Lecture 2 Milk Fermentation.Pdf
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بسم هللا الرحمن الرحيم Dairy Microbiology Karima Galal Abdel Hameed Professor of Milk Hygiene Faculty of Vet. Med. S V U Milk fermentation Milk fermentation Def.: The process by which a change is produced in milk through microbial activity General Specific fermentation fermentation 1. Germicidal (Destruction phase) 1. Souring 2. Souring & gassiness 2. Souring phase 3. Proteolysis 3. Neutralization 4. Alkaline fermentation 4. Putrefactive phase 5. Ropiness General fermentation Changes occur in a milk sample immediately after milking & kept at room temp. (24hs) 1. Germicidal (Destruction phase) 2. Souring phase 3. Neutralization 4. Putrefactive phase Neutralization phase Germicidal phase 1. Germicidal (Destruction phase) No growth or decrease no. of m.os due to Lactinine I & II Anticoliforms Length: few min. to few hrs According to *Heating of milk 1. Temp: 3h at 30-37ºC at 60-80°C for 1/2h. 6 h at 25ºC 24 h at 5-10ºC 2. Milk of different age 3. Milk of different quarters Destruct this phase 4. Milk of different seasons 2. Souring phase: Continue to hrs or days Due to growth of milk souring bacteria change lactose to lactic acid & curdling of casein e.g. Sterptcoccus & Lactobacillus ❑ Factors affecting: 1. Type of bacteria 2. No. of bacteria 3. Temp. Reach maximum when acid 0.8-1% & bacteria inhibited 3.Neutralization phase Due to acid yeast & mold growth ( A thick mat of molds on the surface Involve several days or weeks 4. Putrefactive phase: Medium neutral or slight alkaline due to yeast & mold Putrefactive bacteria grow Attack protein (casein) Putrefactive yeasts & molds Decompose the solid constituents of milk Result in a clear fluid Specific fermentation 1. Souring: common fermentation of milk Lactose Lactic acid Lactic acid bacteria b) Lactobacillus a) Strept. lactis (10-37ºC) Up to 1% lactic acid bulgaricus acidophilus casei 2-4% lactic acid Evidence of milk fermentation Curd Sour flavor firm Jelly like smooth Used in manufacture of fermented milk and cheese 2. Souring & gassiness Characterized by acid gas The curd smooth Gelatinous with gas puncture ( spongy) Most common m.os Coliforms: e.g. E. coli Aerobic spore forming: e.g. B. polymyxa Clostridium spp. Lactose fermenting yeast: e.g. Candida , Torula cremoris & Torula lactis Gas producing microorganisms *Escherchia coli *Aerobacter aerogenes *Anaerobic bacteria: Clostridium perfringens Clostridium butyricum Clostridium sporogenes *Yeasts and Molds Torula cremoris Torula lactis Proteolysis: is broken into Casein Water soluble compounds Classification of proteolysis: ➢ Acid proteolysis: e.g. Micrococci, Strept. fecalis ➢ Proteolysis with little acid or alkaline ➢ Slow proteolysis due to endoenzymes of bacteria after autolysis ➢ Sweet curdling in pasteurized & sterile milk Coagulation of milk without acid due to rennin like enzyme 1- Aerobic spore bacilli: Bacillus subtilis, Bacillus cereus, Bacillus cereus varmycoides, Bacillus mycoids, Bacillus coagulens and Sterilized milk Bacillus calidolactis 2. Non spore forming cocci: e.g. Strept. liquefaciens Sources: ▪ Dust ▪Feed ▪Soil ▪Water ▪Utensils 4. Alkaline fermentation: Cause: Pseudomonas fluorescens Secrete enzyme saponify fat Alkaligenes fecalis a t t a k Milk is whey like substance Citrates forming Alkaline carbonates Alkaline Ropiness 5. Ropiness: RopinessAbnormal is a flavorform of in bacterial sweat creamspoilage & in milk milk that makes the milk highly viscous or sticky. Ropy milk has characteristic silk-like threads Surface ropiness thatTypes: may vary in length from a few inches to several feet. 1. Bacterial Ropiness: Alkalligenes viscosus Slime producing Cause: Strept. cremoris varhallandicus bacteria AerobacterThroughout aerogens ropiness Micrococcus freudenreichii 2. Non bacterial Ropiness: Throughout ropiness Gargety milk (mastitic milk) leucocytes & fibrin Stringiness ( Thin film of casein or lactalbumen at low temp.) Surface ropiness Sliminess (thickness of cream) Ropiness Ropiness is a form of bacterial spoilage in milk that makes the milk highly viscous or sticky. Ropy milk has characteristic silk-like threads that may vary in length from a few inches to several feet. You would test for this by dipping a pointed device (like a needle) on the surface of the milk (after incubating for 12-48 hours) and raising the needle to see if there is presence of a “rope.” This defect affects milk, cream and whey. The effect is very significant in milk and cream meant for the market. Ropiness can be classified as either bacterial or non-bacterial in nature Bacterial Ropiness Caused by a slimy capsular material produced by the bacterial cells (gums or mucus). It further classify bacterial ropiness under these two categories: Surface Ropiness: – observed at the top of the milk and is caused by Alcalegenes viscolactis, it found in the soil & water. Ropiness observed throughout the milk: – caused by: Some coliforms, (e.g. Enterobacter aerogenes, Enterobacter cloacae, and in rare curcumstances Escherichia coli.) Some species of LABs, (e.g. Streptococcus lactis, Lactobacillus bulgaricus, Lactobacillus cereus, Streptococcus cremoris, and Lactobacillus plantarum). Most of these microorganisms grow in chains, which leads to the formation of the ropes in milk. Other microbes like Micrococci and Bacilli. Non-bacterial Ropiness This form of bacterial spoilage in milk may occur as a result of: Presence of mastitis in the milk, especially fibrin and leucocytes from the cow’s blood present in the milk. Thick cream at the top of the milk Casein film or lactalbumin that occur in milk during cooling .