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Microorganisms and food
To the general public, the association of microorganisms and food conjures up negative images of rotten fruit or food poisoning. On reflection, many people may remember that yeast is involved in bread and beer production, but how many realise that microor-ganisms play a part in the manufacture of soy sauce, pepperoni and even chocolate? In the following pages, we shall look at the contribution of microorganisms to the contents of our shopping baskets before considering one of the negative associations referred to above, the microbial spoilage of food.
The production of foodstuffs as a result of microbial fermentation reactions pre-dates recorded history. The accidental discovery that such foods were less susceptible to spoilage than fresh foods must have made them an attractive proposition to people in those far-off days. Of course, until relatively recent times, nothing was known of the part played by microorganisms, so the production of beer, cheese and vinegar would not have been the carefully controlled processes that are used today. Indeed, it was only with the development of isolation techniques towards the end of the nineteenth century, that it became possible to use pure cultures in food production for the first time. The fermentation of foodstuffs, hitherto an art, became a science.
There is evidence that alcoholic drinks, including beer and wine, were being produced thousands of years be-fore the Christian era, making them among the earliest known examples of the exploitation of microorganisms by humans. Ethanol results from the fermentation pro-cess, because the conversion of sugar to carbon dioxide and water is incomplete:
C6H12O6−−−−−−−→ 2CH3CH2OH + 2CO2
Although, in principle, wine can be made from almost any fruit juice with a high sugar content, the vast major-ity of commercially produced wines derive from the fer-mentation of the sugar present in grapes (Figure 17.1). Such fermentation reactions may be initiated by yeasts naturally found on the grape skin; however the results of such fermentations are erratic and may be unpalatable. In commercial winemaking the must (juice) resulting from the crushed grapes is treated with sulphur dioxide to kill off the natural microflora, and then inoculated with the yeast Saccharomyces cerevisiae, variety ellipsoideus. Specially developed strains are
used, which produce a higher percentage of alcohol (ethanol) than naturally occurring yeasts. Fermentation proceeds for a few days at a temperature of 22–27 ◦ C for red wines (lower for whites), after which the wine is separated from the skins by pressing. This is followed by ageing in oak barrels, a process that may last several months, and during which the flavour develops. Malolactic fermentation is a secondary fermentation carried out on certain types of wine. Malic acid, which has a sharp taste, is converted to the milder lactic acid, imparting smoothness to the wine.
COOH-H2OC-H2C-COOH (Malic acid) −−−−−−−→ CH3-H2OC-OOH (Lactic acid) + CO2
A secondary product of malolactic fermentation is di-acetyl, which imparts a ‘buttery’ flavour to the wine. Spir-its such as brandy and rum result from the products of a fermentation process being concentrated by distillation. This gives a much higher alcohol content than that of wines.
Beer is produced by the fermentation of barley grain. The procedure varies according to the type of beer, but follows a series of clearly defined steps (Figure 17.2). Grain, unlike grapes, contains no sugar to serve as a sub-strate for the yeast, so before fermentation can begin, it is soaked in water and allowed to germinate. This stim-ulates the production of the enzymes necessary for the conversion of starch to maltose (‘malting’). An additional source of starch may be introduced during the next stage, mashing, in which the grains are ground up in warm wa-ter, and further digestion takes place. The liquid phase or wort is drained off and hops are added. They im-part flavour and colour to the finished product and also possess antimicrobial properties, thereby helping to prevent contamination. The mixture is boiled, inactivating the enzymes, precipitating proteins and killing off any microor-ganisms. In the next stage, the wort is filtered and transferred to the fermentation vessel where yeast is introduced.
Two species of yeast are commonly used in the brewing process, both belonging to the genus Saccharomyces. S.cerevisiae is mainly used in the production of darker beerssuch as traditional English ales and stouts, whereas S.carlsbergensis (no prizes for guessing where this one wasdeveloped!) gives lighter coloured, less cloudy, lager-type beers. Cells of S. cerevisiae are carried to the surface of the fermentation by carbon dioxide bubbles (top fermenters), while S. carlsbergensis cells form a sediment at the bottom (bottom fermenters). ‘Spent’ yeast may be dried, and used as an animal food supplement.
Fermentation takes about a week to complete, at a temperature appropriate for each type of yeast (S. carlsbergensis prefers somewhat lower temperatures than S. cerevisiae). Following fermentation, the beer is allowed to age or ‘rest’ for some months in the cold. Beers destined for canning or bottling are filtered to remove remaining microorganisms.
Beers typically have an alcohol content of around 4 per cent. Small amounts of other secondary products such as amyl alcohol and acetic acid are also produced, and con-tribute to the beer’s flavour. ‘Light’ or low-carbohydrate beers are produced by reducing the levels of complex carbohydrates. The yeast do not possess the enzymes necessary to cope with these branched molecules, so a supplement of debranching enzymes may be added to aid their breakdown.
Milk can be fermented to produce a variety of products, including butter, yoghurt and cheese (Figure 17.3). In each case, acid produced by the fermentation process causes coagulation or curdling of the milk proteins.
In cheese-making, this coagulation is effected by the addition of the protease rennin, or by the action of lac-tic acid bacteria (especially Streptococcus lactis and S. cremoris). Coagulation allows the separation of thesemisolid curd from the liquid whey. The subsequent steps in the cheese-making process depend on the spe-cific type of cheese (Table 17.2). Following separation, the curd of most cheeses is pressed and shaped, removing excess liquid and firming the texture. During the ripening process, salt is often added, and flavour develops due to continuing microbial action on the protein and fat components of the cheese. In some cases, a fresh inoculation of microorganisms is made at this point, such as the addition of Penicillium spores to Camembert and Brie. The length of the ripening period varies from a month to more than a year according to type, with the harder cheeses requiring the longer periods.
Yoghurt is another milk derivative. Thickened milk is exposed to the action of two bacteria, Streptococcus thermophilus and Lactobacillus bulgaricus, both of which fer-ment lactose present in milk into lactic acid. In addition, L. bulgaricus contributes aromatics responsible for imparting flavour to the yoghurt.
Other dairy products, such as soured cream and buttermilk, are also produced by means of the fermentative properties of species of streptococci and lactobacilli.
The biological agent responsible for bread production is yeast. In fact baker’s yeast and brewer’s yeast are just different strains of the same species, Saccharomyces cerevisiae. In breadmaking, aerobic, rather than anaerobic conditions are favoured, so sugar present in the dough is converted all the way to carbon dioxide rather than to alcohol. It is this that causes the bread to rise. Any small amount of ethanol that may be produced is evaporated during the baking process.
Many other popular foodstuffs are the result of microbial fermentation processes (see Table 17.3). These include vinegar, soy sauce and sauerkraut. Silage is animal fodder made from the fermentation of grass and other plant material by the action of lactic acid bacteria.
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