The molecular, vegetative animalcule
Throughout the centuries, yeast has been an important commodity in domestic and industrial life. Consequently, its fermenting ability has been the focus of many different hypotheses and paradigm shifts. Fermentation was once thought to be the consequence of a chemical reaction by some kind of substance and never imagined to be the metabolic activity of a living organism. During the Biblical era, due to lack of knowledge, there would have been no conception that microscopic bugs were responsible for fermentation.
Yeasts are abundant in the environment, especially in the soil where they are transferred, by insects or other means, on to the skins of fruit and animals, including humans. There exists a wide variety of naturally occurring yeasts, from those that cause fungal infections (Candida spp.) to industrially important species used in baking and wine production (Saccharomyces spp.). Yeast belongs to the kingdom Fungi and division Ascomycota, derived from the Greek for ascus meaning “sac” or “wineskin”, a name inspired by their characteristic spore cases. In recent times, it has become a major player in biological research and is now one of the most studied organisms on Earth. It was the first eukaryote to have a fully sequenced genome, the majority of its genes have been researched and functionally analysed and many of these genes have analogues in other multicellular organisms. It is, therefore, possible to study molecular processes from mammalian systems within a unicellular, model eukaryote, making yeast a favourable alternative to animal model systems. Furthermore, the simplicity of its life cycle and the ease in which it is cultured has made it a valuable resource in medical and biotechnological applications. Yeast has make a great impact on 21st century anthropology, which has influenced social behaviour and scientific research. Humanity would most probably have evolved differently if this organism ceased to exist.
The term leaven originates from the latin word levere, the French equivalent being the noun levure meaning to rise. The English term yeast, probably originates from the Greek term for fermenting which was zestos. Derivatives from later European influences resulted in the medieval English terms zeest, which eventually became yest. Leaven when mentioned in the Bible is probably used to describe fermented dough or sourdough. So called because, along with yeast cells, it contained acidifying bacteria that produce lactic and acetic acid, giving the bread a unique tangy flavour. Natural microbial contaminants of milled grains and fruit were probably used for alcohol production and leavening in the Biblical era. This microbial flora would have included wild yeasts that were naturally found on cultivated crops. In just one hundred grams of flour there are up to ten billion microbes of these about thirty thousand are natural yeasts.
The historical steps taken to elucidate the metabolic processes and characteristics associated with yeast and fermentation follow an intriguing journey of scientific discovery that spanned nearly four centuries. A journey that commenced with the discovery of microscopy in the 17th century and is now meandering through the complex science of molecular biology in the 21st century. This chapter looks at the history of Science in relation to the discovery of yeast, exploring how Biblical text has influenced the principles and directions of scientific investigation, for instance, the Biblical version of creation greatly differs from scientific theories of evolution. The ensuing debates this creates typically illustrates divides that exist between religious and scientific theory.
It’s generally accepted that for an enquiry to be viewed as scientific it must involve the gathering of observable, empirical and measurable evidence. The scientific method involves the collection of this data to formulate and test hypotheses. A number of proven hypotheses, from various published and recorded sources, can then be strung together in a wider context to form theories. The practice of distributing and therefore sharing data is often referred to as full disclosure as it permits evidence to be scrutinised by others thereby allowing the interpretation of results to be challenged.
The Bible indicates that a divine being must have in some way directed the creation of Life in order to account for its existence on Earth and complexity. An understandable viewpoint, as even a simple single-celled microscopic organism such as yeast is an intricate living structure encoded by over 6000 genes. It’s difficult to visualise where DNA originated and how complexity could have happened gradually over time. The 21st century scientific method would not defend the concept of human origin as presented in the Bible as it’s largely based on theories without the support of tangible evidence. Although the ancient scientific method may well have defended it when it was first written, given the lack of scientific research at the time. Ultimately though, it is impossible for anyone living in the 21st century to know, with any certainty, how the World was created without use of a time-machine. There is certainly no compelling reason why there should be Life on Earth, so various scientific and theological arguments must be considered before determinate conclusions are reached.
Based on archaeological evidence, it is thought that the Earth was created approximately 4.6 billion years ago and that life originated in the form of bacteria 3.8 billion years ago. Multicellular organisms, in various microscopic forms, are thought to have first existed about a billion years ago and are thought to have given rise to simple animals, such as sponges and anemones. More complex forms of animals started to appear 550 million years ago (mya). Arthropods are thought to have appeared first, followed by fish, land plants, insects, amphibians, reptiles, mammals, birds and flowering plants respectively. The first primates that resembled humans arrived on the scene 2.5 mya with humans resembling the present form arriving just 200,000 years ago. Neanderthals disappeared fairly recently, only 25,000 years ago. A short time in evolutionary terms, but still a vast time-scale for a human with an average lifespan of 62.5 years to visualise. When the Bible was written human lifespan was only 28 years.
The Bible condenses the time-scale that humans arrived on earth within the space of a few days. On the first day God created light which was divided into day and night. The next day, water were divided from the sky and earth was divided into land and sea. It was not until the third day that Life first appeared when God created vegetation. During the fourth day planets and stars were added with time being divided into seasons, days and years. On the fifth day God creates birds and sea creatures, commanding them to be fruitful and multiply. Finally, on the sixth day he adds wild beasts, livestock and reptiles, creating humanity in his own “image”. The humans are instructed to be fruitful, multiply, fill the earth and subdue it. In the book of Genesis the Bible draws a distinction between animals and humans, with emphasis on human superiority:
God said, ‘Let us make humans in our own image, in the likeness of ourselves, and let them be masters of the fish of the sea, the birds of heaven, the cattle, all the wild animals and all the creatures that creep along the ground.’
[Gen 1. 26]
There are a number of anomalies in the account of creation within the Bible. We now know that the stars and planets must have existed before the introduction of life. Seasons would prerequisite the existence of vegetation. We are also aware that reptiles existed before mammalian species on account of dinosaur fossil records. These restrictions in knowledge also resulted in no reference to the creation of microbes within Genesis, but there are several references to disease and of fermentation processes throughout the Bible.
Ancient civilisations would be completely unaware of the causes to many diseases. In general, injuries resulting from accidents or animal bites were understood and treated by various dressings, including sesame oil, wine and balsams, some of which contained naturally occurring antibiotics. In contrast, the mechanisms behind diseases involving parasites or microbes were a complete mystery and thought to have been placed in the body by evil forces. Consequently, these illnesses were treated by making the body hostile to the invader through intense cleansing with various noxious substances. Diseases were largely viewed as a punishment brought through sin or disobedience:
The Lord said, ‘If you will not obey my commands, you will be punished. If you refuse to obey my laws and commands and break the covenant I have made with you, I will punish you. I will bring disaster on you- incurable diseases and fevers that will make you blind and cause your life to waste away.
As microbes where viewed as some kind of mysterious, unexplained power they were also used to represent the spread of sin and corruption. For instance, in his first letter to the Corinthians, Paul the Apostle uses the permeating character of leaven to illustrate the spreading of corruption within a community:
You know the saying, “A little leaven makes the whole batch of dough rise.” You must remove the old leaven of sin so that you will be entirely pure. Then you will be like a new batch of dough with no leaven, as indeed I know that you actually are. For our Passover Festival is ready, now that Christ, our Passover lamb, has been sacrificed. Let us celebrate our Passover, then, not with bread having the old leaven of sin and wickedness, but with the bread that has no leaven, the bread of purity and truth.
[1 Cor. 5.6-9]
Fermentation procedures and the art of making leavened bread was first introduced by the Egyptians over 4000 years ago. In ancient Egypt, brewing and baking were practiced together on the same premises. Wooden and plaster models depicting Egyptians working in a brewery and bakery have been recovered from a tomb dated around 1975 BC during the reign of Amenemhat I. Egyptian hieroglyphs that show pictures of dough rising next to bread ovens have also been found. The Egyptians were, therefore, aware that both processes used the same substance. To the Egyptians, bread was an important commodity used as a type of currency for trading and for the payment of services. In fact, the workers who built the pyramids were believed to have been paid in bread. Bread was also used to pay homage to Osiris the God of grain. The Egyptians believed that Osiris had given them the gift of leavened bread to make it lighter to carry, especially in the journey to the afterlife.
In contrast to the Egyptians who revered fermented dough, the ancient Palestinians were extremely suspicious of it. Perhaps this could be due to the different methods by which the bread was leavened. Palestinians would reserve a piece of leavened dough from a previous baking to make the next batch of bread. This dough could easily have been contaminated by harmful microbes which may not have been completely destroyed by the baking processes. If a person died all leaven was thrown out of the house and from all neighbouring houses because Palestinians thought that it may have contributed in some way to the individual’s demise, they believed that the angel of death may have thrust his sword into it.
In Britain during the 15th century, as in ancient Egypt, brewing also took place on the premises where bread was baked. The thick layer of dormant yeast cells that sank to the bottom of the brewing or wine vessel was usually referred to as the lees, while froth from the top of beer was known as barm. Barm from a good batch of beer would be reused to make more beer and to ferment bread dough. Barm, the brewer’s equivalent of leaven, used to be referred to as Godisgoode because it was thought to be a gift from God. Yeast used in brewing and baking seemed to be received in a different manner to leaven that was used to make sourdough. In fact, during the 17th century the Paris Faculty of Medicine tried to ban leaven because in the New Testament St. Paul had signified that it was a substance that denoted corruption.
So it seems that fermentation generated through leaven was not viewed in the same way or as a similar process to the fermentation that produced wine. It is clear from passages in the Bible that leaven is used figuratively to symbolise the permeating influence of certain types of human behaviour, especially corruption. Traditionally unleavened bread is used to symbolise the purity of Christ’s body whereas wine symbolises the blood of Christ. This is not without some irony, as wine is made in a similar process and perhaps more likely to lead to acts of sin and corruption when consumed than is bread. Indeed, some religions do recognise this and at the beginning of the 20th century, alcohol became prohibited for a time in a number of Nordic countries and North America due to Protestant led objections. Alcohol is also prohibited in some countries that follow Islamic laws. The contradictory views relating to fermentation in wine and bread will be discussed at greater length in a later chapter.
Although its meaning still may remain a mystery, Life in itself is hard work and requires a lot of energy. It’s now well established that the initial source of this energy is provided by the Sun in the form of light, which is absorbed by a photosensitive pigment called chlorophyll found in plants and other photosynthetic organisms. The energy is then trapped in molecules of glucose, a carbohydrate compound composed by a series of chemical reactions involving carbon dioxide and water. Plant consumers then transfer the energy stored within the glucose carbon source along the food chain. When the glucose is broken down it produces adenosine triphosphate (ATP), the compound required to release the energy that powers most cellular functions. The most efficient way for an organism to synthesise ATP, thereby releasing energy, is by an oxygen requiring process called cellular respiration. In humans, oxygen is transferred into the body from the surrounding atmosphere by respiring, it is extracted from air in the lungs by haemoglobin, which is then circulated around the system in the blood. As it contains the oxygen required for anaerobic energy production humans cannot survive without blood. Blood was, therefore, considered of extreme importance in the Biblical era, as it was the substance thought to contain an animal’s character and life force.
Every living thing is a package of consumable energy, but not every organism can boast a sophisticated circulatory system that enables cellular respiration. Microbes and other lower life forms have to adopt fairly basic means to generate their energy. The energy generating processes of yeast produces by-products that have been exploited by human civilisations for centuries. One of the ways yeast requires its solar produced energy is by fermentation; a biochemical transformation that converts carbon sources such as glucose or sucrose into energy, producing alcohol and giving off carbon dioxide in the process.
Fermentation is not as efficient in producing ATP as aerobic respiration, but enables yeast to convert glucose into energy without the aid of oxygen. Scientifically defined, fermentation is a catabolic process that makes a limited amount of ATP from glucose without an electron chain (supplied by oxygen) producing a characteristic end product, such as, ethyl alcohol or lactic acids. During fermentation, yeast not only generates energy from the carbon source but it also breaks it down into an industrially and socially important commodity, namely alcohol. Yeast also has the ability to perform aerobic respiration to give off carbon dioxide, but this process does not produce alcohol. Being able to live with or without oxygen is undoubtedly ecologically advantageous to this microbe. Certainly explaining why it inhabited the Earth long before humans did and why it will still be here long after our fragile species has disappeared.
The mysteries surrounding fermentation were once, and to some extent still are, the subject of great scientific endeavour. It was mainly assumed that the reaction was chemically induced because investigators were unaware that miniscule creatures unseen by the human eye could exist. The yeast commercially responsible for transforming carbohydrate rich ingredients, like flour and fruit juice, into loaves of bread or alcoholic drinks is predominately Saccharomyces cerevisiae also known as baker’s, brewer’s or budding yeast. When sugar is plentiful the metabolic route that this type of yeast chooses is fermentation. During fermentation cells multiply rapidly by budding, when all carbon resources are depleted cells either enter a stationary phase of non-division or produce spores. Budding yeasts can also reproduce sexually. Adjacent cells of opposing mating types fuse together, in response to pheromones, by forming protruded structures called shmoos. The end product is a slightly larger round diploid cell that contains two sets of chromosomes; a method of introducing genetic variability into the cell. This diploid cell can either continue budding or enter meiosis to produce four ascospores.
Performing meiosis to generate ascospores is a risky business to budding yeast as it has to temporarily stop increasing population size. Therefore, it only faces this challenge when nutrients are low and its survival is threatened. Ascospores and quiescent cells are resistant to stress and can remain dormant for several months, years, decades or even centuries. While dormant they lie at the bottom of the fermenting vessel to form a thick layer of pale brown sediment. Some cells or spores die, but many retain the ability to begin dividing again when conditions improve, for instance when more sugar becomes available. This mode of survival allows them to remain viable in the face of adversity. They are well suited to harsh industrial conditions and, also, the arid environment that forms the backdrop of the Biblical Testaments.
Like other simple life forms, yeasts such as Saccharomyces cerevisiae are fully self-contained within one microscopic cell. S. cerevisiae cells are round and, providing they are well nourished with carbohydrates, spend most of their life cycle reproducing vegetatively by growing buds. Buds separate from the parental cell when they reach a certain size in order to follow an individual pattern of growth. Upon maturity these too can start budding; each cell produces about thirty progeny. The loss of the bud leaves a scar on the parental cell that can be visualised with fluorescent dyes or electron microscopic techniques. The pattern and number of scars can reveal a lot about the condition and age of the yeast cell. Some yeasts do not reproduce by budding, but by forming a cross-wall rather like the mitotic cell division observed in higher eukaryotes. Schizosaccharomyces pombe or fission yeast is an example of this. It divides in a similar way to human cells and therefore is used as a model system to study many human diseases, especially cancer.
The concept that living organisms produced leaven wasn’t seriously considered until Erxleben, in 1818, proposed that leaven and barm consisted of living vegetative organisms responsible for fermentation. Prior to this, in 1680, Leeuwenhoek, with his early microscope, observed yeast cells in fermenting beer. He referred to most of these single-celled creatures as animalcules because they were believed to be immature forms of larger animals. These first observations of microscopic cells were not further investigated for another century. Leeuwenhoek’s contemporaries were largely preoccupied with the argument centred on spontaneous generation, a belief that animals could materialise from other living or mineral things. Before the groundbreaking experiments by Louis Pasteur in the mid 19th Century, which showed that excluding particles from sterile broth prevented contamination by microbes, many theorists believed in spontaneous generation.
Different theories and speculations concerning the creation of organic things occur in every religion, as most feel that the complexity of the natural world could not have arisen by chance. Many investigators began to challenge the image of creation as depicted in the Bible. Perhaps the most compelling of these arguments was the theory of natural selection presented by Charles Darwin in the mid 19th century. His book entitled the Origin of Species created tensions between the church and science because it questioned a popular and largely accepted image of creation.
Religious devotees perhaps saw science as being not only a threat to their faith but to their social acceptance and respect. Science innovation threatened to ridicule the basis of their fundamental beliefs and values. It is, therefore, understandable that there was a need to retain Biblical teachings in some form. In the 19th century, the paradigm shift that was rapidly evolving science was too extreme to evoke an equally rapid change in religious faith. In order to fully commit to a belief requires a great deal of conviction. This conviction can be impenetrable leading believers to imagine that an evil being is responsible for any deviancy from a steadfast commitment. Any element of uncertainty in religious belief seems to lead to the evolution of new religious theories to give meaning to situations that are too difficult to comprehend. In the New Testament an interesting method is used to quell sceptics and doubting critics. Individuals who questioned the ideals proposed by Jesus were thought to be influenced by the devil:
After spending forty days and nights without food, Jesus was hungry. Then the devil came to him and said, “If you are God’s Son, order these stones to turn into bread.”
But Jesus answered, “The scripture, says, Man cannot live on bread alone, but needs every word that God speaks.”
[Matt. 4.1-11; MK. 1.12-13; Lk. 4.1-13]
This not only discourages doubt from those with religious faith but also prevents others from persuading them away from their convictions. It is not surprising that scientific hypotheses that question religious beliefs are subject to contention.