Processing with Microbes

Microorganisms such as bacteria, yeasts, and molds are common contaminants of food, but some of them also are critical to processing foods. Much of our usual concern in process development and sanitary design is how to kill and prevent the growth of microbes. What about when we want the benefits of microbes? Oversimplifying somewhat, we need to know what the favorable growth conditions for the microbe of interest are, and we had better know what they do for us. Here we will consider some of the foods made by fermentations or microbial actions and discuss what we know about such processes.

Beverages
Beer, wine, and whiskeys are all made by yeast fermentations of various substrates. Differences among the products depend on differences among substrates, strains of yeast, fermentation conditions, and subsequent treatment of the material. It is likely that a type of beer (known to the ancient Egyptians) is one of the oldest manufactured foods.

The Egyptians discovered that if they wet baked barley loaves, the liquid would be converted to a tasty, stable, and nutritious beverage. Even today, people describe beer as liquid bread, because it is made from various cereal grains in which some of the starch has been converted to fermentable sugars by allowing the grain to sprout. This is called malting. Barley is still a preferred grain, even though it is more expensive than some others, such as corn and rice. Barley produces effective saccharifying (sugar-making) enzymes that convert its own starch as well as that from adjuncts, such as corn, wheat, or rice.

The type of yeast as well as the exact recipe of grains and the degree of toasting applied to the barley all affect the final product, yielding the range of beers and ales, from very light in color and flavor to dark and heavier. Subsequent treatments include filtering, addition of other flavors, and addition of hops or hop extracts. Hops are a bitter flower that originally was used as a preservative, but now is a characteristic flavor of beers and ales. Ales differ from beer by being top fermented, while beer yeasts dwell at the bottom. Ales also are usually darker than beer because malted barley that has been heated longer is used for ale.

Wine is made from fermenting fruit, most commonly grapes, but honey, apples, and some other fruits can be used. The sugars in fruits are immediately available for fermentation, as compared with starch in cereals, which must be cooked and enzymatically converted. People can detect differences in the flavor of wines made from different varieties of grapes, and from the same varieties grown in different areas; the terroir—or combination of soil, climate, and cultivation practices—dictates the final product quality. Red wines are fermented from juice that includes the skins of the grapes, while white wines are fermented from juice that is quickly separated from the skins, where the pigments reside. Red wines are typically fermented at higher temperatures than white wines, which are usually fermented in refrigerated vessels. Fermentation of sugar to ethyl alcohol and carbon dioxide gives off heat, and if the temperature rises too much, off-flavors can result. Sparkling wines retain much of the carbon dioxide to provide the stimulating small bubbles, while still wines permit the gas to escape.

Wines are clarified by settling and filtration and then are aged, usually in wooden barrels made of oak. The barrels are re-used four or five times and may be rejuvenated by cleaning and charring. Aging involves an exchange of material between the wine and wood. Some compounds are absorbed by the wood, while components of the wood are extracted by the wine. There is also some slow oxidation and evaporation, because the wood is porous. Wine continues to change, usually for the better, after bottling in glass and closure by a cork. Exactly what these changes are, chemically, is not well known, but undesirable ones can be accelerated by high temperatures so wine is best stored below about 50° F.

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Whiskeys and other distilled beverages start out much like beer, as a cooked mash of malted cereal. After fermentation, the mash is distilled by heating to remove the alcohol. Commonly, the distillation, in chemical engineering terms, is batch with one theoretical plate. Some traditional whiskeys are re-distilled as much as three times, with the residue being recycled to the next batch. Yield and efficiency is low.

White beverages, such as vodka, gin, and rum, may be distilled in continuous multi-stage columns. Whiskeys get their color and flavor from aging in wood barrels. Bourbon must be made from more than 51% corn and is aged at least four years in new, charred oak barrels. Irish, Canadian, and Scotch are aged in used barrels that may have previously held bourbon or wine. Blended whiskeys contain single-grain whiskey, made from barley malt or corn, and grain neutral spirits, which are very mildly flavored, unaged distillates made from corn. Rum is made from sugar cane juice or molasses. Gin is made from grain neutral spirits to which various herbs are added for flavor.

Bread
Bread, in all its variety, is made by fermenting a cereal dough, normally containing wheat flour, with a pure strain of yeast. As with other fermentations, heat is generated, and so dough mixers and proof boxes, where fermentation occurs, are temperature-controlled. Trapped carbon dioxide causes bread dough to expand, contributing to its ultimate texture. Often proofed (fermented) dough is “punched down” to release gas and then allowed to rise again. The longer and more slowly bread dough rises, generally the better the flavor and texture, but economics often dictates that the process be accelerated.

Variations of bread processes include some in which only a portion of the flour is fermented in a thin slurry that can be pumped to which the balance of flour is later added. Mixing is vitally important to bread texture and usually is performed in horizontal, heavy bladed mixers, but there are high speed mixers as well. The key is to hydrate a unique protein in wheat called gluten, which then forms a network that supports the foam of starch and carbon dioxide.

The fermented dough is formed into loaves or other shapes (buns, rolls, doughnuts) and baked or fried. Baking or frying removes some of the water, darkens the outer layer (the crust) and causes expansion of the shape, as water turns to steam and trapped carbon dioxide expands. Bread is chemically and physically unstable and relatively quickly changes texture from soft to brittle, as starch recrystallizes.

Biogas
Food and other organic wastes can be decomposed by a mixed culture of microbes in aerobic and anaerobic waste water treatment. Aerobic treatment is also called activated sludge because it maintains the culture in settled solids that are partially recycled and partially removed for disposal, often by land application. Aerobic treatment requires supply of air by mechanical agitation and yields carbon dioxide, water, and cell mass.

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Anaerobic treatment occurs in the absence of air and produces methane, water vapor, and carbon dioxide. Removal of the water gives biogas, which is combustible and can be used as fuel. Will Burke, President of Sol-simple (phone 312-637-9831, [email protected]), described a practical system installed at a banana processor in Nicaragua, where he dries mangoes. He and I are evaluating such a system for a project in Haiti.

Vegetables
Sauerkraut, kim chee, and some types of pickles are made by lactic acid fermentation of cabbage and cucumbers, with added salt. The salt creates an environment favoring the desirable bacteria to the exclusion of spoilage and pathogenic microbes. The acid produced further restricts what can grow and also creates the unique flavor and texture of the final product. Most fermented foods keep best if refrigerated. They usually do not need cooking or inoculation; the desired microbes are in the air and establish themselves once the conditions are favorable. Kim chee is a popular Korean condiment made from cabbage with added chili peppers, garlic, salt, and sometimes a bit of vinegar.

Meat
Italian salami is made from ground pork, salt, and some other flavors. It is stuffed into a porous casing and hung in a controlled environment for up to thirty days. The pork is first frozen to kill any trichinosis parasites that might be present, as a legal requirement and precaution. (Modern pork rarely has this contaminant.) The sausage is made edible and safe by a combination of phenomena: salt lowers the water activity; during the curing, the meat loses moisture, further lowering water activity; and also during curing, lactic acid bacteria grow, lowering pH, competing with pathogens for nutrients, and developing characteristic flavor. Yield is reduced the longer the meat cures so accurately projecting demand is critical for scheduling manufacturing.

Space does not permit discussing fermented dairy products such as cheese and yogurt in this column. Clearly, these tiny organisms that we so often consider our invisible enemies in food processing are often our partners in producing tasty and safe foods.

J. Peter Clark,
Contributing Editor, Consultant to the Process Industries, Oak Park, Ill.
[email protected]