Food Microbiology & Safety
Helen Billman-Jacobe
Sources of microorganisms in food
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Ray and Bhunia Chapter 3
Intended learning outcomes
Differentiate between normal flora in plant and animal derived foods, contaminants, spoilage organisms, pathogens and functional organisms
Relate the intrinsic and extrinsic factors of food substances to how microorganism can grow and survive
optimal growth preferences
Ability to compete with other microbes
Sensitivity to physical and chemical damage
pH and buffering Redox potential
Antimicrobial factors Nutrient content
Metabolic capacity
Implicit factors
Growth rate
Factors affecting microbial growth and survival in food
Extrinsic factors
Intrinsic factors
Gaseous atmosphere
temperature
Water activity
FOOD MICROBES
Intrinsic factors: pH changes
Intrinsic factors: nutrients
Intrinsic factors: Aw
Nutrient content: water, protein and amino acids, minerals, fats and fatty acids, vitamins and other bioactive components, and small quantities of carbohydrates
Aw: high Aw. Drying meat is a common way of preserving it
Intrinsic: Eh
Eh: oxidative deterioration of meat leads to off‐flavor development.
Oxidation of ferrous‐oxymyoglobin (Fe2+) to ferric‐metmyoglobin (Fe3+) occurs in the presence of some reactive species and produces discoloration of meat
Meat: Spoilage organisms
Cause organoleptic changes in products • Off odor
• Off flavour
• Bad taste
• Change in colour
Spoilage occurs at a bacterial density of ~107 cfu/g
One product is skatole made from tryptophan. It is foul smelling. The amino acid arginine can be degraded anaerobically to generate the amine putrescine
Amines such as putrescine and cadaverine formed by decarboxylation of amino acids are good chemical indicators of meat spoilage
Decarboxylation reactions of several other amino acids yield amines such as tyramine and histamine
Anaerobic degradation of sulphur containing amino acid yields H2S
Intrinsic: Eh
Eh: oxidative deterioration of meat leads to off‐flavor development.
Oxidation of ferrous‐oxymyoglobin (Fe2+) to ferric‐metmyoglobin (Fe3+) occurs in the presence of some reactive species and produces discoloration of meat
Spoilage organisms
Packaging influences what can grow
– Pseudomonas
• Vacuum packed
– Lactobacillus
– Brochothrix thermophatica
• Modified atmosphere packaging (MAP)
– Lactobacillus
– Brochothrix thermophatica
Pseudomonas
• Predominant bacteria that are often associated with spoiled meat are Pseudomonas, which are motile, Gram –negative, rod shaped, aerobic bacteria.
• Digest proteins in meats into amino acids and foul‐smelling compounds such as ammonia, amines, and hydrogen sulfide
• Pseudomonas fragi growing on meat surface uses compounds such as glucose, free amino acids, and lactate.
• Prefer sugars and metabolise amino acids when sugars are depleted glucose>lactate>citrate>glutamate>creatine‐creatinine
Brochothrix thermosphacta
Gram‐positive, non‐spore forming, non‐motile, catalase‐positive, facultatively anaerobic, regular rod shaped bacteria. The optimal temperature for growth is 20‐25o C.
Can grow at at temperatures a low as 0°C and under conditions of low oxygen concentration and high C02 concentration
Brochothrix thermosphacta can grow aerobically in the presence of 210 mM L‐lactate It can grow anaerobically at pH values down to at least 5.5 provided there is no lactate B. thermosphacta is sensitive to undissociated lactic acid
Postrigor meat usually contains sufficient lactic acid to select against the anaerobic growth of B. thermosphacta.
Clostridium sp
Anaerobes Spore formers Toxins
Gas production Psychrophiles
C. estertheticum – blown pack spoilage‐ H2 and CO2 gas
C. perfringens is commonly found on raw meat and poultry. It prefers to grow in conditions with very little or no oxygen, and under ideal conditions can multiply very rapidly. Some strains of C. perfringens produce a toxin in the intestine that causes illness.
Yeasts and molds
Cladosporium (black spot) ‐dark mycelia which may be brown to blackish‐brown or gray‐green in color.
Pencillium corylophillum (blue green mold) Thamnidium elegans (whiskers)
Spoilage. Rarely cause disease
https://doi.org/10.1016/j.meatsci.2014.10.003
Food Safety News
http://www.foodsafetynews.com/2016/08/fsa‐finds‐brits‐not‐up‐to‐the‐ mark‐on‐best‐burger‐practices/#.W1vTWtIzY2w
Try the same exercise with this scenario
We are making a chicken casserole. Chicken was bought from the supermarket and then cut into pieces and coated with flour and seasoning ( salt‐ and‐pepper). The next step is to fry the flour‐ coated chicken in oil to brown it. The browned chicken will be added to a casserole with vegetables, herbs and stock and cooked in an oven at 180°C for 45 min .
Identify the sources of microbial contaminants of each of the ingredients used in this meal
Ingredients = chicken, flour, vegetables, herbs, salt‐and‐pepper, stock
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