Fermentation and
Other Uses of
Microorganisms
Fermentations occur when microorganisms consume susceptible organic substrate as part of their own
metabolic processes. Such interactions are fundamental to the decomposition of natural materials, and to the
ultimate return of chemical elements to the soil and air without which life could not be sustained.
Natural fermentations have played a vital role in human development and are probably the oldest form of food
preservation. Although the growth of microorganisms, in many foods is undesirable and considered spoilage,
some fermentations are highly desirable. Fruit and fruit juices left to the elements acquired an alcoholic flavor;
milk on standing became mildly acidic and eventually became cheese; cabbage turned to sauerkraut. These
changes tasted good and so early civilizations encouraged the conditions that permitted them to occur.
Sometimes the desired results were obtained repeat
repeatedly, but this was not always so. It soon was also discovered that certain alcoholic fruitjuices and sour
milks would keep well, and so part of the food supply was converted into these forms as a means of preservation.
Today, other methods of food preservation are superior to fermentation as means of preserving many foods. In
technically advanced societies the major importance of fermented foods has come to be the variety they add to
diets. In many less developed areas of the world, however, fermentation and natural drying are still the major food
preservation methods, and, as such, are vital to survival of much of the world's population.
The various preservation methods discussed thus far, based on the applications of heat, cold, removal of
water, application of radiation, and other principles, all have the common objective of decreasing the numbers of
living organisms in foods, or at least holding them in check against further multiplication. In contrast,
fermentation, whether for preservation purposes or not, encourages the multiplication of microorganisms and
their metabolic activities in foods. But only selected organisms are encouraged, and their metabolic activities and
end products are highly desirable. A partial list of fermented foods from various parts of the world is given in
Table 12.1. The increasing application of biotechnology and genetic engineering techniques to food production is
bringing added importance to food fermentations.
Table 12.1. Some Industrial Fermentations in Food Industries
Lactic acid bacteria
Vegetables and fruits
cucumbers - dill pickles, sour pickles, salt stock
olives - green olives, ripe olives
cabbage --- >sauerkraut
turnips ---> sauerriiben
lettuce ---> lettuce kraut
mixed vegetables, turnips, radish, cabbage ~-> Paw Tsay
mixed vegetables in Chinese cabbage ---> Kinichi
vegetables and milk - Tarhana
vegetables and rice - Sajur asin
dough and milk - Kislik
coffee cherries ---> coffee beans
vanilla beans --> vanilla
taro ---> poi
Meats --> sausages such as salami, Thuringer, summer, pork roll, Lebanon bologna,
cervelat
Dairy products
sour cream
sour milk drinks-acidophilus, yoghurt, cultured buttermilk, Bulgarian, skyr, gioddu,
leban, dadhi, taette, mazun
butter-sour cream butter, cultured butter, ghee
cheese-unripened --- > cottage, pot, schmierkase, cream
whey ---> mysost, primost, ricotta, schottengsied
ripened --> Cheddar, American, Edam, Gouda, Cheshire, provolone
Lactic acid bacteria with other microorganisms Dairy products with propionic acid bacteria-Emmenthaler, Swiss,
Samso, Gruy~re cheeses with surface-ripening bacteria-Limburger, brick, Trappist, Munster, Port de Salut with
yeasts -kefir, kumiss or kumys with molds-Roquefort, Camembert, Brie, hand, Gorgonzola, Stilton, Blue
Vegetable products with yeasts-Nukamiso pickles with mold-tempeh, soya sauce
Acetic acid bacteria-wine, cider, malt, honey, or any alcoholic and sugary or starchy
products may be converted to vinegar
Yeasts
malt --- > beer, ale, porter, stout, bock, Pilsner
fruit --> wine, vermouth
wines ~--> brandy
molasses ---> rum
grain mash - whiskey
rice ---> sak6, sonti
agave ---> pulque
bread doughs --- > bread
Yeasts with lactic acid bacteria
cereal products --- > sour dough bread, sour dough pancakes, rye bread
ginger plant ----> ginger beer
beans --> vermicelli
Yeasts with acetic acid bacteria cacao beans citron
Mold and other organisms soybeans-miso, chiang, su fu, tamari sauce, soy sauce fish and rice-lao, chao
Courtesy of C. S. Pederson.
Definitions
The term fermentation has come to have somewhat different meanings as its underlying causes have become
better understood. The derivation of the word fermentation signifies a gentle bubbling condition. The term was
first applied to the production of wine more than a thousand years ago. The bubbling action was due to the
conversion of sugar to carbon dioxide gas. When the reaction was defined following the studies of Gay-Lussac,
fermentation came to mean the breakdown of sugar into alcohol and, carbon dioxide. Pasteur later demonstrated
the relationship of yeast to this reaction, and the word fermentation became associated with microorganisms, and
still later with enzymes. The early research on fermentation dealt mostly with carbohydrates and reactions that
liberated carbon dioxide. It was soon recognized, however, that microorganisms or enzymes acting on sugars did
not always evolve gas. Further, many of the microorganisms and enzymes studied also had the ability to break
down noncarbohydrate materials such as proteins and fats, which yielded carbon dioxide other gases, and a wide
range of additional materials.
Currently, the term fermentation is used in various ways which require clarification. When chemical change is
discussed at the molecular level, in the context of comparative physiology and biochemistry, the term
fermentation is correctly employed to 2~ describe the breakdown of carbohydrate materials under anaerobic
conditions. In a somewhat broader and less precise usage, where primary interest is in describing the end
products rather than the mechanisms of biochemical reactions, the term fermentation refers to breakdown of
carbohydrate and carbohydratelike materials under either anaerobic or aerobic conditions. Conversion of lactose
to lactic acid by Streptococcus lactis bacteria is favored by anaerobic conditions and is true fermentation;
conversion of ethyl alcohol to acetic acid by Acetobacter aceti bacteria is favored by aerobic conditions and is
more correctly termed an oxidation rather than a fermentation. Common usage frequently overlooks this
distinction and considers both types of reactions to be fermentation& In this and subsequent chapters the
common usage of the term fermentation, referring to both the anaerobic and aerobic breakdown of carbohydrates,
will be followed.
But the word fermentation also is used in a still broader and less precise manner. The term fermented foods is
used to describe a special class of food products characterized by various kinds of carbohydrate breakdown; but
seldom is carbohydrate the only constituent acted upon. Most fermented foods contain a complex mixture of
carbohydrates, proteins, fats, and so on, undergoing modification simultaneously, or in some sequence, under the
action of a variety of microorganisms and enzymes. This creates the need for additional terms to distinguish
between major types of change. Those reactions involving carbohydrates and carbohydratelike materials (true
fermentations) are referred to as "fermentative." Changes in proteinaceous materials are designated proteolytic or
putrefactive. Breakdowns of fatty substances are described as lipolytic. When complex foods are "fermented"
under natural conditions, they invariably undergo different degrees of each of these types of change. Whether
fermentative, proteolytic, or lipolytic end products dominate will depend on the nature of the food, the types of
microorganisms present and environmental conditions affecting their growth and metabolic patterns. In specific
food fermentations, control of the types of microorganisms and environmental conditions to produce desired
product characteristics is necessary.
Benefits of Fermentation
In addition to the roles of fermentation in preservation and providing variety to the diet, there are further
important consequences of fermentation. Several of the end products of food fermentation, particularly acids and
alcohols, are inhibitory to the common pathogenic microorganisms that may find their way into foods. The
inability of Clostridium, botulinum to grow and produce toxin at pH values of 4.6 and below has already been
cited. Increasing the acidity of foods by fermentation is very common. Foods as diverse as yogurt, hard
sausages, and sauerkraut all contain acid as a result of fermentation.
When microorganisms ferment food constituents, they derive energy in the process and increase in numbers.
To the extent that food constituents are oxidized, their remaining energy potential for humans is decreased.
Compounds that are completely oxidized by fermentation to such end products as carbon dioxide and water retain
no further energy value. Most controlled food fermentations yield such major end products as alcohols, organic
acids, aldehydes, and ketones, which are only slightly more oxidized than their parent substrates, and so still
retain much of the energy potential of the starting materials. Fermentation processes are attended by temperature
increases. The energy dissipated as heat represents a fraction of the total energy potential of the original food
material no longer recoverable for nutritional purposes.
Fermented foods can be more nutritious than their unfermented counterparts. This can come about in at least
three different ways. Microorganisms not only are catabolic, breaking down more complex compounds, but they
also are anabolic and synthesize several complex vitamins and other growth factors. Thus, the industrial
production of such materials as riboflavin, vitamin B12 and the precursor of vitamin C is largely by special
fermentation processes.
The second important way in which fermented foods can be improved nutritionally has to do with the liberation
of nutrients locked into plant structures and cells by indigestible materials. This is especially true in the case of
certain grains and seeds. Milling processes do much to release nutrients from such items by physically rupturing
cellulosic and hemicellulosic structures surrounding the endosperm, which is rich in digestible carbohydrates and
proteins. Crude milling, however, practiced in many less developed regions, often is inadequate to release the full
nutritional value of such plant products; even after cooking, some of the entrapped nutrients may remain unavailable to the digestive processes of humans. Fermentation, especially by certain molds, breaks down
indigestible coatings and cell walls both chemically and physically. Molds are rich in cellulose-splitting enzymes;
in addition, mold growth penetrates food structures by way of its mycelia. This alters texture and makes the
structures more Permeable to the cooking water as well as to human digestive juices. Similar phenomena result
from the enzymatic actions of yeasts and bacteria.
A third mechanism by which fermentation can enhance nutritional value, especially of plant materials, involves
enzymatic splitting of cellulose, hemicellulose, and related Polymers that are not digestible by humans into simpler
sugars and sugar derivatives. This goes on naturally in the rumen of the cow through the enzymatic action of
protozoa and bacteria. It also occurs in the process of preparing silage for animal feeding. Cellulosic materials in
fermented foods similarly can be nutritionally improved for humans by the action of microbial enzymes.
Of course, such changes are accompanied by gross changes in texture and appearance of the starting food
materials, just as all fermented foods are markedly altered from their unfermented counterparts. Such changes
are not looked upon as quality defects. Quite the contrary; particularly in areas of the world where most of
human nutrients are derived from plant sources, food materials markedly altered by fermentation commonly are
more frequent and relished items of diet than are the natural plant components.
Microbial Changes in Foods
The normal microbial flora associated with foods can produce a very wide range of breakdown products.
Depending on the major food substrates attacked, these microorganisms are designated proteolytic, lipolytic, or
fermentative. Because of their generally broad complement of enzymes, few types of microorganisms are
exclusively proteolytic, lipolytic, or fermentative. Rather, most types exhibit varying degrees of each property,
depending on environmental conditions and other factors. Nevertheless, many organisms are characteristically
dominant in one or another of these three basic kinds of change produced in food.
Proteolytic organisms, which break down proteins and other nitrogenous compounds,' give rise to putrid and
rotten odors and flavors considered undesirable beyond certain rather low levels. Similarly, lipolytic organisms,
which attack fats, phospholipids, and related materials give rise to rancid and fishy odors and flavors not desired
in most foods beyond minor levels. On the other hand, fermentative organisms convert carbohydrates and
carbohydrate derivatives largely to alcohols, acids, and carbon dioxide. These end products are not generally
offensive to our tastes and add zest to many J foods. Moreover, when produced in sufficient concentrations, the
alcohols and acids resulting from fermentation inhibit many proteolytic and lipolytic organisms that are capable of
food spoilage if not controlled. Here in lies the principle of preservation by fermentation: encourage the growth
and metabolism of alcohol and/or acid-forming, microorganisms and suppress or control the growth of proteolytic
and lipolytic types. Once the fermentative organisms are heavily established, they limit growth of the other types,
not only by virtue of their production of alcohol and acid but also because they compete for and consume certain
constituents of the food that otherwise would be utilized by the proteolytic and lipolytic organisms.
Fermentation technology is not as simple as the above indicates. It is complex, due to the large number of
microorganism types and enzymes on the one hand and the diversity of food systems on the other. Processors
rarely deal with systems in which one or two organism types work on one or two food constituents; nor do they
generally want only alcohol or acid production to the total exclusion of protein and fat breakdown. The clean, tart
taste of fresh cottage cheese is largely due to the conversion by fermentation of lactose into lactic acid. On the
other hand, the more complex flavors of Cheddar and Limburger cheeses are due to different degrees of protein
and fat breakdown in addition to lactic acid fermentation. To obtain these balanced flavors in certain foods, the
fermentation processes must be controlled to balance the microorganism types that may grow in the foods.
Some of the more common and significant types of microbial activity in foods are indicated below. The complex
intermediate steps leading to the final results are omitted.
Sugar fermented by yeasts, such as Saccharomyces cerevisiae and Saccharomyces ellipsoideus, yields ethyl
alcohol and carbon dioxide in accordance with the following overall reaction:
C6H12O6------------2C2H5OH+2CO2.
yeast
This is the basis of wine and beer production and the leavening of bread.
Alcohol from yeast-fermented cider, in the presence of oxygen, will be further fermented by bacteria such as
Acetobacter aceti to acetic acid as in the reaction
C6H5OH+O2---------------------CH3 COOH+H2O.
Acetobacter aceti
This is the mechanism of vinegar production.
Lactose (milk sugar), fermented by Streptococcus lactis bacteria, gives lactic acid, which curdles the milk to
yield cottage cheese and curd from which other cheeses can be made.
Acids produced from fermentation, in the presence of oxygen, can be further broken down by molds. When this
happens, the preservative action of the acid against other microorganisms is lost.
Proteins broken down by proteolytic bacteria such as Proteus vulgaris and other organisms yield a wide range
of nitrogen-containing compounds that give putrid, fishy, or decayed odors to food.
Lipids broken down by lipolytic bacteria such as Alcaligenes lipolyticus and other organisms yield fatty acids.
These and their subsequent breakdown products contribute to rancid odors or the characteristic odors of some
aged cheeses.
Low-acid foods supporting growth of Clostridium, botulinum may contain toxins produced by this bacterium.
This food-poisoning organism will not grow in fermented foods high in acid.
The types of activities indicated can lead to many interesting and highly significant sequences of reactions.
These sequences are either prevented or encouraged, as dis cussed in the next section, depending on the type of
fermented food being produced.
Controlling Fermentations in Various Foods
Among the many factors that influence microorganism growth and metabolism, the most common for
controlling food fermentations include level of acid, level of alcohol, use of starters, temperature, level of oxygen,
and amount of salt. These factors also determine the types of organism that may grow in a fermented food on
later storage.
Acid
The inhibitory effects of acid are exerted whether acid is added directly to the food, is a natural constituent of
the food, or is produced in the food by fermentative microorganisms. If not a natural constituent of the food (as
it is in oranges or lemons), then acid must be added or formed by fermentation quickly, before spoilage or other
harmful microorganisms have a chance to increase substantially in numbers and produce their effects.
Food containing acid may be in a state of preservation, but if oxygen is available and surface molds grow and
further ferment the acid, its preservative power is lost. In this way, proteolytic and lipolytic activity may gradually
develop on the surface of such food. This can occur during the ripening of Cheddar cheese and constitutes a
defect. Acid level can be effectively decreased by neutralization also. Certain yeasts will tolerate moderately
high-acid conditions and produce alkaline end products, such as ammonia, from the breakdown of protein. These
neutralize previously formed acid and permit subsequent growth of proteolytic and lipolytic bacteria. This is
desirable and is encouraged in the surface ripening of Limburger cheese.
These types of changes also occur when raw milk is allowed to ferment naturally (Fig. 12. 1). Raw milk generally
will be contaminated with a wide variety of microorganisms. After a short period during which freshly drawn raw
milk fails to support microbial growth (period of germicidal action), Streptococcus lactis dominates the fermentation and produces lactic acid. Eventually, this organism is inhibited from further, growth by its own acidity.
Bacteria of the genus Lactobacillus, also common to milk are still more acid tolerant than Streptococcus lactis. The
lactobacilli now take over the fermentation and produce still more acid until the new level becomes inhibitory to
their further growth. In the high-acid environment, these lactobacilli gradually die off and acid-tolerant yeasts and
molds become established. The molds oxidize acid and the yeasts produce alkaline end products from proteolysis,
both of which gradually decrease the acid level to the point where proteolytic and lipolytic spoilage bacteria find
the medium satisfactory. The growth of these organisms, especially the increased proteolytic activity, decreases
the milk's acidity to the point where it can become more alkaline than the original raw milk. During the period of
Streptococcus and Lactobacillus growth, the milk clots and the curd becomes firm, with little evidence of gas
accumulation or development of off-odors. Mold and yeast growth followed by proteolytic and lipolytic bacterial
growth digest this curd, produce a gassy condition, and develop off-odors characteristic of putrefaction.
In bread-making, the sugars of dough are fermented with yeast, producing alcohol, carbon dioxide, and minor
fermentation products. In typical white bread, the fermentation is not intended for preservation purposes and
provides little protection of this kind. Here we are interested 'in the leavening power of the carbon dioxide gas and
the flavors from fermentation. However, there are many varieties of sour breads where the yeast fermentation is
accompanied by lactic acid fermentations from organisms of the Lactobacillus group. In addition to imparting
characteristic flavor, the acid inhibits growth of spore-forming bacteria of the genus Bacillus in the dough and
later in the bread. Spores of this genus if present in the dough survive the temperatures of baking.
They them may produce a gummy condition known as “ropy” bread when non-acid bread is stored under damp
conditions. This rarely occurs in sour breads.
Alcohol
Like acid, alcohol is a product of some fermentations and can be a preservative, depending on its
concentration. The alcohol content of wines depends, in part, on the original sugar content of the grapes, the
type of yeast, fermentation temperature, and level of oxygen. Just as with organisms producing acid, yeasts
cannot tolerate their own alcohol and other fermentation products beyond certain levels. For many yeasts this
occurs in the range of 12-15% alcohol by volume. Natural wines generally will contain 9-13% alcohol from
fermentation. This is not sufficient in itself for complete preservation, and so such wines must receive, in addition,
a mild pasteurization treatment. Fortified wines are natural wines to which additional alcohol is added to bring the
final alcohol concentration up to about 20% by volume. Such wines may not require further pasteurization.
Use of Starters
When a particular type of microorganism is present in large numbers and is multiplying, it usually dominates its
environment and keeps down the growth of other types of microorganisms. In early times, a winemaker or
cheesemaker used this principle, without quite knowing why, when part of a previous batch of wine was poured
back into fresh grape juice, or cheese. milk into fresh milk for the next batch. Such practices continue today in
many areas of the world. Fig. 12.2 illustrates one kind of primitive cheese-making currently practiced in Nepal in
the Himalayas. Milk from the yak ox ferments under natural conditions until sufficient acid is produced to
coagulate curd. The curd is squeezed through the fingers into noodlelike forms, which then are dried in the sun.
The fermented milk from one day's operation is used as a starter to initiate fermentation of the next day's
production.
In technologically advanced countries, starters of pure cultures obtained from commercial laboratories are used
to help ensure controlled fermentation during cheesemaking. These cultures, available in dehydrated and in
concentrated frozen form, have been developed from selected strains of lactic acid organisms outstanding for
their quick and dependable acid production under cheesemaking conditions. Such strains often are resistant to
traces of antibiotics and pesticide residues, which may find their way into cheese milk from farm operations, and
to bacterial viruses (phages), all of which could otherwise interfere with starter activity. Similarly, special cultures
are available for the production of wine, beer, vinegar, pickles, sausage, bread, and other fermented foods.
Frequently, the food is heated to inactivate detrimental types of contaminating organisms prior to starter addition.
Temperature
Various microorganisms may dominate a mixed fermentation depending on the fermentation temperature. The
sauerkraut fermentation is particularly sensitive to temperature. The effects temperature can have in this
fermentation on final acid concentration and time to reach various acidities are indicated in Table 12.2.
In sauerkraut production, three major types of organisms convert the sugar of cabbage juice to acetic acid, lactic
acid, and other compounds. These bacteria include Leuconostoc mesenteroides, Lactobacillus cucumeris, and
Lactobacillus pentoaceticus., Leuconostoc mesenteroides produces acetic acid, some lactic acid, alcohol, and
carbon dioxide. The alcohol and acids also combine to form esters, which contribute to final flavor. Lactobacillus
cucumeris produces additional lactic acid when Leuconostoc mes-, enteroides leaves off. Lactobacillus
pentoaceticus produces still more lactic acid after Lactobacillus cucumeris ceases to be active. The desirable
sequence of these fermentations is indicated in Fig. 12.3. Leuconostoc mesenteroides requires cool temperatures
of about 21ºC for optimum growth and fermentation in sauerkraut manufacture. lactobacilli tolerate higher
temperatures.
If temperatures much above 21ºC are employed in the initial stages of the fermentation, the lactobacilli easily
outgrow L. mesenteroides and then their high levels of acid production further prevent growth and fermentation
of L. mesenteroides. Under these conditions, acetic acid, alcohol, and other desirable products of the L.
mesenteroides. fermentation are not formed. The sauerkraut fermentation, therefore, employs initial low
temperatures, which then may be increased somewhat in the later stages of fermetation. This is but one example of
manipulating temperature to favor the type 1 organism desired.
Oxygen
The aerobic nature of molds has been discussed. The acetobacter important in vinegar making also requires
oxygen, but the yeast that produces alcohol from sugar does it better in the absence of oxygen. Clostridium
botulinum is a strict anaerobe. Food processors provide or remove air or oxygen as required to encourage or
inhibit particular microorganisms.
An organism may have different requirements with respect to oxygen for growth than it has for fermentation
activity. Bakers' yeast (Saccharomyces cerevisiae) and yeast (Saccharomyces ellipsoideus) are good examples of
this. Both grow better produce greater cell masses under aerobic conditions, but they ferment sugars more rapidly
under anaerobic conditions. Thus, in the commercial production of bakers' the yeast is grown under aerobic
conditions by bubbling air through a yeastated olasses solution in large tanks. Fermentation is favored in the
breadmaking operation (after sufficient yeast population is established) by the relatively anaerobic conditions of
large dough masses.
In traditional vinegar manufacture, the fermentations are separated principally on the basis of the relationships
of the fermenting organisms to oxygen. In this two-step process, the first step-involving conversion of the sugar
of apple juice to alcohol may be started under aerobic conditions to stimulate yeast growth and increased cell
mass. But conditions are soon made anaerobic to favor the fermentation of the sugar to alcohol. The second step
involving the conversion of alcohol to acetic acid is promoted by highly aerobic conditions, since this
transformation is really an oxidative fermentation. This conversion of alcohol to acetic acid is commonly carried
out in a vinegar generator. Vinegar generators differ in design but generally consist of large tanks or vats packed
with wood shavings to provide a large aerobic surface area. The alcoholic cider, after heavy inoculation with
vinegar bacteria, is trickled through the wood shavings while air is blown up through the shavings. The vinegar is
removed from the generator when its acetic acid concentration reaches 4% (or somewhat higher), since this is the
minimum legal level for acetic acid in vinegar. Operation of a vinegar generator demands close control. Aerobic
conditions can encourage mold development, and, as has been pointed out, molds can further break down acid. In
addition, excessive aeration can itself oxidize acetic acid further to carbon dioxide and water.
Salt
Microorganisms can be separated on the basis of salt tolerance. The lactic acidproducing organisms used in
fermenting olives, pickles, sauerkraut, certain meat sausages, and similar products generally are tolerant to
moderate salt concentrations of the order of 10-18%. Many proteolytic and other spoilage organisms that can
infect pickle and sauerkraut vats are not tolerant to salt above about 2.5%, and especially are not tolerant to a
combination of salt and acid.
In these fermentations, added salt gives the lactic acid-producing organisms an advantage in getting under
way even if proteolytic types are present on the cucumbers or cabbage. Once underway, the acid produced by
the lactic acid organisms plus the salt strongly inhibits proteolytic and other spoilage types. The salt added to
vegetable fermentations also draws water and sugar out of the vegetables. The sugar entering the salt brine
provides readily available carbohydrate for continued fermentation in the brine, which complements fermentation
within the vegetable tissue from inward diffusion of lactic acid microorganisms. In this way, salt makes the
difference between desirable fermentations and outright spoilage.
Water drawn from the vegetables also tends to dilute the brine; thus salt must be frequently added to maintain
the brine's preservative salt level. In the production of sauerkraut, approximately 2.0-2.5% salt generally is added
to the cabbage, the major preservative effect coming from the acidity formed. Olives are placed in salt brines of
about 7-10%, and cucumbers commonly are fermented in brines maintained at about 15-18% salt.
Quite the same principle applies in the making of cheese. It is common practice to salt cheese curd to control
proteolytic organisms during the long ripening periods, which may be in excess of a year for certain types of
cheese. In this case, various salttolerant lactobacilli continue to produce acid and further modify the cheese curd
during the ripening period.
Many types of sausage and other fermented meats owe their unique flavors to fermentations by strains of
Leuconostoc, Lactobacillus, and Pediococcus bacteria. Generally, fermentations by these organisms in meat
products produce a less acid condition than is common in fermented vegetables. Such products as fermented
sauerkraut and pickles have acidities in the range of about pH 2.5-3.5. Fermented meat sausages commonly have
acidities in the range of pH 4.0-5.5. This degree of acidity would be marginal as a preservative were it not
augmented by the presence of salt and other curing chemicals in the sausages, plus the effects from smoking,
cooking, and partial drying of certain of these products.
The desire to reduce the salt content of some fermented products must be undertaken with caution, as this can
encourage the growth of undesirable microorganisms including food-borne disease causing organisms.
Reduction in salt content must often be accompanied by other methods of inhibiting undesirable organisms while
still promoting desirable ones.
MICROORGANISMS AS DIRECT FOODS
Quite apart from the use of microorganisms to produce desirable changes in foods, microorganisms of various
types are grown, harvested, and further processed to yield animal feed and human food. Strains have been
selected for rapid growth on specific substrates, nutrient content including amount and quality of their protein,
organoleptic properties, and other attributes. In some cases, the protein from microorganisms has been isolated
and used in foods. The term single-cell protein (SCP) has been introduced to designate high-protein food from
yeast and other microorganisms, although the practice of growing yeast for food goes back many years. The term
single-cell protein can be misleading because it suggests a product that is all protein. Although food yeasts may
contain at least one-third protein on a dry basis and it is possible to extract this protein and produce nearly pure
protein isolates, this is not commonly done and the entire yeast cell is more often utilized as a food or feed
supplement.
Brewers' yeast (Saccharomyces cerevisiae or Saccharomyces uvarum), a by-product of beermaking, and bakers'
yeast (S. cerevisiae), commonly grown on molasses and produced mainly for its leavening property, have long
been used as sources of nutrients. These and other yeasts have different carbon and nitrogen assimilation
patterns (Table 12.3) and, therefore, can be grown on a wide range of agricultural and industrial byproducts, such
as hydrolyzed plant tissues, cheese whey, ethanol, petroleum hydrocarbons, and other materials appropriately
supplemented with nitrogen and mineral salts.
Yeast solids normally contain about 7-12% of nucleic acids, which can produce harmful effects when yeast is
consumed in large amounts. Several methods involving extraction procedures and autolytic degradation by the
yeast cell have been developed that can decrease nucleic acids to about 1% yet retain much of the protein. Such
Procedures need not be employed where the quantities of yeast consumed would contribute less than about 2 g
of yeast nucleic acid per day to the adult diet.
GENETIC ENGINEERING
Humans have been breeding food animals, plants, and microorganisms in order to improve characteristics such
as yield, disease resistance, appearance, processing attributes, and fermentation characteristics for centuries.
Traditional breeding is accomplished by mating a male and a female in hopes that the offspring will have the
desired characteristics. In the case of plants and microorganisms this is sometimes accomplished by direct
mutation of the genes. Genes contain all the inheritable traits of living organisms. Such breeding is, in reality,
selecting and directing the genetic makeup of the animal, plant, or microorganism.
The problems with conventional breeding and mutation as methods of selecting for desirable traits are that
they are not always predictable nor successful and can be time-consuming. It is also not possible to cross the
species barrier with conventional breeding; that is, desirable traits of oranges, such as ability to produce high
amounts of ascorbic acid, cannot be transferred to apples.
In recent years, techniques for more directly manipulating the genetic characteristics of organisms, commonly
referred to as genetic engineering, have been developed, following major advances in molecular biology. Genetic
engineering through the use of recombinant DNA techniques, cell hybridization, spheroplast or protoplast fusion,
and other methods can now remove genes from cells of one organism and reinsert them into the cells of
organisms and program them to do specific functions. Progress to date has been greatest with microorganisms,
including yeasts, and plants, but progress has also been made with animals.
All of these processes are similar in that they identify the specific genes responsible for desirable traits in one
species or type of organism and then transfer these specific genes to a different organism. In this way, the
recipient organism acquires these traits. For example, in humans, pancreatic cells produce the protein insulin
which is required to control blood sugar. People whose pancreas does not make insulin have diabetes and must
take insulin. The genes from human cells which tell the pancreas to make insulin have been transferred to bacteria.
The bacteria containing the genes for insulin are thus able to make insulin in culture. This insulin is collected,
purified, and used to treat diabetics.
In the food industry, these new techniques are being used to improve yields of traditional fermentation
products; convert underutilized raw materials into useful substrates; produce new and improved enzymes,
flavoring agents, sweeteners, gums and other food ingredients; and improve performance of cultures under
economical processing conditions. For example, virus-resistant strains of important fermentation
`microorganisms’ have been developed, as well as organisms which produce enzymes used to make foods such
as cheese. In the brewing industry, cell hybridization has been used to produce improved yeast strains. As
outlined in Fig. 12.4, the process involves removing cell walls from two yeast strains possessing different
desirable attributes, promoting interchange of genetic material through fusion of their spheroplasts, and
providing a medium and conditions for cell wall regeneration. The new yeast, which is capable of division and
replication, can ferment maltose, dextrin, and lactose and thus has a wider range of utilizable substrates than
either starting strain. Further, the property of flocculation facilitates removal of the yeast from the fermentation
wort, making for better clarification of fermented beverages and more efficient reuse of yeast. Other important
characteristics of the hybridized yeast resulting from initial strain selection include alcohol tolerance, production
of desirable flavor compounds, and genetic stability.
Genetic engineering is also finding uses in agriculture. Genes from bacteria which can kill certain insects but are
harmless to humans have been transferred to plants. The plants then produce the protein that is toxic to the
insect, so that when the insect cats the plant, the insect dies. These proteins have no effect on humans because
they are inactivated in the human stomach. This type of genetic engineering may lead to a large reduction in the
use of synthetic pesticides.