Vinegar E-Book

Caroline Gilmartin

• Herausgeber: The Crowood Press
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 315

Veröffentlichungsjahr: 2024

Beschreibung

In this book, fermented foods expert Dr Caroline Gilmartin details the production of vinegar from start to finish, covering a variety of methods for a range of skill levels. Learn how to make your own vinegars, from apple cider to raisin, in your familiar home environment, from the simplest low-intervention processes to more technical ones. The origins and development of this well-loved condiment remain a mystery to many, so a rundown of vinegar's history is provided. It takes an in-depth look at some of the world's most famous vinegars that will motivate you and inspire your own creations.

Leseprobe

The Tacuinum sanitatis is an eleventh-century illustrated handbook written by Ibn Butlan of Baghdad, in which he presents the elements needed for a healthy happy life. He particularly recommends vinegar for problems with bile and the gums. A woman stands on a ladder filling a pitcher with vinegar while a man watches.WIKIMEDIA COMMONS

CONTENTS

Preface

1 Introduction

2 A Potted Vinegar History

3 The Fermentation Process

4 Reasons to Make your Own Vinegar

5 General Equipment Requirements and Cleaning

6 How to Make Your Own Vinegar

7 Optimising Acetification

8 Monitoring Acetification

9 Troubleshooting the Vinegar-Making Process

10 Vinegar Recipes Using Ready-Made Alcohols

11 Preparing Alcohol Bases for your Vinegars

12 Alcohol Base Recipes

13 All-in-One

14 Scrap Vinegars

15 Vinegar Food and Flavour

16 Using Your Home-Made Vinegars

17 Vinegar for Health – Is It a Superfood?

18 Vinegars in the Home

Vinegar for Fun

A Summary of the Dos and Don’ts of Vinegar Making

Appendix: Producing Vinegar for Business

References

Glossary

List of Suppliers

Further Reading

Index

Acknowledgements

PREFACE

What do most of us know about vinegar? Not a lot, it seems. Though we might have five or six types in the cupboard, our ubiquitous and ancient condiment is largely taken for granted. While we instinctively reach for it to add the acidity our palates seek, few of us know the back story – complex microbial interactions and 10,000 years of history!

Home-made and shop vinegar are a world apart.

Alcohol and vinegar are inextricably linked: without the former you can’t have the latter. The process is as ancient as life on earth, an intricate dance between plants, yeasts and bacteria that will happen whether we intervene or not, but which we have learned over the millennia to control.

Handmade vinegars bear little resemblance to the mass-produced versions on supermarket shelves: they will be complex, delicious and unique.

This is a pastime that cannot be rushed because nature will not allow this to happen, but we can coax it along with a little understanding of the process. Vinegar making can be as simple or as complicated as you like; there are various starting points, also parameters, such as acidity, that you can measure or ignore. Vinegar can be produced with minimal scientific knowledge and specialist equipment, or it can be monitored closely for more precise results. This book will explain the science and history behind the vinegar, talk you through the most successful methods for home and small-scale craft production, and give you some delicious recipes to try.

While I have endeavoured to keep to the vernacular, there is a glossary at the back for jargon busting – and please don’t be put off by the maths: why not embrace the challenge? Fear not, because although it is helpful for measurements, vinegar making can be done without it.

CHAPTER 1

INTRODUCTION

WHAT IS VINEGAR?

I asked 380 adults (via an Instagram poll) if they knew where vinegar came from. Respondents could answer a) yes, b) I’ve a vague idea, or c) I’ve not a clue. The results can be seen in the chart.

Bearing in mind that a large proportion of those who replied work in the fields of either food or health, and as only 12 per cent knew the answer, let’s start at the very beginning! And as a starting point, here is a simple definition:

The breakdown of Instagram poll responses to the question ‘do you know how vinegar is made?’.

Vinegar is the result of a natural transformative process that is essentially a tale of two microbes: yeasts that turn sugar into alcohol, and acetic acid bacteria (AAB) that subsequently turn the alcohol into vinegar. Without alcohol, you can’t make vinegar.

We can expand this for some more detail:

Vinegar is a solution that contains 4–9 per cent acetic acid in water. It is produced in a two-step fermentation process. Firstly, yeasts break down sugar in fruit juice or grain mash to produce alcohol. Secondly, AAB convert the alcohol to vinegar in the presence of oxygen. It contains a range of bioactive components including organic acids, amino acids and phenolics, and has culinary, medical and household uses.

Etymology

Working backwards, our English version of the word comes from the French vin aigre, which means sour wine. In turn, this was derived from the Latin vinum acer, meaning the same. When malt vinegar first became popular in the fifteenth century it was known as alegar, as opposed to vinegar, as it was produced from ale, not wine, although this term has now died out (almost – see page 85).

ACETIC ACID

Acetic acid is an organic chemical compound (the scientific meaning of organic is ‘carbon containing’). It also has another name that you might come across: ethanoic acid. It is the component that gives vinegar its characteristic sour taste and pungent smell. The written formula and the molecular structure can be seen in the illustration.

Pure and undiluted it is called ‘glacial’ acetic acid because of the ice-like crystals that form at just below room temperature. In its glacial form it is highly corrosive, and its vapour is irritating to the eyes, nose, throat and respiratory system. Even at the low concentration found in vinegar, acetic acid can cause coughing, burning and streaming eyes!

Acetic acid can be produced in two distinct ways: in a laboratory using chemical reagents, or biologically with microbes.

Chemical Production of Acetic Acid

Vinegar existed thousands of years before anyone had heard of acetic acid, which wasn’t identified as its major component until the eighth century, by Persian alchemist Jabir ibn Hayyan (see box). During the Renaissance, German alchemists extracted pure acetic acid from the pigment copper acetate, and as early as 1648 an industrial process had been developed to isolate it by burning wood in a vacuum.

The molecular structure of acetic acid, where the C stands for carbon, the H for hydrogen, and the O for oxygen.

Until the late 1700s, it was assumed that the acetic acid found in vinegar was different to the pure glacial form; however, a scientist called Pierre Adet realised that it was the same thing, but dilution in water changed the characteristics.

Nowadays, industrial chemical processes such as methanol carbonylation and aldehyde oxidation are used for rapid acetic acid production on a grand scale, with a global market worth a massive USD 20.6 billion.1 This is a consequence of its versatility: it is used in the production of plastic bottles, wood glue, synthetic fabrics, chemical compounds, descaling and cleaning products, and as an acidity regulator in food with the additive code E260.2 It can also be safe for human consumption when diluted (see below,page 158).

The biological production of acetic acid is the focus for the rest of the book – the natural result of the action of yeasts and AAB on suitable carbohydrate substrates, resulting in vinegar. We’ll be looking in greater detail at the microbes and processes involved in Chapter 2.

What Vinegar is Not

When you go to your local chippy and they ask you if you’d like salt and vinegar on your chips, did you know that this is not technically vinegar? It is usually a diluted solution of industrially produced acetic acid mixed with caramel as a colouring agent, and as such is devoid of the complexity of flavour and nutritional compounds that result from natural fermentation processes. This product must be labelled NBC – non-brewed condiment, to distinguish it from vinegar. This was the result of legal action pursued by vinegar manufacturers in 1950, which went all the way to the House of Lords,3 as they attempted to protect their businesses from this cheap and non-authentic competition.4 The result was that anything described as vinegar must be the product of the double fermentation process (alcoholic followed by acetic), performed by microbes.

Fish and chips and non-brewed condiment! This cannot be called vinegar as it is made from industrial (food safe!) acetic acid coloured with caramel. DryWhite is the UK’s best-selling brand. DRYWHITE

JABIR IBN HAYYAN

Jabir Ibn Hayyan developed methods for distillation, and discovered citric acid (the sour component of citrus fruits) and tartaric acid (from wine lees), as well as hydrochloric and nitric acids. In those days, the dream of the alchemist was to be able to create pure gold – however, his combination of hydrochloric and nitric acids was able to dissolve gold, but the making of it evaded him. He was a prolific discoverer of processes we still use today, including distillation and the use of manganese dioxide in glass manufacturing to prevent the green tinge produced by iron. He also noted that upon boiling wine, a flammable vapour was released, paving the way for the later discovery of ethanol, the other essential part of our vinegar story, by another Iranian, Abu Baki al Razi, a few years later.

CHAPTER 2

A POTTED VINEGAR HISTORY

It would be remiss of me to gloss over vinegar’s fascinating history, as the processes we will be using have developed over thousands of years – so here is a whistlestop tour. There are two tales here: first, how the relationship between yeasts and AAB arose; and then the human perspective – how vinegar became so integral to our lives that we completely take it for granted.

MICROBES AND VINEGAR

Vinegar microbes have a long history of symbiotic association; you can see this in action as a mother of vinegar forms on the surface of a freshly made batch. This is a type of biofilm formed by a mass of microbes growing together within a cellulose matrix, similar in principle to a kombucha SCOBY or kefir grains. These are fascinating manifestations; bacteria and yeasts themselves can’t be seen without a microscope, yet they can produce these tangible and very visible entities.

Prokaryotes, including anaerobic bacteria, developed about 3.5 billion years ago. The evolution of Cyanobacteria, which could photosynthesise and produce oxygen, led gradually to the presence of aerobic bacteria, such as AAB, around 3.1 billion years ago (according to a new genetic analysis of dozens of families of microbes).5

Symbiotic relationships exist in other fermented foods: kombucha, water kefir, milk kefir and mother of vinegar are all examples of biofilms.

More complex, single-cellular yeasts (eukaryotes) developed about 2.7 billion years ago through endosymbiosis, whereupon smaller microbes became incorporated into larger ones. After the appearance of fruiting bodies of plants, about 125 million years ago, yeasts developed the ability to rapidly convert simple sugars into ethanol. This gave them a selective advantage because ethanol is toxic to many microbes, but yeasts can tolerate quite high levels. Over time, yeast ethanol metabolism and AAB use of alcohol as an energy source became aligned. Today both AAB and yeasts are ubiquitous in nature, on and in plant surfaces, soil and air.

HUMANS AND VINEGAR

While much is known about the origins of alcohol production, it is harder to pinpoint exactly when vinegar ‘began’. At first people didn’t know how to stop wine from turning into vinegar, and in many instances there might not have been much difference between the two – it’s fair to say that ancient wines would have been rather challenging for our palates.

Researchers at the University of Pennsylvania discovered 9,000-year-old Neolithic jars at Jiahu (Henan province, China) in which were detected traces of the earliest known alcoholic beverage. It appears to have been a delicious sounding mixture of wild grapes, hawthorn berries, rice and honey. Remnants of early wine manufacture are also scattered throughout the Middle East, but it’s not clear who, if anyone, can claim the rights to the ‘invention’ of vinegar.

Alcohol traces were found in fragments of jars from the neolithic period. ADOBE STOCK

The earliest known wines: a delicious-sounding mixture of wild grapes, hawthorn berries, rice and honey.

Vinegar Over the Ages

The Babylonians

The first written record of vinegar has been identified as dating from Babylonian times. By 3000bc, Babylonian civilisation was well established, and the deciphering of cuneiform symbols inscribed upon clay tablets tells us that they were great innovators, with winemaking an important industry.

Although vines were grown, dates grew better, so date beer/wine was the mainstay. The Babylonians knew that vinegar was able to prevent the deterioration of foodstuffs, and it was extensively used in preservation (more so than as a seasoning). Preservation was an activity that occupied much of our forebears’ time, as the seasonality of produce meant that storage was of paramount importance. As soured date-beer vinegar was abundant, it was more economical than using salt (see page 129 to find out how to make your own date or raisin vinegar). As viticulture spread throughout the Mediterranean, so did the presence of vinegar.

The Romans

In Roman times, diluted vinegar known as ‘posca’ was the drink of slaves and soldiers. It was safer than drinking plain water as the effect of vinegar in terms of water purification was known, even if the agents of disease were not – and one supposes that it made for a sober workforce and army too!

An acetabulum was found at every Roman feast (Museum August Kestner, Hannover). MARCUS CYRON

It was common in Roman households to have a vinegar-containing dish called an acetabulum on the table at mealtimes. As we know, the Romans were great feasters, and between courses it was usual to dip bread into it and consume this as a palate cleanser. This is interesting, given what we have recently learned about the ability of vinegar to help regulate glucose levels (see page 156).

Lucius Columella, who lived during the first century, in his text De Re Rustica (Farming Topics), presents the very first written recipes for the use of vinegar both in the kitchen and as a medicine (see page 126 for Columella’s fig vinegar recipe, which you can try yourself).

Rather unfortunately for them, the Romans also developed sapa, a delicacy of sweetened, boiled grape syrup. This they prepared by boiling fermented grape must in lead pots. Acetic acid in the must reacted with the pots, causing high concentrations of lead acetate in the sapa, and consequently, lead poisoning among the aristocracy.

An example of cuneiform tablets that were deciphered to reveal the first written mention of vinegar – sadly not the exact ones. WIKIMEDIA COMMONS

The Romans even had a verb for the act of boiling down grape must into a syrup: defrutare. It is likely that this tradition of boiling must eventually developed into the production of balsamic vinegar (see page 138).

The Ancient Greeks

The ancient Greeks had their own, far more beneficial version of a vinegar beverage. Oxymels were comprised of water, vinegar, honey and herbs. The physician Hippocrates, also known as ‘the father of modern medicine’, prescribed oxymels as salves for wounds and sores, and to be imbibed for the treatment of respiratory diseases. The Greek scholar Theophrastus (371–287bc6) described how vinegar reacted with metals to make mineral pigments such as white lead and verdigris from copper, for artistic use.

A paintbox with mineral pigments; vinegar was reacted with metals to make them. DADEROT

Ancient Islamic Civilisation

By ad700, the use of vinegar in ancient Islamic civilisation was also well established. The prophet Mohammed said: ‘Allah has put blessings in vinegar, for truly it was the seasoning used by the prophets before me.’ Although alcohol is considered haram, or forbidden according to the laws of Islam, vinegar is halal, or permitted.

This put a different perspective upon its production, because while the alcoholic starting material, most likely date beer, would have been without value, the opposite was true of vinegar. A one-step process was used for production, whereby fruit juice was given optimal conditions to turn to vinegar via simultaneous development of alcohol and acetic acid. (See page 125 for how to set up all-in-one vinegars.)

Europe

By the end of the fourteenth century, vinegar was firmly on the map as a genuine industry, centred in Orléans, France. This had grown hand-in-hand with the development of the French wine industry: the aristocracy had developed a taste for the fine wines of the Bordeaux, Loire and Rhône regions, most of which were barrelled and then transferred to barges that travelled up the River Loire to Orléans, the nearest major river port to Paris, for distribution via local wine merchants.7

Boats transported wine barrels along the Loire to Orléans where they were checked before leaving for Paris – taxes were payable, and no one wanted to pay taxes for undrinkable wine. ALAIN DARLES

The range of vinegars available from Martin Pouret, the last traditional vinegar maker in Orléans. MARTIN POURET

Once unloaded, the wine would be inspected by a team of piquers-jureurs, or quality control inspectors. Anything failing to make the grade was sold to local vinegar and mustard makers: vinegar was in high demand as a food preservative. With Orléans as a hub for over-oxidised fine wines, these became the bases for similarly fine vinegar. The process used was to lie aerated barrels on their sides to age for several months; this became known as the ‘Orléans method’, and is still used today. Orléans vinegar maintained its reputation until the French Revolution, whereafter industrialisation, and the use of cheap distilled spirits and global competition, caused its demise. From 300 producers pre-Revolution, today just one of the original vinaigriers, Maison Martin Pouret, survives.

Across the pond in the UK, in 1845 there were 65 London-based vinegar makers, using products including raisins, beer, gin and wood as bases for vinegar production. These days, big names including Sarson’s, Aspall and Manor Vinegar produce much of the regular malt and distilled fare, although there has been an upsurge in the production of raw apple cider vinegar (ACV) with the mother from both large and smaller producers.

After the Industrial Revolution to the Present Day

Since the Industrial Revolution vinegar production has, like everything else, been automated, but traditional methods and principles are still very much in evidence. Of the four processes described below, three are still in use in industry today, and we will be adapting these to make our own vinegar later (see section on page 44).

The Orléans Process (Surface Method)

The Orléans process is still used today for traditional vinegars such as balsamic and Jerez. Wooden barrels are laid on their sides to increase the surface area, with covered orifices for aeration to provide oxygen for the AAB. A covering of cellulose forms, and as the oxygen moves across this barrier, a concentration gradient is set up, with continuous diffusion of finished vinegar downwards. This can be tapped off at intervals, and fresh wine added. This is done using an in-situ long funnel that can add the alcohol to the bottom of vessel without disturbing the mother of vinegar. The process takes time – between months and many years, leading to excellent flavour development. Operating on a continuous culture basis, long slow evaporation and ageing, coupled with enrichment of successive barrels, leads to complex, flavoursome vinegars.

A series of barrels showing the Orléans process. Barrels are laid on their sides for increased surface area. On each you can see the air holes and level indicator. MARTIN POURET

A diagram showing the set-up of an Orléans vinegar barrel.

A diagram showing the Boerhaave process.

The Boerhaave and Trickly Methods (Quick Process Acetifiers)

The first major upgrade to the Orléans method was the work of Dutch scientist Herman Boerhaave (1668–1738).8 The principles of his method are increased aeration and surface area for optimal AAB activity. Wooden sticks or old vines were placed in each of two barrels, with plenty of room for liquid to be poured between. Something would be put across the sticks to keep them in place. The first barrel would be three-quarters filled with wine; the second, a quarter filled. Twice daily the wine in the fuller cask would be emptied into an emptier one, leaving one quarter behind. The constant tipping to and fro of the wine would oxygenate it, the sticks providing a much larger surface area for AAB to grow on than just the surface of still wine. The wine left in the bottom of the cask would prevent the sticks drying out. This would speed up the process immensely, and wine vinegar could be made within a month. In Chapter 7 we will see how to make a small-scale Boerhaave system – this is my favourite way to make vinegar at home.

A diagram showing the set-up for the trickling acetifier.

Modern acetators are not to be confused with acetifiers! These highly regulated machines include cooling columns, aeration, stirring, and strict control of every imaginable parameter to make vinegar very quickly! ADOBE STOCK

German chemist Karl Schüzenbach (1793–1869) further mechanised the Boerhaave process. Instead of two barrels, he filled just one with wine to the level of a false bottom. This was recirculated through a pipe with a rotating sprinkler attachment, so it would trickle down over the packing, thus continuously exposing the wine to AAB, and the bacteria to the air. The heat produced from the reaction could increase the efficiency to produce vinegar of 10 per cent acidity within three or four days. Packing materials such as these are still commonly used in industrial systems – for example the Welsh larchwood wool used by Sarson’s Malt Vinegar Company to this very day.

The Continuous Submerged Method (Acetator)

Submerged fermentation is the pinnacle of modern industrial production. High-grade vinegar can be produced in just 24 hours when specially selected strains of AAB are stirred with pumped pure oxygen, rather than just being left to grow on the surface.

Oxygen is fed directly into a tank containing a source of alcohol and microbes, which is stirred with an impeller. Nutrient levels, temperature, aeration, circulation rate and the ethanol:acetic acid ratio must be closely controlled. If aeration is too high, evaporation occurs, potentially lowering the yield. If there is a problem with the oxygen supply for as little as a minute, mass death of microbes can occur and a complete restart is needed. This method has been further adapted and refined by Heinrich Fring, founder of the company that today supplies acetators to the vinegar industry worldwide.

VINEGAR MYTHS AND LEGENDS

Vinegar is at the root of many famous tales; the following are some of the most interesting, if not the most credible!

The Inventor of Vinegar?

There is an ancient Chinese tale of a wine maker, Heita son of Du Kang (approximately 2000bc), who apparently invented vinegar. He thought it wasteful to discard wine lees, so he stored some in a jar. When he reopened it sometime later, the aroma of sweetness and sourness filled his nostrils and he couldn’t help but taste it… Thus, vinegar as a condiment was born, and perhaps even helped to form the basis of Chinese medicine.9

Heita, son of Du Kang. Did he invent vinegar? Perhaps – in any case it’s an interesting theory.

Cleopatra’s Pearls

In his Natural History, Pliny the Elder relates that Cleopatra, the last of the Egyptian queens, owned the two largest pearls of all time. They were worth millions of sesterces. To demonstrate ultimate superiority in extravagance over her lover, the Roman leader Marc Antony, she bet that she could spend ten million sesterces on a single meal. He accepted the wager; she fed him a sumptuous meal, which was delicious but clearly not that valuable, and he laughed, thinking he’d won. She explained that the best was yet to come. For the next course, servants brought a dish containing vinegar, into which she plunged one of the pearls. When it was dissolved, she swallowed it, thus winning the bet.

Cleopatra – a queen not to be reckoned with. HANNES GROBE/AW1

This had been dismissed as fiction; recently, however, classicist Prudence Jones of Montclair State University in New Jersey conducted an experiment to show that a supermarket strength vinegar (5 per cent w/v acetic acid) takes 24 to 36 hours to dissolve a solid pearl weighing about a gram. When the pearl is crushed and the vinegar warmed, the reaction time is reduced to about ten minutes. Pearls are composed of calcium carbonate, which is soluble in acetic acid. It didn’t happen quite as quickly as Pliny’s report would have us believe, but it really did work! It is certainly possible that Cleopatra, who was known to have carried out toxicological experiments, might have pre-softened the pearl to speed things up.10

Hannibal and the Impassable Pass

Hannibal, the great Carthaginian general, undertook his famous trek across the Alps from Spain to conquer Rome. According to the Roman historian Livy (Titus Livius 59bc–ad17), Hannibal’s army used the technique of fire-setting to clear rocks that were obstructing their path. Fires were set against a rockface to heat the stone, which was then doused with vinegar, the thermal shock causing the stone to fracture. Limestone rocks can be dissolved by the acetic acid in vinegar, but it would require a lot of it; in this case it is most likely that the physical reaction was more effective than the chemical one. Perhaps the soldiers were carrying vinegar for drinking and used this instead of water.

A Bronze Age example of how rocks were hollowed, in the Austrian Alps. ANDREAS G. HEISS

The Crucifixion

Vinegar has several mentions in the bible, but the best known occurs in the gospel descriptions of the crucifixion (Matthew 27:48, Mark 15:36, and John 19:29). Soldiers offered Jesus vinegar on a stick soaked in a hyssop sponge. This was extremely likely to have been posca, the diluted ‘sour wine’ drunk by soldiers, which would probably have been nearby for their refreshment. This interpretation rather changes the perspective of the offer from one of cruelty to one of kindness.11

Vinegar and Jealousy

The Chinese word for eating vinegar is 吃醋, which is made up of 吃, the verb ‘to eat’, and 醋, the word for ‘vinegar’. However, as a phrase this doesn’t simply mean ‘eat vinegar’ – it means ‘to be jealous’. During the Tang Dynasty (ad618–907) there was a prime minister called Fang Xuanling. He was a good man, reputedly very much in awe of his wife, Lady Lu. As a reward for his hard work, the emperor gave him two concubines – but Fang was so scared of his wife’s reaction that he sent them back to the palace.

The emperor was very upset at the rejection of his lovely gift, and called the couple to the palace for a conference. He gave Lady Lu two options: either to allow her husband to take the two ladies home, or to drink a glass of poison. With no hesitation, Lady Lu grabbed the glass of poison and downed it in one. Fang Xuanling was beside himself, but soon realised that the whole assembly was laughing, because it was not in fact poison, but vinegar, and the emperor had been testing Lady Lu! One assumes that after this, she was able to go home with just her husband, leaving the concubines behind.12

CHAPTER 3

THE FERMENTATION PROCESS

To recap, vinegar is a product of two separate microbial fermentations: alcoholic fermentation by yeasts, then acetic fermentation by AAB. The stages can occur either sequentially or concurrently, depending on which method you choose (more on this later, see page 44). First, let’s look at the microbes and the biological reactions in some more detail.

Microbes, which are so called because of their microscopic nature, can be thought of as tiny factories. Each makes all the components it needs to create a whole new microbe whilst producing a myriad of other things, including alcohols, acids, vitamins and enzymes. AAB and yeasts are both microbes, but they are fundamentally different creatures.

YEASTS AND THEIR FUNCTION

Yeasts are members of the fungus kingdom. Even though they still only exist as single cells, they are, like us, eukaryotes, with a nucleus of DNA contained in a membrane. They also contain organelles – tiny sub-factories where protein manufacture and energy production occur. Yeasts are an important part of everyday life, and essential for processes including beer-, wine- and breadmaking as well as biofuel production – and, of course, Marmite.

There are about 1,500 identified species, but the one that is most important in this context is Saccharomyces cerevisiae. A study examining yeasts on grapes used for making a natural wine found nine species present at the beginning of fermentation (such as Pichia, Candida, Zygosaccharomyces, Hanseniaspora), but by the end of fermentation only S. cerevisiae remained. It almost always dominates because it is extremely efficient at metabolising sugar. In fact the name Saccharomyces can be translated as sugar fungus, with the cerevisiae part meaning ‘of beer’ in Latin. There are many different strains with various characteristics, making them suitable for brewing or baking, for example – we’ll look at this in detail later (see page 104).

An overview of how vinegar is made: sugars in the juice are converted by yeast into alcohol. Then alcohol is converted to vinegar by acetic acid bacteria (AAB).

The structure of a yeast cell, showing the nucleus that contains the DNA; the organelles, including the mitochondria, which generate energy; and the golgi apparatus that packages proteins.

Louis Pasteur (1822–1895) – one of the greatest microbiologists of the Victorian age – invented pasteurisation, discovered acetic acid bacteria as spoilage agents in wine, ascribed fermentation to yeasts, set up a microbiology institute… PAUL NADAR

Electron micrograph of reproducing yeast cells – new ones form by budding, and here small buds are forming that will grow in size until they split off. ADOBE STOCK

In 1857 Louis Pasteur formally identified yeasts as living creatures responsible for making alcohol in wine; hitherto a chemical process was thought to be responsible, and yeast cells incidental.

Yeasts are facultative anaerobes, which means they can produce energy through different pathways, depending on whether oxygen is present (respiration) or absent (alcoholic fermentation). They produce nine times more energy through aerobic respiration.13 Despite this, if sugar is in plentiful supply, some yeasts will still make alcohol even in the presence of oxygen, a phenomenon called the Crabtree effect. This seems very inefficient and the reasons aren’t clear – maybe they do it for fun!

Yeast growth dynamics: lag phase, during which sterols are produced, leading to exponential growth phase, followed by stationary phase, where nutrient availability and growth are stabilising, then the death phase where nutrient depletion occurs. MICHAEL KOMORNICZAK

When yeast is added to a must, it enters a lag phase, during which it makes sterols (lipids) that are essential for membrane reinforcement; yeasts with strong membranes are more resistant to alcohol and more efficient at fermenting and replicating. Sterol building requires oxygen. The microbes then enter a growth phase: under optimal conditions, they multiply exponentially, doubling every 100 minutes or so, and can live for about 25 cycles before dying, and sinking to the bottom of the jar.

How Yeasts Produce Alcohol

Yeast can use a variety of sugars to make alcohol. Glucose and fructose can be used straight away, but sucrose and maltose need to be cleaved into simple sugars via the enzyme invertase. Glucose and fructose are transported into the yeast by hexose transporters – membrane proteins that act like gates in the cell wall.

Then begins a complex series of events, and yeasts will either ferment or respire depending on their environment. This reaction occurs quite happily at room temperature. It also generates heat that isn’t noticeable on a small scale, but in industrial settings, cooling can be required.

The invertase reaction, which splits sucrose into glucose and fructose (and maltose into glucose).

Fermentation pathway in yeast, through which alcohol is produced.

ACETIC ACID BACTERIA (AAB)

AAB are prokaryotes, the simplest of creatures – a level down from eukaryotes, we could say, with just the most basic cellular machinery. They are a large family of microbes, four types of which are of particular interest to us, because they are directly involved in the manufacture of vinegar and kombucha. These are Acetobacter, Gluconobacter and the succinctly named Komagataeibacter and Gluconacetobacter. While they are diverse enough to have different names, they all oxidise ethanol to acetic acid.

Acetic acid bacteria are prokaryotes, with minimal cell machinery.

An electron micrograph of Acetobacter aceti in pairs (coloured). ADOBE STOCK

Celluose production is common in AAB, but not all strains are producers.

AAB are rod-shaped or oval, occurring singly, in pairs or in chains. They do not form spores, and often have flagella, which are structures like tadpole tails that can help them swim in liquids. They are aerobes, requiring oxygen for growth and reproduction, and thrive in acidic environments, between pH3 and 6.5. Most prefer temperatures between 26 and 30°C, though they can make vinegar at lower temperatures more slowly.

Other characteristics include, for some, the production of copious quantities of cellulose, which we see as vinegar mother, or kombucha SCOBY. This cellulose matrix serves as a support framework to enable the microbes to grow at the air-liquid interface, and also helps to protect them from harsh environmental conditions.

Louis Pasteur can also claim the credit for establishing the importance of AAB in fermentation. In 1856 he was commissioned by a manufacturer of beetroot wine to determine why the product kept spoiling. When he examined the microbes in the wine he found oval, plump yeast cells in normal beet juice, but in spoiled batches there were also smaller oval-shaped microbes. He identified them as Acetobacter sp., the agents of wine spoilage and vinegar production.

AAB are ubiquitous in nature, even in soil where they promote plant growth through nitrogen fixing. They are used commercially for the bioproduction of cellulose; bacterial cellulose is strong, and much purer than the plant-based version, which is usually entangled with lignin. Due to its excellent water-holding capacity, high tensile strength and biocompatibility, it has therapeutic applications including for burn, wound and ulcer repair.

Curiously, many strains of AAB are not that tolerant to the acetic acid they produce and can struggle to survive in strong vinegar – the same goes for high concentrations of alcohol, and above 10 per cent they die off. Acetic acid bacteria have a far slower doubling time than yeasts – up to twelve hours at room temperature, compared with about 90 minutes for brewer’s yeast. This explains why vinegar can take so long to form, and why so much effort has been expended in optimising the process (see page 58).

This chemical equation will underpin much of the optimisation of the alcohol-making process. One mole of glucose makes two moles of ethanol and two moles of CO2. One mole of glucose weighs 180g. One mole of ethanol weighs 46g. One mole of CO2 weighs 44g. Calculations show that 1kg of glucose can produce about 500g of both ethanol and carbon dioxide. If you put that kilo of sugar into 4ltr of water, you can produce about 130g/ltr ethanol, which equates to 13 per cent. This is where the idea that 250g/ltr sugar is the required quantity for making wine. The equation also shows that for every 1 per cent of alcohol produced, 19.23g of sugar is required.

HOW AAB USE ETHANOL TO PRODUCE ACETIC ACID

In the second stage of the process, AAB convert alcohol into acetic acid. Oxygen (air) is required, and there are two separate steps. In the cytoplasm are thousands of different enzymes. Two of these, alcohol dehydrogenase and aldehyde dehydrogenase, are involved in breaking down alcohol – this can be seen in the illustration.

The two steps in the conversion of alcohol to acetic acid. The reaction also requires a co-factor called purroloquinoline (PQQ) and calcium.

Firstly, alcohol is oxidised to acetaldehyde and water; then with more oxygen, acetaldehyde is oxidised to acetic acid. This reaction produces heat, and although it can occur slowly well below room temperature, the microbes prefer a warmer 24–28°C. Some acetic acid will also react with the remaining alcohol to make ethyl acetate, emitting strong solventy odours – this is a reversible reaction, and is a sign that fermentation is underway but not yet complete.

Some of the resulting acetic acid is used to generate energy for the cell, the rest is exported by a transporter mechanism in the cell wall, to the external medium, thus making vinegar. Both actions reduce the intracellular acetic acid concentration, which if it rose too high, would kill the bacterium.14

In the absence of alcohol, AAB can oxidise sugars for energy – and if they’re really starving, they can use their own acetic acid that they’ve made, oxidising it to carbon dioxide and water; we will talk about how to avoid this reaction destroying all your hard work later (see page 66).

The Yield of the Reaction

The theoretical yield of the vinegar-making process is as follows: one quantity of sugar can be fermented into almost equal quantities of alcohol and CO2. This quantity of alcohol can then be converted into an equal quantity of vinegar. However, in real life the process isn’t this efficient. Working backwards, to end up with vinegar at 5 per cent acetic acid, about 7 per cent alcohol is recommended. In turn, to make a 7 per cent alcohol mash, approximately 14 per cent sugar is needed at the start. The bottom line is, you need enough sugar to make enough alcohol convert to vinegar.

MOTHER OF VINEGAR