List of six important fermented foods:- 1. Vinegar 2. Fermented Milk Products 3. Bread 4. Single Cell Protein 5. Fermented Vegetables 6. Alcoholic Beverages.
Fermented Food # 1. Vinegar:
Different substances like fruits (oranges, grapes, apples, pear etc.), vegetables (potatoes), malted cereals (barley, wheat, rye etc.), sugary syrups (honey, molasses etc.) and many other substances are used in the commercial production of vinegar.
The production takes place in two steps:
The first step involves the initial anaerobic fermentation of the carbohydrate foodstuffs into alcohol by Saccharomyces cerevisiae, followed by second step, the secondary transformation of alcohol to acetic acid by Acetobacter and Glucobacter.
Process of industrial production:
The commercial production of vinegar can be done by the following two methods:
i) Slow or “let home” method:
During this-process, initially alcohol is synthesised at a concentration of 11 -13% due to natural alcoholic fermentation. Then acetic acid bacteria is seeded into the alcoholic liquid and alcohol becomes slowly converted into acetic acid.
A film of vinegar bacteria, i.e., mother of vinegar, grows on the surface of the liquid, indicates that ethanol becomes oxidised into acetic acid. Inferior quality of vinegar is produced in this process.
ii) Quick or “French Orleans” process:
In this process, a barrel is filled about 1/4th of the total volume with raw vinegar from previous run to provide active inoculum. The barrel is then filled up to half of the volume with malt liquor, hard cider or wine and temperature is maintained between 24° to 29°C.
Air is left in the barrel and acetic acid bacterium is grown as a layer on the top of the liquid. The complete conversion to acetic acid takes place within a few weeks to several months. The recovery of vinegar is made by replacing it from the barrel and the barrel is again filled with equal amount of alcohol to continue the process.
The production of vinegar becomes retarded due to dropping of film of vinegar bacteria. Thus a raft of floating framework is provided to continue the filter.
To increase the rate of production, vinegar generator can be used where alcohol containing liquid is trickled over a surface film of vinegar bacteria. The bacterial film is maintained on wood clips.
Alcoholic liquid is sprinkled on the wood clips and during the slow trickling, the liquid is poured down through the generator, thus the alcohol is gradually converted to acetic acid. Air is supplied from the bottom of the generator (Fig. 2.35).
Pests like mites, fruit flies and vinegar eel (Anguillula aceti) attached with the film of vinegar bacteria, cause to sink the film, deteriorate vinegar and thus reduce the yield. Some bacteria like Lactobacillus and Leuconostoc may contaminate the vinegar and thus reduce its quality. In addition to the above, by storing in copper, tin, steel or iron container, the vinegar becomes cloudy.
Fermented Food # 2. Fermented Milk Products:
i) Sour cream:
To prepare the sour cream, butter is soured by lactic acid bacteria. Bacteria like Streptococcus lactis or S. cremoris are used to produce lactic acid and Leuconostoc citrovorum is used to develop necessary flavour. The cream is inoculated and incubated until the desires acidity develops.
ii) Yogurt/Curd:
Initial substrate used in the preparation of yogurt is the artificially concentrated milk. The fermentation of the concentrated milk is done by mixing it with Lactobacillus bulgaricus and Streptococcus thermophilis at a temperature of 42°C for 4 hours, followed by fermentation with yeast. Other organisms like Lactococcus cremosis, L. lacties and L. diactotylactis are also used in this process.
Flavour of the yogurt appears due to accumulation of lactic acid and acetaldehyde.
Sweetness can be done by addition sugar in the concentrated milk, otherwise it will be sour in taste.
For preparing good quality of yogurt, recently prepared yogurt of satisfactory quantity is added with the concentrated milk.
iii) Cheese:
The cheese is produced from the curd by proper shaking and separating it from whey (the liquid portion).
Natural production involves lactic acid fermentation with mixtures of different species of Lactobacillus and Streptococcus. The flavour of cheese varies with the microbial starter cultures, incubation time and inclusion of secondary microbial species at the later part. During this process, unripe cheese are inoculated with fungal spores and incubated for different period of time.
Cheese may be categorised as:
i) Unripe:
This type of cheese is prepared from single step fermentation, e.g., cream and cottage cheese. It is also soft in nature and contains 50-80% water and called soft cheese.
ii) Ripe:
Unripe cheese are added with different microorganisms and incubated for different period of time to achieve different texture, taste and aroma. The ripe cheese may be soft, contains about 50-80% water, e.g., Camembert and Brie, which require 1-5 months ripening.
The semisoft type cheese contains about 45% water such as Roquifort, Blue etc. which require 1-12 months ripening. The hard type cheese contains less than 45% water, e.g., Colby and Cheddar which require 3-12 months’ ripening. The Camembert cheese is produced by Penicillium camemberti, Roquefort cheese by P. roqueforti and Swiss cheese by Propionibacterium shermanii.
The process of cheese making involves nine steps:
Fermented Food # 3. Bread:
The baker’s yeast used in bread baking is the different strains of Saccharomyces cerevisiae. The S. cerevisiae is mixed with bread dough to ferment sugar, thereby CO2 is produced. The leaven lightens the bread by forming gas bubbles in it and also gives characteristic flavour. The leavening is caused by the different enzymes secreted by the 5. cerevisiae.
The enzymes are:
i) Amylase, that hydrolyses starch to maltose,
ii) Maltase, that degrades maltose to glucose, and
iii) Zymase, causes fermentation of glucose and produces ethyl alcohol and CO2. Both the substances evaporate and make the bread spongy and soft.
Fermented Food # 4. Single Cell Protein (SCP):
The term single cell protein refers not only to the isolated cell protein, but also to any microbial biomass, from uni- or multi-cellular microorganisms that can be used as food or feed additives. These are called single cell proteins, because a single type of microorganism is artificially cultivated to get such proteins.
The organisms involved in SCP production include both prokaryotic (Bacteria, Cyanobacteria) and eukaryotic members (algae, yeasts and some filamentous fungi). The mushrooms, where fruit bodies are eaten as food, are also considered in this group.
This group of organisms came into consideration as animal food, because of:
(a) High protein content,
(b) High growth rate,
(c) Ability to utilise various organic substances available at cheaper rate, and
(d) Ability to utilise agricultural and industrial wastes.
For the above, the microbes get the importance and thus SCP industries have developed in different corners of the world.
The traditional system of crop production is based on season, but the production of SCP is independent on climatic and seasonal variation. The microbial biomass contains about 45-55% protein and other essential nutrition and some of them contain all essential amino acids required for human beings.
Regarding the rate of production, it is shown that 50 kg of yeast is able to produce 5,000 kg protein in 24 hours which is much higher than the traditional crop production through agriculture. The production of SCP was initially started during 1960s by an oil company, but now-a-days, it is more popular and attractive.
During SCP production, both autotrophs and heterotrophs are used.
Autotrophs:
Autotrophs like algae such as Spirulina, Chlorella, Scenedesmus etc., have been grown in different warm ponds. The organisms use the energy source from the sun instead of fuel, thus reduce the cost of SCP production. In India, Central Food Technology Research Institute (CFTRI), Mysore, Karnataka, has been working on Spirulina.
They cultured Spirulina and the harvested algae are dried, blended and sold as tablets, which contain 60% protein along with unsaturated fatty acids and vitamins.
The Imperial Chemical Industries (ICI) has been researching on the use of a bacterium, Methylophilus methylotrophus, capable of utilizing methanol for production of SCP in the trade name of ‘Pruteen’ to be used as animal feed. Thus, by feeding animals with SCP, the animals could supply proteins for human consumption.
Heterotrophs:
Mushrooms are being cultivated almost throughout the world for their high protein, less fat and having an essential amino acid lysine by utilising agricultural wastes (straw etc.). Besides this, other renewable carbon sources like molasses, effluents of distilleries and confectionaries, whey, etc., are used for the production of SCP.
Fermented Food # 5. Fermented Vegetables:
(i) Pickles. Natural microflora associated with cucumbers is used to ferment cucumbers under controlled condition, with appropriate temperature and salt concentration. This can also be achieved by the bacteria like Lactobacillus plantarum and Pediococcus cerevisiae for six to nine weeks. Gradually salt (NaCI) concentration has to increase to 15-16%.
The colonisation of microbial organisms varies with the gradual increase in salt concentration:
(a) During low NaCI concentration, bacteria like Pseudomonas and Bacillus are able to grow.
(b) With the increase in salt concentration, Streptococcus faecalis and Leuconostoc mesenteroides are found to grow.
(c) With more increase in salt concentration, bacteria like Lactobacillus plantarum becomes dominant.
(d) At the final fermentation stage, Candida, Pichia and Debaryomyces appear and the concentration of lactic acid becomes reduced. The sour taste of pickles is due to lactic acid present in the medium.
(ii) Soya sauce:
It is a brown, salty and tangy sauce produced from the mixed substrate consisting of soya beans, wheat and wheat bran. Initially, the mixture of soya beans and wheat is inoculated with spores of Aspergillus oryzae and undergo koji fermentation (dry fermentation).
The fungus A. oryzae grows on the surface and accumulates various enzymes like proteinases and amylases. Different lactic acid bacteria also dominate at this stage.
The starter is then dried and extracted. The extract is then mixed with a mixture containing sterile soya beans, steamed wheat bran and autoclaved and crushed wheat. The mixture is then incubated at 30°C for some days and then soaked with concentrated brine.
Now, the mixture is called maromi, incubated for 10 weeks to 1 year. Maturation begins with lactic acid bacteria and later by alcoholic fermentation with yeasts like Saccharomyces rouxii and Torulopsis spp. Important organisms used in this process are Aspergillus oryzae and different species of Lactobacillus.
(iii) Poi:
Poi is a type of fermented food of Hawaiian Islands. It is prepared from the Taro plant. The plants are steamed, ground and kept for 1-6 days for fermentation. In the first few hours, Pseudomonas and other microbes dominate, but they are successively replaced by Streptococcus, Lactobacillus and Leuconostoc and finally, Geotricbum candidum and different yeasts dominate.
Proper flavour, texture and aroma become available due to the presence of ethanol and different acids like acetic acid, lactic acid and formic acid.
(iv) Tempeh:
Tempeh is a type of food of Indonesia produced from soya bean seeds. Water soaked seeds are ground and inoculated with the spores of Rhizopus. The mixture is then incubated for 20-24 hours at 32°C. After incubation, the product is added with salt and fried in oil before eating.
(v) Tofu:
It is a Japanese food prepared from soya bean seeds. In China, it is called Sofu. It is a cheese-like substance developed by fermentation of soya bean seeds by Mucor spp. The soaked soya bean seeds are ground and added with Mg or Ca salts and the mass is incubated at 14°C for one month after inoculating with Mucor.
(vi) Idly:
Idly is prepared from rice and black gram (Vigna mungo) seeds in the proportion 1:1, especially in the southern part of India. The seeds are soaked in water for two hours and then ground properly. The mixture is kept overnight. A small amount of salt and baking powder is added to it.
The mixture is baked under steam.
The organism involved in this process is Leuconostoc mesenteroides.