Paper No.: 02 Paper Title: Principles of The food processing & preservation Module 21: Food Fermentation Paper Coordinator: Dr. P. Narender Raju, Scientist, National Dairy Research Institute, Karnal, Haryana Content Writer: Dr. Kuna Aparna, Asst. Professor, Prof. Jayashankar Telangana State Agricultural University, Hyderabad
Module 21 FOOD FERMENTATION
Introduction Fermentation could be described as a process in which microorganisms change the sensory (flavor, odor, etc.) and functional properties of a food to produce an end product that is desirable to the consumer.
History Fermentation, along with salting, cooking, smoking, and sun drying, is one of the earliest ancient traditions developed by cultures all around the world to extend the possible storage time of foods. Before the initiation of preservation technology, humans frequently had to choose between starvation and eating spoilt foods and then suffer the possible consequences of this. For thousands of years, raw animal and plant ingredients have been fermented. Fermented fruits were probably among the first fermented foods eaten. The methods for fermentations were developed by trial and error and from the experiences of many generations.
Fermentation as a Preservation Method As new preservation techniques have been developed, the importance of fermentation processes for food preservation has declined. Yet fermentation can be effective at extending the shelf life of foods and can often be carried out with relatively inexpensive, basic equipment. Therefore, it remains a very appropriate method for use in developing countries and rural communities with limited facilities. To reduce or prevent microbial spoilage of food, four basic principles can be applied: Minimize the level of microbial contamination onto the food, particularly from high-risk sources (asepsis) Inhibit the growth of the contaminating microflora Kill the contaminating microorganisms Remove the contaminating microorganisms
Factors Produced by the Metabolic Activity of Microorganisms That Can Contribute to the Increased Stability and Safety of Fermented Foods Low ph Organic acids, e.g., lactic acid, acetic acid, and formic acid Low redox potential Nutrient depletion Accumulation of inhibitors, e.g., toxins, bacteriocins, antibiotics, lactococcins, nisin, natamycin, hydrogen peroxide Ethanol Diacetyl Carbon dioxide
Microbial Contamination of Foods Foods are derived from other living organisms and during their development and preparation they are continuously exposed to microbial contamination. The resultant contaminating microflora can have different effects on the food. These include negative effects such as spoilage, where the food becomes unfit for human consumption or health risks when infectious or toxigenic microorganisms are present. Negligible effects on the food occur when the microflora does not cause disease or any detectable changes in the food. However, benefits can also be reaped from the action of the microorganisms when their activity brings about improvements in the appeal of the food. In developed countries, the improved appeal is the major reason for microbial fermentations of foods continuing today.
Examples of Microbial Metabolic End Products Used in Fermented Foods Metabolic End Product Carbon dioxide Ethanol Acetic acid Lactic acid Flavor compound - Diacetyl Flavor compound Acetaldehyde Example of Uses Leavening bread Alcoholic beverages Vinegar Fermented vegetables Dairy products Yogurt
Potential Benefits of Fermented Foods Increased Safety Health benefits Retail value Nutritional value Digestibility Lowered Toxicity Cooking time Production costs Equipment needs Levels of anti-nutritional factors Suitability for subsequent processing Sensory properties Ease of storage and transportation Shelf life
Microorganisms Used in Food Fermentations A variety of groups of microorganisms are frequently used in fermented foods Microbial Group Lactic acid bacteria (LAB) Acetic acid bacteria Yeasts Molds Product Lactic acid Acetic acid Alcohol and Co2 Enzymes
Lactic Acid Bacteria LAB perform an essential role in the preservation and production of wholesome foods. Examples of lactic acid fermentations include (a) fermented vegetables such as sauerkraut, pickled cucumbers, radishes, carrots, and olives; (b) fermented milks such as yogurt, kefir, and cheeses; (c) fermented/ leavened breads such as sourdough breads; and (d) fermented sausages
Potential Health Benefits from Lactic Acid Bacteria Benefits From foods Improved nutritional value, e.g., production of vitamins or essential amino acids Reduced toxicity, e.g., by degradation of noxious compounds Increased digestibility and assimilability of nutrients From colonization Control of intestinal infections Improved digestion of lactose Inhibition of tumor growth Lowering of serum cholesterol levels Immune stimulation
Acetic Acid Bacteria A second group of bacteria with importance in food fermentations are the acetic acid producers. Acetic acid is one of the oldest chemicals known; it is named after the Latin word for vinegar acetum. The acetic acid bacteria are acid tolerant, grow well at ph levels below ph 5.0, are Gram-negative, motile rods, and are obligate aerobes. They are found in nature where ethanol is produced from the fermentation of carbohydrates by yeasts, such as in plant nectars and damaged fruits. The acetic acid bacteria consist of two genera, Acetobacter and Gluconobacter. Acetobacter can eventually oxidize acetic acid to carbon dioxide and water using Krebs cycle enzymes referred to as overoxidation.
Yeasts Yeasts are widely distributed in natural habitats that are nutritionally rich and high in carbohydrates, such as fruits and plant nectars. Yeasts are rarely toxic or pathogenic and are generally acceptable to consumers. After extensive study, yeasts have been classified into about 500 species. However, only a small number are regularly used to make alcoholic beverages. Yeasts are used to produce ethanol, CO2, flavor, and aroma.
Molds The majority of fungal species have filamentous hyphae and are referred to as molds. They are grouped into four main classes based on the physiology and production methods of their spores. Molds are aerobic and have the greatest array of enzymes. Some molds are used in the food industry to produce specific enzymes such as amylases for use in bread making. They are relatively tolerant to extreme environments and are able to colonize and grow on most foods. Molds are important to the food industry, both as spoilers and preservers of foods and in particular in fermentations for flavor development.
Starter Cultures Fermented foods may be produced by the action of fermentative microorganisms naturally found on the raw materials or in the production environment. However, to improve reliability starter cultures are frequently used. Starter cultures may be pure or mixed cultures. Using mixed starter cultures can reduce the risks of bacteriophage infection and improve the quality of the foods when the organisms are mutually beneficial. Food fermentations frequently involve a complex succession of microorganisms induced by dynamic environmental conditions. Fermentative microorganisms must be safe to eat even in high numbers and must produce substantial amounts of the desired end product(s). For practical reasons, the organisms should be easy to handle and should grow well, enabling them to outcompete undesirable microorganisms. In many traditional fermentations, the natural micro flora were used for the fermentation. Starter cultures are increasingly used to improve not only the reliability, but also the reproducibility and the rate at which the fermentation is initiated.
Classification of Fermented Products Fermented foods are classified in a number of different ways. They may be grouped based on the microorganisms, the biochemistry, or on the product type. Campbell-Platt (1987) identified seven groups for classification, namely, (1) beverages, (2) cereal products, (3) dairy products, (4) fish products, (5) fruit and vegetable products, (6) legumes, and (7) meat products [25], whereas Steinkraus (1997) classified fermentations according to the type of fermentation.
Fermented Products 1. Alcoholic Beverages: Throughout history alcoholic beverages have had a place in most cultures. They require the alcoholic fermentation of fruits or other high-sugar materials by yeasts. Examples of alcoholic beverages are: Beer Wine
2. Distilled Spirits The fermented products discussed above can only produce a maximum alcohol content of about 17%. Concentrations in excess of this inhibit the metabolism of the yeasts. To obtain higher alcohol concentrations, the fermented product must be subsequently distilled. Whiskey, gin, vodka, rum, and liqueurs are examples of distilled spirits. Although the process for producing most products of these types is quite similar to that for beers, the content of alcohol in the final products is considerably higher.
3. Lactic Acid Products 3.1 Dairy Products Yogurt: Yogurt is a coagulated milk product obtained by lactic acid fermentation through the action of Streptococcus thermophilus and Lb. delbrueckii subsp. bulgaricus. Cheese: Cheese is a concentrated milk product obtained after coagulation and whey separation of milk, cream or partially skimmed milk, buttermilk, or a mixture of these products.
3.2 Fermented Vegetables A large number of vegetables are preserved by lactic acid fermentation around the world. The most important commercially fermented vegetables in the west are cabbage (sauerkraut), cucumbers, and olives. Others include carrots, cauliflower, celery, okra, onions, and peppers. Typically, these fermentations do not involve the use of starter cultures and rely on the natural flora.
3.3 Fermented Animal Products The primary reason for developing methods to ferment meats and fish was to extend the shelf life of these highly prized, perishable foods. Gram-positive micrococci have an important role in these fermentations. Several products became popular, including fermented sausages Fermented fish Fish sauces Fish pastes
4. Combined Fermentations The release of carbon dioxide by microorganisms has two major roles in food fermentations: (i) it can act as a leavening agent and (ii) it can be used for carbonation of beverages. One of the most common uses of carbon dioxide is to leaven dough during bread making.
4.1 Bread The use of yeasts thousands of years. to produce bread dates back Breads have relatively short shelf lives; therefore, the primary reason for their production was not preservation, but to improve the digestibility and eating appeal of grains. The yeasts contribute to the flavor and provide an appealing aroma. When the loaves are baked the proteins are denatured, fixing the structure, and the low levels of ethanol produced by the yeasts evaporate.
4.2 Sourdough Sourdough bakery products have an extended shelf life. The fermentation combines the metabolic activity of LAB for souring and yeasts for leavening. Methods for their fermentation date back thousands of years. The sourness of the product depends on many factors, including fermentation temperature and time, type of grain, and the strains of yeast and LAB. The metabolism of LAB and yeasts also provides a range of desirable aroma products.
4.3 Vinegar Vinegar is one of the oldest known culinary products. It is thought that it was discovered by accident from spoilt wine, in fact it is named after the French term vin aigre meaning sour wine. Vinegar is classified as a condiment that contains a minimum of 4% w/v (40 g/l) acetic acid and has a ph value between 2.0 and 3.5. The strength of vinegars may also be quoted in grains, with 10 grains being equivalent to a concentration of 1% acetic acid. Higher strength vinegars may be used for pickling; spirit vinegar is made from an alcoholic solution that has been distilled. The shelf life of a wide range of foods is extended by storing the product submerged in vinegar; this includes pickled vegetables such as gherkins, olives, and onions.
4.4 Kefir Kefir is produced by an acid/alcohol fermentation of pasteurized milk with a mixture of LAB, yeasts, and other bacteria. The final product is acidic, slightly alcoholic, liquid to semiliquid, and effervescent, and is consumed as a beverage. Kefir grains are used to inoculate the milk. Kefir grains comprise proteins, polysaccharides, and a mixture of microorganisms, mainly lactose-fermenting yeasts, and aroma bacteria and LAB. The yeasts consist mainly of Candida kefir and Saccharomyces kefir, while the LAB comprise mainly of Lactobacillus kefir, Leuconostoc species, and L. lactis.
5. Oriental Fermented Products The production of soy sauce, miso, and saki involves koji fermentation. Koji comprises soybeans or grains on which molds grow to produce enzymes such as proteases, lipases, and amylases. The fungal enzymes produced digest proteins, carbohydrates, and lipids into nutrients that are used by microorganisms in subsequent fermentations. Koji is produced in many varieties depending on the products to be manufactured. Koji differs in terms of the molds, the substrate, the method of preparation, and the stage of harvest.
5.1 Soy Sauce Soy sauce is a dark brown liquid produced by the fermentation of soybeans and wheat in a salt brine. 5.2 Tempeh Tempeh is a protein-rich food that is considered one of the world s first meat analogs. Tempeh production is not a means of improving shelf life, but it does improve the acceptability and the nutritional quality of its raw material (soybeans).
Conclusion Fermentations should not be expected to sterilize substandard raw products, but rather should use highquality substrates. Microorganisms can improve their own competitiveness by changing the environment so that it becomes inhibitory or lethal to other organisms while stimulating their own growth, and this selection is the basis for preservation by fermentation. Fermentation improves the safety of foods by decreasing the risks of pathogens and toxins achieving the infective or toxigenic level, and extends the shelf life by inhibiting the growth of spoilage agents.
Glossary Fermentation: Transformation of organic substances into smaller molecules by the action of a microorganism; yeast ferments glucose to carbon dioxide and alcohol. Homofermenters are the ones that produce lactic acid as the major or sole product from glucose. Heterofermenters are the ones that produce equimolar amounts of lactate, carbon dioxide, and ethanol.
Suggested Readings Gustavo V. Barbosa-Canovas, Maria S. Tapia, M. Pilar Cano, 2004, Novel Food Processing Technologies, CRC Press. Lothar Leistner and Grahame Gould, 2002, Hurdle Technologies: Combination Treatments for Food Stability, Safety, and Quality. Michael J. Lewis and Neil. Heppell, 2000, Continuous Thermal Processing of Foods: Pasteurization and UHT Sterilization.