Microbiology Of Fermented FoodProduction
Fermentation Involves exposing the raw or starting food materials to conditions that favor growth and metabolism of specific anddesirable microorganism.As the desirable microorganisms grow, they utilize some nutrients and produce some end products.These end products, along with theunmetabolizedcomponents of the starting materials, constitute the fermented foods having desirable acceptance qualities, many of which are due to the metabolic end products.
Raw Or Starting Materials
A large number of raw materials from plant and animal sources are used to produce fermented foods. These include:MilkMeatFishEggsVegetablesFruitsCereal grainsLentilsBeansSeeds
Many species and strains of bacteria, yeasts and molds are associated with fermentation of foods.Depending on a product, fermentation may be achieved by a single species and strain.In most fermentations , a mixed population of several bacterial species and strains , or even bacteria and yeasts or bacteria and molds is involved.
When a fermentation process involves a mixed population, the members should not be antagonistic toward one another, rather, they should preferably be synergistic.Maximum growth of a desirable microorganism and optimum fermentation rate are dependent on environmental parameters such as nutrients, temperature of incubation, oxidation-reduction potential andpH.
Fermentation Process1- Natural Fermentation
Many raw materials used in fermentation contain both desirable and associated microorganisms.The conditions of incubation are set to favor rapid growth of the desirable types and no or slow growth of the associated types(many are undesirable).A product produced by natural fermentation can have some desirable aroma resulting from the metabolism of the associated flora.
Because the natural microbial flora in the raw materials may not always be the same, It is difficult to produce a product with the same characteristics over a long period of time.There is a chance of product failure because of the growth of undesirable flora and foodbornediseases by the pathogens.
2- Back Slopping
Some products from a successful fermentation are added to the starting materials and conditions are set to facilitate the growth of the microorganisms coming from the previous culture.Retention of product characteristics over a long period may be difficult because of changes in microbial types. Chances of product failure and foodborne diseases are also high.
3- Controlled Fermentation
The starting materials which may be heat-treated are inoculated with a high population(one million cells/ml) of a pure culture of single or mixed strains or species of microorganism(starter culture).Incubation conditions are set for the optimum growth of the starter culture.There is less chance of product failure and foodborne diseases.There may be no growth of desirable secondary flora, as a result, a product may not have some delicate flavor characteristics.
Fermented Food Groups
There are more than 3500 types of fermented foods worldwide, the following are the major groups:Dairy products………Cheeses,Yogurt,SourcreamMeat Products…Pepperoni,Salami,PickledmeatCereal products…….Breads, Pancake, PizzaFruits and vegetable products…….Pickled fruits, pickled vegetables, OlivesLegume products………Soy sauce, fermented soymilk
Fish products………Fish sauces, Pickled fishBeverages………Coffee, tea, cocoa, BeerStarch crop products……Fermented products from potato, sweet potato, BananasMiscellaneous products……Vinegar, fermented eggs
Fermented Dairy Products
Fermented dairy products can be broadly divided into two groups;Fermented milk productsCheesesIn fermented milk products, all milk constituents are retained in the final products.In cheeses, a large portion of milk constituents is removed in whey to obtain the final product.
Milk Composition and Quality
The growth of desirable microorganisms and the quality of a fermented dairy product are influenced by the composition and quality of milk used in a fermentation process.Cows milk contains approximately 3.2% protein, 4.8% lactose, 3.9% lipids, 0.9% minerals, traces of vitamins and 87.2% water.Casein is present as calciumcaseinateat higher concentration than albumin and globulin.Lactose is the main carbohydrate, lipids are dispersed as globules of different sizes in emulsion.
Minerals are present in solution and as colloid with casein.Total solids: 12.8%.The whey contains the water soluble components, some fat and water.Milk contains natural antimicrobials; agglutinins and thelactoperoxidaseisothiocynatesystem.The agglutinins can induce clumping of starter culture cells and slow their growth and metabolism.Thelactoperoxidase-isothiocynatesystem can inhibit starter culture.Antimicrobials can cause problems only when raw milk is used, because both are destroyed by heating milk.
Milk can also contain antibiotics, either used in the feed or used to treat animals for some infections such as mastitis. Their presence can also affect the growth of starter culture.Some milk can contain heat stable proteases and lipases produced by somepsychrotrophicbacteria such as Pseudomonas species during refrigerated storage of raw milk before pasteurization. These enzymes remain stable after heating and can cause product defect(low yield of cheese, proteolysis, rancidity).Before milk is used for fermentation, these aspects should need to be considered.
Fermented Milk Products
Yogurt is made withStreptococcusthermophilusandLactobacillusdelbrueckiisubsp.Bulgaricus.Buttermilk is made with lactobacillus spp. Without or withLeuconostoccremoris.Acidophilus milk is made from Lactobacillus acidophilus.
Microbiology Of Yogurt Fermentation
Plain yogurt has a semisolid mass due to coagulation of milk(skim, low, or full fat) by starter culture bacteria.It has a sharp acid taste withfalvorsimilar to walnuts and a smooth mouth feel.The flavor is due to combined effect of acetaldehyde,lactate,diacetyland acetate.About 90% of flavor is due to acetaldehyde.Many types of yogurt are available in the markete.g. plain Y., fruit Y., flavored and colored Y. and sweetened Y.
Batch process for a low fat2%) plain yogurtHomogenized milk (12% TS) plus stabilizer (1%), the stabilizer is added to give desired gel structure.Heating to 85C for 30 min, and cooled to 43.3C, heating helps to destroy vegetative microbes and slightly destabilize casein for good gel formation.Starter is added , incubated at 43.3C and pH 4.8 for 6h.Quickly cooled to 29.4C for 30.min. to slow down further starter growth and acid production especially by Lactobacillus spp., agitated and pumped to filler machine.Packaged in containers, cooled by forced air to 4.4C to stop the growth of starter. Held for 24h, pH drops to 4.3.
In a good product, the two starter species should be added at aStrep:Lactoratio of 1:1, in the final product, the ratio should not exceed 3:2.For balanced growth of the two species, the fermentation is conducted at 43.3C, at this temperature both acid and flavor compounds are produced at the desired level.If the temperature is raised above 43.3C lactobacillus spp. Predominates, causing more acid and less flavor production.Attemperturebelow 43.3C growth of Streptococcus spp. Is favored, forming a product containing less acid and more flavor.
The two species show symbiotic growth while growing together in milk.Initially Streptococcus spp. Grows rapidly in the presence of oxygen and produces formic acid and CO2.The anaerobic condition , formic acid and CO2 stimulate growth of lactobacillus spp. Which has goodexoproteinaseand peptidase systems and produce amino acids and peptides from milk proteins.
Some of the amino acids such as glycine,valine, histidine,leucineand methionine are necessary for good growth of Streptococcus spp. Which lacks proteinase enzymes.Streptococcus spp. Grows rapidly until pH drops to 5.5 at which time the growth of Streptococcus spp. slows down.However, growth of Lactobacillus spp. Continues fairly rapidly until the temperature is reduced to 29.4C following a drop in pH to 4.8.
Lactose MetabolismBoth species have a constitutiveβ-galactosidasesystem and lactose is hydrolyzed to glucose andgalactose.Both species arehomofermentativeand produce lactate from glucose by glycolysis.Both species do not metabolizegalactose, as a result,galactoseis excreted outside causing its accumulation in yogurt.
Flavor ProductionThe major flavor in yogurt is acetaldehyde with somediacetyland acetate.Acetaldehyde is produced from glucose via pyruvate by Streptococcus spp. And from threonine by Lactobacillus spp.FormateProductionFormate(necessary for Lactobacillus growth) is produced by Streptococcusthermophilusfrom pyruvate by the action offormatelyase.Pyruvate--------------Formate+ Acetate
Slime Formation(Glycan).Β-galactosidasein some strains of streptococcusthermophiluspolymerizes glucose to produce oligosaccharides and glycan, which may give aviscioustexture to yogurt.Milk proteins in the presence of proteinases and Lactobacillus yields peptides.Excess accumulation of peptides , some of which cause bitter flavor.Peptides by the action of peptidases produces amino acids necessary to Streptococcus, also threonine can be used to produce flavor by Lactobacillus.
Microbial ProblemsIn plain yogurt, flavor problems can be associated with the concentration of acetaldehyde. A low concentration gives a chalky and sour flavor. Too much acetaldehyde gives a green flavor.Too muchdiacetylgives a buttery aroma.Too much acid production during storage are associated with bitter flavor.
Production ofexopolysaccharidesby the starter can give avisciousand ropy texture.Growth of yeast during storage can also produce fruity flavor especially in yogurt containing fruits and nuts.During long storage, molds can grow on the surface.