Microbes, Milk, and Humans PDF

Microbes, Milk, and Humans Significance of Microbes in Milk - Information on microbial load can be used to assess its sanitary quality and the conditions of production - Bacteria cause spoilage of milk - Milk is potentially susceptible to contamination with pathogens. - Certain microbe...

Microbes, Milk, and Humans Significance of Microbes in Milk - Information on microbial load can be used to assess its sanitary quality and the conditions of production - Bacteria cause spoilage of milk - Milk is potentially susceptible to contamination with pathogens. - Certain microbes produce chemical changes that are desirable in the production of cheese, yogurt and fermented milk products. MORPHOLOGY AND CLASSIFICATION OF DAIRY BACTERIA - divided on the basis of their action on milk constituents, for example acid- former, proteolytic, saccharolytic. - classification is based on the morphology of these micro-organisms. Classification Based on morphology (Shape, Size, Arrangement of Cells) - Cocci Bacterial cells with spherical or ellipsoidal shape are called as cocci and may have the following type of arrangement of cells: Diplococci: (e.g. Neisseria), Streptococci: (e.g. Streptococcus), Tetrads: (e.g. Pediococci) Staphylococci: - Bacilli Bacteria with cylindrical or rod like cells are called as bacilli and these may have the following type of arrangement of cells. Diplobacilli, Streptobacilli: Lactobacillus bulgaricus or Bacillus anthracis. - Spirilla (rods) Bacteria with spiral or helical shaped cells (Vibrio, Spirochetes) Classification Based on Temperature - Mesophillic: 20 and 40 C with the optimum growth temperature of 37 C (e.g. S. aureus, E. coli). - Psychrotrophic: Microorganisms that are capable of growing at or below 7 C with the optimum growth temperature of 15-20 C. - Thermophilic: Organisms capable of growing over 50 C with optimum growth temperature of 55 C. - Thermodurics: Organisms capable of withstanding pasteurizing temperatures of 63C/30 min. with optimum growth temperature of 35- 37C Classification Based on Oxygen Requirement - Aerobic: grow in a standard air atmosphere of 21% oxygen: bacillus - Anaerobic: grow in the presence of CO 2: clostridium. perfrongens - Facultative: grow either in the presence or absence of oxygen: Lactococcus lactis ssp. lactis - Microaerophillic: Organisms which grow best at 1-15% of O2: Campylobacter jejuni Physiological Grouping - Acid producers: fermenting lactose to form lactic acid, Lactic acid coagulates milk by producing precipitation of Casein at 4.6 pH. Homofermenters: Lactococcus, few Lactobacilli Heterofermenters: Few Lactobacilli, Lueconostoc spp., - Gas producers: producing CO2 or \ and H2 from lactose fermentations yeast, clostridium Proteolytic: degrade milk proteins into soluble components by enzymes known as proteinases or proteases. (Bacillus and pseudomonas spp). Lipolytic: degrade milk fat by enzymes such as lipases liberating glycerides and fatty acids. (Pseudomonas spp, Achromobacter lipolyticum). Sweet curdling: curdling of milk by rennin like enzyme before the development of sufficient acidity (B. subtilis). Ropiness: change in the viscosity of milk or forming threads (Alcaligenes viscosus) Flavour producing Many organisms are capable of producing different flavours in milk during their growth. Some of the flavours and associated organisms are listed below: Fruity: Pseudomonas fragi Malty: Lactococcus lactis subsp. lactis biovar maltigenes Fishy: Proteus icthyosmius Unclean: E. coli Colour fermentations: producing different colors in milk during their growth by producing chromogenic compounds. Yellow: Pseudomonas synxantha Blue: Pseudomonas cyanogenes Green: Penicillium roqueforti Black: Pseudomonas nigrifaciens Spores forming bacteria: (bacillus and clostridium). Rate of Reproduction (generation time) Nutritional Requirements Inhibition of Growth and Destruction of Microorganisms: - Modulation of favorable bacterial conditions to control bacteria. - pH (6-8) cottage cheese with a pH of 5.0 is likely to keep significantly longer than cottage cheese with a pH of 5.2 - water activity: (lypholization). - Salt % - fatty acid content: acetic acid (two carbons), inhibit and even destroy salmonellae, oleic acid stimulate growth of lactobacilli. - antibiotics: from microorganisms inhibits others. Groups of Bacteria Important to the Dairy Industry Lactic Acid Producers Lactobacillus - Lactobacillus is Gram-positive facultative anaerobic or microaerophilic, rod-shaped bacteria - convert lactose to lactic acid - production of lactic acid makes its environment acidic that inhibits the growth of harmful bac - aerobic/ facultative anaerobe; nonmotile - divided into three: - Thermobacterium, - Streptobacterium - Betabacterium lactobacillus bulgaricus - is one of the cultures used for production of yogurt and that also found in other naturally fermented products. - It utilizes lactose to produce lactic acid, which is used to preserve milk. - It is Gram-positive rod, non-motile, and does not form spores. - This bacterium is regarded as aciduric or acidophilic, requires a low pH (around 5.4-4.6) to grow. - The bacterium has complex nutritional requirements, including the inability to ferment any sugar except lactose. - thermophilic microbe whose optimum temperature is 40˚C. Lactobacillus bulgaricus cont… used with Streptococcus salivarius subsp thermophilus as a starter in making yogurt. The two species work in synergy, with Lactobacillus bulgaricus producing amino acids from milk proteins that are used by S. thermophilus. Both species produce lactic acid that gives yogurt its tart flavour and acts as a preservative. The resulting decrease in pH also partially coagulates the milk proteins, such as casein, resulting in thickness of yogurt. While fermenting milk, L. bulgaricus produces acetaldehyde, one of the main yogurt aroma components. Some strains of L. bulgaricus also produce bacteriocins named bulgaricin (stable at 100˚C/hr), that kill undesired bacteria.