Food Fermentations. Beer, wine and cheese

The discovery that microorganisms could positively affect the organoleptic quality and storage life of foods, a process that includes numerous chemical and physical changes, was accidental. In general, the fermentation process involves the breakdown of complex macromolecules and the conversion of the resulting substrate into other metabolic end products.

In most cases this specifically involves the breakdown of a complex carbohydrate and the production of an acid or alcohol as the end product. The development of a particular acidogenic or alcoholic fermentation process may be more a function of the interest in and exploitation of these types of fermentation rather than a fundamental thermodynamic driving force. Fermentative microorganisms carry out this process to alter their environment by reducing the available substrate, as well as to generate a concentration of metabolite that would discourage the growth of competitors.

Fermentations may also be a combination of enzymatic and microbial processes, with the former contributed by the raw ingredients and the latter by endogenous microflora. Modern adaptations of these natural fermentations have mastered these previously haphazard events to ensure the uniformity and predictability of the processes.

A. Beer. As mentioned previously, beer fermentation owes its origins to the ancient Egyptians, who first brewed beer using bread as a substrate. Throughout the intervening years, beer brewing has evolved through a constant alteration in the substrates used in the process and an understanding of the microbiology of the fermentation. Most modern brewing practices are derived from the sixteenth-century Bavarians, who discovered the value in using “bottom-fermenting” yeast and hops as key ingredients.

To preserve their product, the Bavarian Purity Law was established, which mandated that only a product composed solely of malted cereals, water, yeast, and hops could be called beer. Today that law is still enforced in Bavaria, although liberal interpretations of the ingredients list are practiced elsewhere.

The brewing process begins with the preparation of the malt. Malt is composed of sprouted grains, which provide amylotic enzymes that assist in the breakdown of starch. The grain is sprouted by soaking in water and allowed to proceed for 4-7 days. After sufficient time is allowed for the sprouting process, it is then halted by heating. The wort is then prepared using a starch base such as corn, wheat, or rice and hydrolysis is carried out by the amylolytic enzymes in the malt. The hydrolyzed starch can then be converted to ethanol upon the addition of the starter culture yeast. Two different types of yeast can be used in the production of beer, a top-fermenting Saccbaromyces cerevisiae and a bottom-fermenting S. carlsbergensis. It is the latter bottom-fermenting yeast that proved to be one of the key improvements in the brewing process introduced during the Middle Ages by Bavarian brewers.

Once the yeast is “pitched” into the wort to start the fermentation process, both ethanol and other minor fermentation end products accumulate over the course of several days. Glucose and larger-molecular-weight dextrans are metabolized by the yeast. The process ends with the depletion of substrate and the accumulation of ethanol, and in most processes the yeast is removed. Some additional finishing is required and the beer may be aged before being packaged.

B. Wine. Wine is the result of a fermentation similar to that of beer except that there are no deliberate substrate processing steps similar to malting. In addition, owing to the absence of any heat treatment prior to fermentation, the flora in the fermentation phase of wine are likely to be more complex. The different varieties of grapes and the yearly differences in the sugar and tannin content of grapes also contribute to the vintage variations. Yeast strains used in the production of wine vary and, as in beer brewing, these different strains are one of the closely guarded secrets of the vintner. The yeast serves to convert the sugar in the grapes to ethanol, but the flavor and aroma are the result of a complex and only marginally defined set of metabolic reactions. Some ancillary reactions are carried out not by the yeast but by the endogenous microflora. The malolactic fermentation carried out by Leuconostoc is of concern as it results in wine spoilage. L-Malic acid is decarboxylated to form lactic acid and carbon dioxide.

C. Cheese. The discovery of fermented dairy products were undoubtedly accidental, the serendipitous result of leaving milk out until it coagulated. The primary flora that would have been responsible are lactic acid bacteria, so called because of their propensity to produce lactic acid as a nearly exclusive metabolite end product. Lactic acid bacteria include members of the genera Lactococcus, Lactobacillus, and Pediococcus. These are gram-positive, non-spore-forming cocci or rods.

Modern dairy fermentations and specifically cheese begin with milk, usually bovine in origin. Other milks from sheep, goats, and buffalo can also be used and products from these milks are common in less developed countries. Depending on the process, the milk may or may not be pasteurized, which has an obvious impact on the microflora in the fermentation. During the fermentation process, lactic acid bacteria (which may be added as a deliberate starter culture and/or be present in the endogenous flora) convert lactose to lactic acid. With these bacteria, the metabolism of lactose is virtually exclusive with greater than 90% substrate to product conversion. In addition to the production of lactic acid, other metabolic products, including diacetyl, contribute to the flavor of cheese. Other reactions that take place include the hydrolysis of triglycerides to free fatty acids and the breakdown of proteins into peptides and amino acids.

The lactic acid bacterial fermentation results in the formation of a curd, which is the coagulated aggregation of the proteins found in milk. The production of lactic acid as well as, in certain cases, the addition of processing aids, including chymosin, denature the caseins in milk leading to the formation of the curd. The nuances in the flavor of cheeses are a function of the starter culture and nonstarter bacteria. For certain cheeses, including blue and Roquefort, fungi are added that carry out secondary reactions usually involving proteolysis or lipolysis. Because of their aerobic nature, these adjunct microorganisms grow on the surface.