1
The Science Underpinning Food Fermentations

Use the word 'biotechnology' nowadays and the vast majority of people will register an image of genetic alteration of organisms in the pursuit of new applications and products, many of them pharmaceutically relevant. Even the Merriam-WebsterDictionary tells us that biotechnology is 'biological science when applied especially in genetic engineering and recombinant DNA technology'. Fortunately the Oxford English Dictionary gives the rather more accurate definition as 'the branch of technology concerned with modern forms of industrial production utilising living organisms, especially microorganisms, and their biological processes'.

Accepting the truth of the second of these, then we can realise that biotechnology is far from being a modern concept. It harks back historically vastly longer than the traditional milepost for biotechnology, namely Watson and Crick's announcement in the Eagle pub in Cambridge (and later, more formally, in Nature) that they had found 'the secret of life'.

Eight thousand years ago our ancient forebears may have been, in their own way, no less convinced that they had hit upon the essence of existence when they made the first beers and breads. The first micro-organism was not seen until draper Anton van Leeuwenhoek peered through his microscope in 1676 and neither were such agents firmly causally implicated in food production and spoilage until the pioneering work of Needham, Spallanzani and Pasteur and Bassi de Lodi in the eighteenth and nineteenth centuries.

Without knowing the whys and wherefores, the dwellers in the Fertile Crescent were the first to make use of living organisms in fermentation processes. They truly were the first biotechnologists. And so beer, bread, cheese, wine and most of the other foodstuffs being considered in this book come from the oldest of processes. In some cases these have not changed very much in the ensuing aeons.

Unlike the output from modern biotechnologies, for the most part we are considering high volume, low value commodities. However for products such as beer, there is now a tremendous scientific understanding of the science that underpins the product, science that is none the less tempered with the pressures of tradition, art and emotion. For all of these food fermentation products, the customer expects. As has been realised by those who would apply molecular biological transformations to the organisms involved in the manufacture of foodstuffs, there is vastly more resistance to this than for applications in, say, the pharmaceutical area. You don't mess with a person's meal!

Historically, of course, the micro-organisms employed in these fermentation processes were adventitious. Even then, however, it was realised that the addition of a part of the previous process stream to the new batch could serve to 'kick off' the process. In some businesses this was called 'back slopping'. We now know that what the ancients were doing was seeding the process with a hefty dose of the preferred organism(s). Only relatively recently have the relevant microbes been added in a purified and enriched form to knowingly trigger fermentation processes.

The two key components of a fermentation system are the organism and its feedstock. For some products, such as beer, there is a radical modification of the properties of the feedstock, rendering them more palatable (in the case of beer, the grain extracts pre-fermentation are most unpleasant in flavour; by contrast, grape juice is much more acceptable). For other products the organism is less central, albeit still important. One thinks for instance of bread, where not all styles involve yeast in their production.

For some products, such as cheese, the end product is quite distinct from the raw materials as a result of a series of unit operations. For products such as beer, wine and vinegar the product is actually the spent growth medium - the excreta of living organisms if one was to put it crudely. Only occasionally is the product the actual micro-organism itself - for example the surplus yeast generated in a brewery fermentation or that generated in a microbial biomass ('single cell protein') operation such as the production of mycoprotein.

The organisms employed in food fermentations are many and diverse. Key players are the lactic acid bacteria, in dairy products for instance, and yeast, in the production of alcoholic beverages and bread. The lactic acid bacteria, to illustrate, may also have a positive role to play in the production of certain types of wines and beers, but equally they represent major spoilage organisms for many such products. It truly is a case of the organism being in the right niche for the product in question.

In this chapter we will focus on the generalities of science and technology that underpin fermentations and the organisms that are involved. We will look at commonalities in terms of quality - for example the Maillard reaction that is of widespread significance as a source of colour and aroma in many of the foods that we are considering. The reader will discover (and this betrays the primary expertise of the authors) that many of the examples given are from beer making. It must be said, however, that the scientific understanding of the brewing of beer is somewhat more advanced than that for most if not all of the other foodstuffs described in this book. Many of the observations made in a brewing context very much translate to what must occur in the less well-studied foods and beverages.

1.1 Micro-Organisms

Microbes can essentially be divided into two categories: the prokaryotes and the eukaryotes.

The former, which embrace the bacteria, are substantially the simpler, in that essentially they comprise a protective cell wall, surrounding a plasma membrane, within which is a nuclear region immersed in cytoplasm (Figure 1.1). This is a somewhat simplistic description, but sufficient for our needs. The nuclear material (deoxyribonucleic acid, ) of course figures as the genetic blueprint of the cell. The cytoplasm contains the enzymes that catalyse the reactions necessary to the growth, survival and reproduction of the organisms (the sum total of reactions of course being referred to as metabolism). The membrane regulates the entry and exit of materials into and from the cell.

Figure 1.1 A simple representation of a prokaryotic cell. The major differences between Gram positive and Gram negative cells concerns their outer layers, with the latter having an additional membrane outwith the wall in addition to a different composition of the wall itself.

The eukaryotic cell (of which bakers or brewers yeast, Saccharomyces cerevisiae, a unicellular fungus, is the model organism) is substantially more complex (Figure 1.2). It is divided into organelles, the intracellular equivalent of our bodily organs. Each has its function. Thus the DNA is located in the nucleus which, like all the organelles, is bounded by a membrane. All the membranes in eukaryotes (and prokaryotes) comprise lipid and protein. Other major organelles in eukaryotes are the mitochondria, wherein energy is generated, and the endoplasmic reticulum. The latter is an interconnected network of tubules, vesicles and sacs with various functions including protein and sterol synthesis, sequestration of calcium, production of the storage polysaccharide glycogen and insertion of proteins into membranes. Both prokaryotes and eukaryotes have polymeric storage materials located in their cytoplasm.

Figure 1.2 A simple representation of a eukaryotic cell.

Table 1.1 lists some of the organisms that are mentioned in this book. Some of the relevant fungi are unicellular, for example Saccharomyces. However the major class of fungi, namely the filamentous fungi with their hyphae (moulds), are of significance for a number of the foodstuffs, notably those Asian products involving solid state fermentations, e.g. sake and miso, as well as the only successful and sustained single cell protein operation (Chapter 17).

Table 1.1 Some micro-organisms involved in food fermentation processes.