs. Another important issue is the growth of demand for organic foods in the United Kingdom and western Europe, which obviously introduces further demands on production methods that are beyond the scope of this chapter.
1.2.3 Deterioration of Raw Materials
All raw materials deteriorate following harvest, by some of the following mechanisms:
Endogenous Enzymes:
Postharvest senescence and spoilage of fruit and vegetables occurs through a number of enzymic mechanisms, including oxidation of phenolic substances in plant tissues by phenolase (leading to browning); sugar–starch conversion by amylases; postharvest demethylation of pectic substances in fruits and vegetables leading to softening tissues during ripening and firming of plant tissues during processing.
Chemical Changes:
These include deterioration in sensory quality by lipid oxidation; non-enzymic browning; and breakdown of pigments such as chlorophyll, anthocyanins, and carotenoids.
Nutritional Changes:
Breakdown of ascorbic acid is an important example.
Physical Changes:
These include dehydration and moisture absorption.
Biological Changes:
Examples are the germination of seeds and sprouting.
Microbiological Contamination:
Both the organisms themselves and their toxic products lead to deterioration of quality, as well as posing safety problems.
1.2.4 Damage to Raw Materials
Damage may occur at any point from growing through to the final point of sale. It may arise through external or internal forces.
External forces result in mechanical injury to fruits and vegetables, cereal grains, eggs, and even bones in poultry. They occur due to rough handling as a result of careless manipulation, poor equipment design, incorrect containerization, and unsuitable mechanical handling equipment. The damage typically results from impact and abrasion between food units, or between food units and machinery surfaces and projections, excessive vibration or pressure from overlying material. Increased mechanization in food handling must be carefully designed to minimize this.
Internal forces arise from physical changes such as variation in temperature and moisture content, and may result in skin cracks in fruits and vegetables, or stress cracks in cereals.
Either form of damage leaves the material open to further biological or chemical damage including enzymic browning of bruised tissue, or infestation of punctured surfaces by molds and rots.
1.2.5 Improving Processing Characteristics through Selective Breeding and Genetic Engineering
Selective breeding for yield and quality has been carried out for centuries in both plant and animal products. Until the twentieth century, improvements were made on the basis of selecting the most desirable looking individuals, while more systematic techniques have been developed more recently, based on greater understanding of genetics. The targets have been to increase yield as well as aiding factors of crop or animal husbandry such as resistance to pests and diseases, suitability for harvesting, or development of climate-tolerant varieties (e.g., cold-tolerant maize or drought-resistant plants) [8]. Raw material quality, especially in relation to processing, has become increasingly important. There are many examples of successful improvements in processing quality of raw materials through selective plant breeding including:
improved oil percentage and fatty acid composition in oilseed rape;
improved milling and malting quality of cereals;
high sugar content and juice quality in sugar beets;
development of specific varieties of potatoes for the processing industry, based on levels of enzymes and sugars, producing appropriate flavor, texture and color in products, or storage characteristics;
Brussels sprouts which can be successfully frozen.
Similarly, traditional breeding methods have been used to improve yields of animal products such as milk and eggs, as well as improving quality – for example, fat/lean content of meat. Again the quality of raw materials in relation to processing may be improved by selective breeding. This is particularly applicable to milk, where breeding programs have been used at different times to maximize butterfat and protein content, and would thus be related to the yield and quality of fat- or protein-based dairy products. Furthermore, particular protein genetic variants in milk have been shown to be linked with processing characteristics, such as curd strength during manufacture of cheese [9]. Hence, selective breeding could be used to tailor milk supplies to the manufacture of specific dairy products.
Traditional breeding programs will undoubtedly continue to produce improvements in raw materials for processing, but the potential is limited by the gene pool available to any species. Genetic engineering extends this potential by allowing the introduction of foreign genes into an organism, with huge potential benefits. Again many of the developments have been aimed at agricultural improvements such as increased yield, or introducing herbicide, pest, or drought resistance. Other developments have aimed to improve the nutritional quality of foods. For example, transgenic “Golden” rice as a rich source of vitamin A; cereal grains with increased protein quantity and quality; oilseeds engineered to contain higher levels of omega-3 fatty acids. However, there is enormous potential in genetically engineered raw materials for processing [10]. The following are some examples which have been demonstrated:
Tomatoes which do not produce pectinase and hence remain firm while color and flavor develop, producing improved soup, paste, or ketchup.
Potatoes with higher starch content, which take up less oil and require less energy during frying.
Canola (rape seed) oil tailored to contain high levels of lauric acid to improve emulsification properties for use in confectionery, coatings, or low-fat dairy products; high levels of stearate as an alternative to hydrogenation in manufacture of margarine; and high levels of polyunsaturated fatty acids for health benefits.
Wheat with increased levels of high molecular weight glutenins for improved breadmaking performance.
Fruits and vegetables containing peptide sweeteners such as thaumatin or monellin.
“Naturally decaffeinated” coffee.
There is, however, considerable opposition to the development of genetically modified foods in the United Kingdom and elsewhere, due to fears of human health risks and ecological damage, discussion of which is beyond the scope of this book. It therefore remains to be seen if, and to what extent, genetically modified raw materials will be used in food processing.
1.3 Storage and Transportation of Raw Materials
1.3.1 Storage
Storage of food is necessary at all points of the food chain from raw materials, through manufacture, distribution, retailers, and final purchasers. Today's consumers expect a much greater variety of products, including non-local materials, to be available throughout the year. Effective transportation and storage systems for raw materials are essential to meet this need.
Storage of materials whose supply or demand fluctuate in a predictable manner, especially seasonal produce, is necessary to increase availability. It is essential that processors maintain stocks of raw materials, therefore storage is necessary to buffer demand. However, storage of raw materials is expensive for two reasons: stored goods have been paid for and may therefore tie up quantities of company money, and secondly, warehousing and storage space are expensive. All raw materials will deteriorate during storage. The quantities of raw materials held in store and the times of storage vary widely for different cases, depending on the above considerations. The “just in time” approaches used in other industries are less common in food processing.
The primary objective is to maintain the best possible quality during storage, and hence avoid spoilage during the storage period. Spoilage arises through three mechanisms:
The main factors that govern the quality of stored foods are temperature, moisture/humidity, and atmospheric composition. Different raw materials provide very different challenges.
Fruits and vegetables remain as living tissues until they are processed and the main aim is to reduce respiration rate without damage to the tissue. Storage times vary widely between types. Young tissues such as shoots, green peas, and immature fruits have high respiration rates and shorter storage periods, while mature fruits and roots, and storage organs such as bulbs and tubers (e.g., onions, potatoes, sugar beets) respire much more slowly, and hence have longer storage periods. Some examples of conditions and storage periods of fruits and vegetables are given in Table 1.1. Many fruits (including bananas, apples, tomatoes, and mangoes) display a sharp increase in respiration rate during ripening, just before the point of optimum ripening, known as the “climacteric.” The onset of the climacteric is associated with the production of high levels of ethylene, which is believed to stimulate the ripening process. Climacteric fruit can be harvested unripe and ripened artificially at a later time. It is vital to maintain careful temperature control during storage or the fruit will rapidly over-ripen. Non-climacteric fruits (e.g., citrus fruit, pineapples, strawberries) and vegetables do not display this behavior, and generally do not ripen after harvest. Quality is therefore optimal at harvest, and the task is to preserve quality during storage.
Table 1.1 Storage Periods of Some Fruits and Vegetables Under Typical Storage Conditions
Harvesting, handling, and storage of fruit and vegetables are discussed in more detail by Thompson [11], while Nascimento Nunes [12] visually depicts the effects of time and temperature on the appearance of fruit and vegetables throughout postharvest life.
With meat storage the overriding problem is growth of spoilage bacteria, while avoiding oxidative rancidity. Cereals must be dried before storage to avoid germination and mold growth, and subsequently must be stored under conditions which prevent infestation with rodents, birds, insects, or molds.
Hence, very different storage conditions may be employed for different raw materials. The main methods employed in raw material storage are the control of temperature, humidity, and composition of atmosphere.
1.3.1.1 Temperature
The rate of biochemical reactions is related to temperature, such that lower storage temperatures lead to slower degradation of foods by biochemical spoilage, as well as reduced growth of bacteria and fungi. There may also be limited bacteriocidal effects at very low temperatures. Typical Q10 values for spoilage reactions are approximately 2, implying that spoilage rates would double for each 10 °C rise, or conversely that shelf life would double for each 10 °C reduction. This is an oversimplification as Q10 may change with temperature. Most insect activity is inhibited below 4 °C, although insects and their eggs can survive long exposure to these temperatures. In fact grain and flour mites can remain active and even breed at 0 °C.
The use of refrigerated storage is limited by the sensitivity of materials to low temperatures. The freezing point is a limiting factor for many raw materials, as the tissues will become disrupted on thawing. Other foods may be subject to problems at temperatures above freezing. Fruits and vegetables may display physiological problems that limit their storage temperatures, probably as a result of metabolic imbalance leading to a build-up of undesirable chemical species in the tissues. Some types of apples are subject to internal browning below 3 °C, while bananas become brown when stored below 13 °C, and many other tropical fruits display chill sensitivity. Less obvious biochemical problems may occur even where no visible damage occurs. For example, storage temperature affects starch/sugar balance in potatoes; in particular, below 10 °C a build up of sugar occurs, which is most undesirable for fried products. Examples of storage periods and conditions are given in Table 1.1, illustrating the wide ranges seen with different fruits and vegetables. It should be noted that predicted storage lives can be confounded if the produce is physically damaged, or by the presence of pathogens.
Temperature of storage is also limited by cost. Refrigerated storage is expensive, especially in hot countries. In practice, a balance must be struck incorporating cost, shelf life, and risk of cold injury. Slower growing produce such as onions, garlic, and potatoes can be successfully stored at ambient temperature and ventilated conditions in temperate climates.
It is desirable to monitor temperature throughout raw material storage and distribution.
Precooling to remove the “field heat” is an effective strategy to reduce the period of high initial respiration rate in rapidly respiring produce prior to transportation and storage. For example, peas for freezing are harvested in the cool early morning and rushed to cold storage rooms within 2–3 h. Other produce, such as leafy vegetables (lettuce, celery, cabbage) or sweetcorn, may be cooled using water sprays or drench streams. Hydrocooling obviously reduces water loss.
1.3.1.2 Humidity
If the humidity of the storage environment exceeds the equilibrium relative humidity (ERH) of the food, the food will gain moisture during storage, and vice versa. Uptake of water during storage is associated with susceptibility to growth of microorganisms, while water loss results in economic loss, as well as more specific problems such as cracking of seed coats of cereals, or skins of fruits and vegetables. Ideally the humidity of the store would equal the ERH of the food so that moisture is neither gained nor lost, but in practice a compromise may be necessary. The water activity (aw) of most fresh foods (e.g., fruit, vegetables, meat, fish, milk) is in the range 0.98–1.00, but they are frequently stored at a lower humidity. Some wilting of fruits or vegetable may be acceptable in preference to mold growth, while some surface drying of meat is preferable to bacterial slime. Packaging may be used to protect against water loss of raw materials during storage and transport (see Chapter 8).
1.3.1.3 Composition of Atmosphere
Controlling the atmospheric composition during storage of many raw materials is beneficial. The use of packaging to allow the development or maintenance of particular atmospheric compositions during storage is discussed in greater detail in Chapter 8.
With some materials the major aim is to maintain an oxygen-free atmosphere to prevent oxidation (e.g., coffee, baked goods), while in other cases adequate ventilation may be necessary to prevent anaerobic fermentation leading to off-flavors.
In living produce, atmosphere control allows the possibility of slowing down metabolic processes, hence retarding respiration, ripening, and senescence as well as the development of disorders. The aim is to introduce N2 and remove O2, allowing a build up of CO2. Controlled-atmosphere storage of many commodities is discussed by Thompson [14]. The technique allows year-round distribution of apples and pears, where controlled atmospheres in combination with refrigeration can give shelf lives up to 10 months, much greater than by chilling alone. The particular atmospheres are cultivar specific, but are in the range 1–10% CO2, 2–13% O2 at 3 °C for apples and 0 °C for pears. Controlled atmospheres are also used during storage and transport of chill-sensitive crops, such as for transport of bananas, where an atmosphere of 3% O2 and 5% CO2 is effective in preventing premature ripening and the development of crown rot disease. Ethene (ethylene) removal is also vital during storage of climacteric fruit.
With fresh meat, controlling the gaseous environment is useful in combination with chilling. The aim is to maintain the red color by storage in high O2 concentrations, which shifts the equilibrium in favor of high concentrations of the bright red oxymyoglobin pigment. At the same time, high levels of CO2 are required to suppress the growth of aerobic bacteria.
1.3.1.4 Other Considerations
Odors and taints can cause problems, especially in fatty foods such as meat and dairy products, as well as less obvious commodities such as citrus fruits, which have oil in the skins. Odors and taints may be derived from fuels or adhesives and printing materials, as well as other foods (e.g., spiced or smoked products). Packaging and other systems during storage and transport must protect against contamination.
Light can lead to oxidation of fats in some raw materials (e.g., dairy products). In addition, light gives rise to solanine production and the development of green pigmentation in potatoes. Hence, storage and transport under dark conditions is essential.
1.3.2 Transportation
Food transportation is an essential link in the food chain, and is discussed in detail by Heap [15]. Raw materials, food ingredients, fresh produce, and processed products are all transported on a local and global level, by land, sea, and air. In the modern world, where consumers expect year-round supplies and non-local products, long-distance transport of many foods has become commonplace, and air transport may be necessary for perishable materials. Transportation of food is really an extension of storage; a refrigerated lorry is basically a cold store on wheels. However, transport also subjects the material to physical and mechanical stresses, and possibly rapid changes in temperature and humidity, which are not encountered during static storage. It is necessary to consider both the stresses imposed during the transport and those encountered during loading and unloading. In many situations transport is multimodal. Air or sea transport would commonly involve at least one road trip before and one road trip after the main journey. There would also be time spent on the ground at the port or airport where the material could be exposed to wide-ranging temperatures and humidities, or bright sunlight, and unscheduled delays are always a possibility. During loading and unloading, the cargo may be broken into smaller units where more rapid heat penetration may occur.
The major challenges during transportation are to maintain the quality of the food during transport, and to apply good logistics – in other words, to move the goods to the right place at the right time and in good condition.
1.4 Raw Material Cleaning
All food raw materials are cleaned before processing. The purpose is obviously to remove contaminants, which range from innocuous to dangerous. It is important to note that removal of contaminants is essential for protection of process equipment as well as the final consumer. For example, it is essential to remove sand, stones, or metallic particles from wheat prior to milling to avoid damaging the machinery. The main contaminants are:
unwanted parts of the plant such as leaves, twigs, husks;
soil, sand, stones, and metallic particles from the growing area;
insects and their eggs;
animal excreta, hairs, and so on;
pesticides and fertilizers;
mineral oil;
microorganisms and their toxins.
Increased mechanization in harvesting and subsequent handling has generally led to increased contamination with mineral, plant, and animal contaminants, while there has been a general increase in the use of sprays, leading to increased chemical contamination. Microorganisms may be introduced preharvest from irrigation water, manure fertilizer, or contamination from feral or domestic animals, or postharvest from improperly cleaned equipment, wash waters, or cross-contamination from other raw materials.
Cleaning is essentially a separation process, in which some difference in physical properties of the contaminants and the food units is exploited. There are a number of cleaning methods available, classified into dry and wet methods, but a combination would usually be used for any specific material. Selection of the appropriate cleaning regime depends on the material being cleaned, the level and type of contamination and the degree of decontamination required. In practice a balance must be struck between cleaning cost and product quality, and an “acceptable standard” should be specified for the particular end-use. Avoidance of product damage is an important contributing factor, especially for delicate materials such as soft fruit.
1.4.1 Dry Cleaning Methods
The main dry cleaning methods are based on screens, aspiration, or magnetic separations. Dry methods are generally less expensive than wet methods and the effluent is cheaper to dispose of, but they tend to be less effective in terms of cleaning efficiency. A major problem is recontamination of the material with dust. Precautions may be necessary to avoid the risk of dust explosions and fires.
Screens
Screens are essentially size separators based on perforated beds or wire mesh by which larger contaminants are removed from smaller food items (e.g., straw from cereal grains, or pods and twigs from peas). This is termed “scalping” (Figure 1.1a). Alternatively “de-dusting” is the removal of smaller particles (e.g., sand or dust) from larger food units (Figure 1.1b). The main geometries are rotary drums (also known as reels or trommels) and flatbed designs. Some examples are shown in Figure 1.2. Abrasion, either by impact during the operation of the machinery, or aided by abrasive disks or brushes, can improve the efficiency of dry screens. Screening gives incomplete separations and is usually a preliminary cleaning stage.
Figure 1.1 Screening of dry particulate materials: (a) scalping and (b) de-dusting.
Figure 1.2 Screen geometries: (a) rotary screen and (b) principle of flatbed screen.
Aspiration
This exploits the differences in aerodynamic properties of the food and the contaminants. It is widely used in the cleaning of cereals, but is also incorporated into equipment for cleaning peas and beans. The principle is to feed the raw material into a carefully controlled upward air stream. Denser material will fall, while lighter material will be blown away depending on the terminal velocity. Terminal velocity in this case can be defined as the velocity of upward air stream in which a particle remains stationary, and depends on the density and projected area of the particles (as described by Stokes' equation). By using different air velocities, it is possible to separate, say, wheat from lighter chaff (Figure 1.3) or denser small stones. Very accurate separations are possible, but large amounts of energy are required to generate the air streams. Obviously the system is limited by the size of raw material units, but is particularly suitable for cleaning legumes and cereals. Air streams may also be used simply to blow loose contaminants from larger items such as eggs or fruit.
Figure 1.3 Principle of aspiration cleaning.
Magnetic Cleaning
This is the removal of ferrous metal using permanent or electromagnets. Metal particles derived from the growing field or picked up during transport or preliminary operations constitute a hazard both to the consumer and to processing machinery (e.g. cereal mills). The geometry of magnetic cleaning systems can be quite variable: particulate foods may be passed over magnetized drums or magnetized conveyor belts, or powerful magnets may be located above conveyors. Electromagnets are easy to clean by turning off the power. Metal detectors are frequently employed prior to sensitive processing equipment as well as to protect consumers at the end of processing lines.
Electrostatic Cleaning
This can be used in a limited number of cases where the surface charge on raw materials differs from contaminating particles. The principle can be used to distinguish grains from other seeds of similar geometry but different surface charge, and has also been described for cleaning tea. The feed is conveyed on a charged belt and charged particles are attracted to an oppositely charged electrode according to their surface charge (Figure 1.4).
Figure 1.4 Principle of electrostatic cleaning.
1.4.2 Wet Cleaning Methods
Wet methods are necessary if large quantities of soil are to be removed, and are essential if detergents are used. They are, however, expensive as large quantities of high purity water are required, and the same quantity of dirty effluent is produced. Treatment and reuse of water can reduce costs. Employing the countercurrent principle can reduce water requirement and effluent volumes if accurately controlled. Sanitizing chemicals such as chlorine, citric acid, and ozone are commonly used in wash waters, especially in association with peeling and size reduction, where reducing enzymic browning may also be an aim [16]. Levels of 100–200 mg l−1 chlorine or citric acid may be used, although their effectiveness for decontamination has been questioned and they are not permitted in some countries.
Soaking is a preliminary stage in cleaning heavily contaminated materials such as root crops, permitting softening of the soil, and partial removal of stones and other contaminants. Metallic or concrete tanks or drums are employed, and these may be fitted with devices for agitating the water, including stirrers, paddles, or mechanisms for rotating the entire drum. For delicate produce such as strawberries or asparagus, or products which trap dirt internally (e.g., celery), sparging air through the system may be helpful. The use of warm water or including detergents improves cleaning efficiency, especially where mineral oil is a possible contaminant, but adds to the expense and may damage the texture.
Spray washing is very widely used for many types of food raw material. Efficiency depends on the volume and temperature of the water and time of exposure. As a general rule, small volumes of high-pressure water give the most efficient dirt removal, but this is limited by product damage, especially to more delicate produce. With larger food pieces it may be necessary to rotate the unit so that the whole surface is presented to the spray (Figure 1.5a). The two most common designs are drum washers and belt washers (Figures 1.5). Abrasion may contribute to the cleaning effect, but again must be limited in delicate units. Other designs have included flexible rubber disks which gently brush the surface clean.
Figure 1.5 Water spray cleaning: (a) spray belt washer and (b) drum washer.
Flotation washing employs buoyancy differences between food units and contaminants. For instance, sound fruit generally floats, while contaminating soil, stones, or rotten fruits sink in water. Hence fluming fruit in water over a series of weirs gives very effective cleaning of fruit, peas, and beans (Figure 1.6). A disadvantage is high water use, thus recirculation of water should be incorporated.
Figure 1.6 Principle of flotation washing.
Froth flotation is carried out to separate peas from contaminating weed seeds, and exploits surfactant effects. The peas are dipped in oil/detergent emulsion and air is blown through the bed. This forms a foam which washes away the contaminating material, and the cleaned peas can be spray washed.
Following wet cleaning it is necessary to remove the washing water. Centrifugation is very effective, but may lead to tissue damage, hence dewatering screens or reels are more common.
Prestorage hot water dipping has been used as an alternative to chemical treatments for preserving the quality of horticultural products. One recent development is the simultaneous cleaning and disinfection of fresh produce by a short hot water rinse and brushing (HWRB) treatment [17]. This involves placing the crops on rotating brushes and rinsing with hot water for 10–30 s. The effect is through a combination of direct cleaning action plus the lethal action of heat on surface pathogens. Fungicides may also be added to the hot water.
1.4.3 Peeling
Peeling of fruits and vegetables is frequently carried out in association with cleaning. Mechanical peeling methods require loosening of the skin using one of the following principles depending on the structure of the food and the level of peeling required [18]:
Steam
is particularly suited to root crops. The units are exposed to high-pressure steam for a fixed time and then the pressure is released causing steam to form under the surface of the skin, hence loosening it such that it can be removed with a water spray.
Lye
(1–2% alkali) solution can be used to soften the skin which can again be removed by water sprays. There is, however, a danger of damage to the product.
Brine
solutions can give a peeling effect but are probably less effective than the above methods.
Abrasion peeling
employs carborundum rollers or rotating the product in a carborundum-lined bowl followed by washing away the loosened skin. It is effective but here is a danger of high product loss by this method.
Mechanical knives
are suitable for peeling citrus fruits.
Flame peeling
is useful for onions in which the outer layers are burnt off and charred skin is removed by high-pressure hot water.
1.5 Sorting and Grading
Sorting and grading are terms which are frequently used interchangeably in the food processing industry, but strictly speaking they are distinct operations. Sorting is a separation based on a single measurable property of raw material units, while grading is “the assessment of the overall quality of a food using a number of attributes” [18]. Grading of fresh produce may also be defined as “sorting according to quality,” as sorting usually upgrades the product.
Virtually all food products undergo some type of sorting operation. There are a number of benefits, including the need for sorted units in weight filling operations, and the aesthetic and marketing advantages in providing uniform-sized or uniform-colored units. In addition, it is much easier to control processes such as sterilization, dehydration, or freezing in sorted food units, and they are also better suited to mechanized operations such as size reduction, pitting, or peeling.
1.5.1 Criteria and Methods of Sorting
Sorting is carried out on the basis of individual physical properties. Details of principles and equipment are given in Saravacos and Kostaropoulos [19], Brennan et al. [20], and Peleg [21]. No sorting system is absolutely precise, and a balance is often struck between precision and flow rate.
Weight is usually the most precise method of sorting, as it is not dependent on the geometry of the products. Eggs, fruit, or vegetables may be separated into weight categories using spring-loaded, strain gauge, or electronic weighing devices incorporated into conveying systems. Using a series of tipping or compressed air blowing mechanisms set to trigger at progressively lesser weights, the heavier items are removed first followed by the next weight category, and so on. These systems are computer controlled and can additionally provide data on quantities and size distributions from different growers. An alternative system is to use the “catapult” principle where units are thrown into different collecting chutes, depending on their weight, by spring-loaded catapult arms. A disadvantage of weight sorting is the relatively long time required per unit and other methods are more appropriate with smaller items such as legumes or cereals, or if faster throughput is required.
