Food Microbiology - Chapter 7 & 8Document Transcript
CHAPTER 7MICROBIOLOGY OF MEAT • Meat is easily spoiled by microorganisms because it contains a lot of nutrient, growth factors etc. the pH for meat is the range of 5.6-7.4. • Microorganism for soil, water and manure make up the dominant flora of meat. During slaughter, the external surface of the animal may contaminate the meat by direct contact through the above sources and equipment, personnel and slaughtering area. • Bacteria generally found in meat: a. pathogenic microorganism E.g. Brucella, Salmonella, Streptococcus, Mycobacterium tuberculosis, Clostridium b. spoilage microorganisms E.g. Acromobacter, Pseudomonas, Bacillus and Staphylococcus c. molds E.g. Aspergillus, Mucor, Penicillium • factors affecting the spoilage of meat a) water content Meat has high water content with dissolved substances such as glycogen, lactic acids and amino acids. All these substance can cause microbial growth which can lead to early spoilage.
b) Redox potential Has a big influence on microbes. Tissue respiration continues after death and uses up oxygen present and produce CO2 . Finally oxygen tension will fall (Eh negative). The inner side of meat becomes anaerobic; together with the production of lactic acids. The bulk of meat becomes anaerobic except on the surface. At the surface, aerobic flora and inside meat, anaerobic flora predominates. But, these microorganisms do not grow readily at low temperature, therefore little growth may be observed.c) pH value pH value of the meat is between 5.5-7.4. Acidic pH (5.5) not suitable for growth of many types of microorganisms. pH of meat depends on amount of lactic acid produced by glycolysis in muscle after death. The more acid produced the lower the pH. But, the amount of acid depends on the amount of glycogen in the muscle at death. Glycogen ↑ Lactic Acid ↑ pH ↓ i. If animal is exercised before slaughter, the glycogen is low, little lactic acid produced and the pH of carcass is relatively high. Muscle will be dry and firm in texture and dark in color (DFD condition). ii. If non-exercised animal is stimulated before slaughter such as stress, glycogen will rapidly turn to lactic acid and low pH is approached before tissue has time to cool. This cause denaturation of some, muscle protein. Meat will
become pale, soft and exudative (PSE condition).• The type and spoilage of meat is determined by: i. Initial numbers presentii. pH value of meat – if pH > 6.0, amino acid is attacked. Microorganisms can grow fast. Meat will putrefy mainly due to Pseudomonas and Aeromonas type.• Two types of spoilage: Aerobic condition Anaerobic condition• Aerobic condition a) Surface slime Accumulation of microorganism on meat surface cause by gram-negative rods such as Pseudomonas and Achromobacter b) Colour change Occur more quickly if stored in oxygen. Red coloured meat (oxidized myoglobin) can change to brown due to production of oxidizing compound such as peroxides or H2S from bacteria e.g. Leuconostoc spp. • Red spots on meat – pigments of Serratia • Yellow spots – pigments of Flavobacterium • Green spots – pigments of Pennicilium • White spots – pigments of Rhizopus
c) Off flavors and odours Due to formation of fatty acids on the meat surface such as formic, butyric and propionic acid. It can also due to production of compounds such as amines, ammonia caused by degradation of amino acid by proteolytic bacteria such as Pseudomonas spp whereas Actinomycetes will give an earthy odour. d) Rancidity Caused by some lipolytic bacteria and molds• Anaerobic condition a. Putrefaction Breakdown of protein with the formation of foul smelling compounds such as mercaptans, H2S, indol, amines and others. This occurs inside the meat such as Proteus, Clostridium and Bacillus b. Souring Can be microbial and biochemical. Microbial souring is due to the formation of fatty acids and lactic acids, with no putrefaction. E.g. Clostridium and Bacillus
CHAPTER 8MICROBIOLOGY OF MILK • Milk and its product contain a large amount of nutrient. Excellent media for growth of spoilage microorganisms. • Before milking, milk is sterile. As soon as it comes out of the udder, it becomes contaminated. Contamination comes from milking utensils, air, personnel etc. • The number of microorganisms present depends on how the milk is handled. Aseptic handling is important in reducing contamination. • Milk collected should be immediately put in tank, then put in cold storage, pasteurize and refrigerate. Make sure aseptic condition after pasteurization otherwise post pasteurization contamination can be occur. • Examples of microflora in milk: a. Pathogenic – Campylobacter fetus, Listeria monocytogenes, Clostridium perfringens b. Spoilage – Pseudomonas spp, Aeromonas spp., Streptococci, Acinetobacter and Flavobacterium • Preservation is commonly done by pasteurization and sterilization.
• Milk spoilage Milk is an excellent media for microbial growth. If the preservation methods are inadequate, different type of spoilage can occur. 1) Souring of milk Milk considered spoilt if it curdles or sours. This is due to the formation of acid caused by lactic acid bacteria. At room temp. : Streptococcus lactis 2) Gas production During fermentation, acid and gas produced, formation of foam on surface can be seen. If too much gas, it cause stormy fermentation. The gas ripped the curd apart, which produces gas. Example: Clostridium, Bacillus, yeast and heterofermentative lactic acid bacteria. 3) Proteolysis Hydrolysis of milk protein with the production of peptides causing bitter flavor. Proteolysis can occur if: Milk store at low temperature When acid forming bacteria are destroyed due to heating, leaving only the proteolytic ones (no competition) Types of proteolysis can be divided into 4 categories: a. Acid proteolysis-acid production occurs together with proteolysis.
b. Sweet curdling-early stage curdling caused by rennin like enzymes produced by bacteria. c. Slow proteolysis-caused by endoenzymes of bacteria which undergo autolysis. Important in cheese making and maturation. d. Residual proteolysis-caused by heat stable proteinases. Proteolytic microorganisms are: Micrococcus, Proteus and Pseudomonas spp.4) Ropiness There are two types of bacterial ropiness: a. Surface – slimy material on surface of food. Develop best at refrigeration temperature. Examples: Alcaligenes viscolactis b. Throughout – the whole milk becomes ropy. Caused by Alcaligenes spp. and Strep. Lactis. These microorganisms reproduce within capsule and form long chain. Ropiness can be reduced if acidity is increased. Acid usually produced at mesophilic temperature.5) Alkali production Due to formation of ammonia from urea, carbonates or organic acids. Caused by Pseud. fluorescens, Pseu. trifolil, Alcaligenes faecalis, Alcaligenes viscolactis
6) Colour change May be due to surface growth of pigmented microorganisms.
MICROBIOLOGY OF FRUITS AND VEGETABLES • 25% of fruits and vegetables are spoilt before harvesting because of infection and disease. • Spoilage after harvesting is due to mishandling, over-ripening etc. Skin becomes soft, makes it easier to be infected by microorganisms.Fruits • Fruits are covered with skin/shell for: a. Natural protection from microbial infection. b. Minimize moisture loss to environment. c. Stabilize enzymatic changes. • Healthy fresh fruits are free from microorganisms. Once skin is broken e.g. insect sting, it will become infected especially if put in place where sanitary condition is not satisfactory. • The kind of microorganisms in fruits depend on various factors: a. Characteristics of fruit – soft ones more prone to infection. b. Nutrient content – sugar and acid. c. Climatic condition. • Bacteria do not play important role in fruit spoilage due to acidity of fruits. • Mold spoilage usually more significant. • Yeast usually initiate spoilage then later taken over by molds. Spoilage by yeast is not extensive.
• Spoilage of fruits: a. Blue mold rot: Penicillium expansum b. Black mold spoilage: Aspergillus spp c. Green mold spoilage: Cladosporium spp. d. Bacteria soft rot: Xanthomonas, Erwinia and PseudomonasVegetables • Vegetables are easily spoilt, just like fruits. • Examples: 40%tomatoes are already spoilt before consumption. • Through genetic engineering, new hybrid of tomato is produced to reduce spoilage. Example: flav sav tomato. • Vegetable contain more available water and less carbohydrate or fat content compared to fruits. High pH and the less acidic content are favoured by bacteria. • Spoilage of vegetables: a. Bacteria soft rot – the vegetables part becomes soft and moist. These bacteria produce pectinase which can hydrolyze pectin such as in tomato, garlic, celery etc. b. Mold spoilage to vegetables: Grey mold rot – Botrytis spp. Sour rot – cause formation of acid. Geotricum candidum.