Food Contamination and Spoilage

• Food is considered contaminated when it contains unwanted microorganisms, which can occur naturally or artificially.

Natural Contamination

• Occurs when microorganisms attach to foods during their growing stages.
• Yeasts often contaminate fruits because they ferment the carbohydrates present.

Artificial Contamination

• Happens when food is handled or processed improperly.
• Example: Faecal bacteria entering food due to poor handling.

Food Spoilage

• Involves disagreeable changes from a food's normal state, detectable through smell, taste, touch, or vision.
• Changes depend on the food's composition, present microorganisms, and chemical reactions from microbial metabolic activities.

Types of Spoilage

• Physical, chemical, and biological factors contribute to spoilage.
• Microorganisms that break down fats cause rancidity in unsalted butter.
• Proteolytic bacteria digest protein in meat, releasing odoriferous products like putrescine and cadaverine.
• Food spoilage can result in a sour taste, such as when milk is kept too long.
• Microbiological food spoilage is caused by the growth of microorganisms which produce enzymes that lead to intolerable byproducts in the food.
• Chemical food spoilage occurs when different components in the food react with each other or with some added component which alter the food’s sensory characteristics.
• Physical food spoilage results when moist foods are excessively dehydrated or dried foods absorb excessive moisture.

Sources of Microorganisms

• The main sources are air, soil, sewage, and animal wastes.
• Foods grown in the ground are contaminated by microorganisms in the soil.
• Meats and fish products are contaminated by bacteria from the animal's internal organs, skin, and feet.
• Ground meat is rapidly contaminated due to bacteria moving into the chopped meat.
• Fish tissues are more easily contaminated than meat because of their looser consistency.
• Canned foods should be sterile, but contamination can occur if sterilization is unsuccessful.

Spoilage Changes

• Not always microbiological in origin
• Can result from insect damage, drying out, discoloration, staling, or rancidity.
• Microbial activity causes most food spoilage through surface slime, colony growth, degradation of structural components, and chemical byproducts.

Spoilage Signs

• Odor: breakdown of proteins (putrefaction) e.g. rotten egg smell
• Sliminess: due primarily to surface accumulation of microbial cells also can be manifestation of tissue degradation
• Discoloration: mould on bread, blue and green mould on citrus fruit and cheese
• Souring: production of acid e.g. sour milk from production of lactic acid
• Gas formation: meat becomes spongy and you also have swollen or bubbling packages and cans

Microbial Determinations

• Standard practice to assess microbial contamination through microbial determinations.
• Standard plate count and spectrophotometric analysis are used to determine bacterial numbers.

Standard Plate Count

• Involves diluting a sample with sterile saline or phosphate buffer until the bacteria are dilute enough to count accurately.
• Final plates should have between 30-300 colonies.

Spectrophotometric Analysis

• Measures transmitted light through a sample to estimate cell population.
• Transmitted light is converted to electrical energy, with the reading (absorbance) reflecting bacterial numbers.

Microbiological Food Spoilage

• Microbial growth is exponential, so spoilage may occur suddenly
• Total microbial counts are poor indicators of spoilage potential.
• Chemical analysis can help indicate spoilage.

Milk

• Fluid secreted by mammals for nourishment of young
• Colostrum is a concentrated liquid secreted immediately after parturition, containing up to 25% total solids.
• Cows are the most commercially important animal for milk production.
• Main components of milk include water, fat, protein (casein and whey proteins), and lactose.
• Also contains calcium phosphate and immunoglobulins.
• Composition varies based on species, lactation stage, time of day, nutritional state, and season.

Typical Milk Composition (%W/V)

Milk Fat

• Lipid content of milk is the most variable feature.
• Mainly consists of C14, C16, and C18 fatty acids.
• Present in the form of fat globules surrounded by a phospholipid-rich layer called the "milk fat globule membrane".
• Fat globules are ~5mm in diameter and milk contains about 10^12 fat globules /L.
• Fat globules rise to top to produce distinct cream line
• Homogenization prevents the cream line by reducing the size of the fat globules.

Milk Protein: Casein Micelles

• The primary group of milk proteins are the caseins.
• Milk contains 3 or 4 caseins in the milk of most species.
• All other proteins are grouped together under the name of whey proteins.
• The major whey proteins in cow milk are beta-lactoglobulin and alpha-lactalbumin.
• Serum albumin and the immunoglobulins are absorbed from the blood.
• A limited amount of immunoglobulin is synthesised by lymphocytes which reside in the mammary tissue (called plasma cells).

Casein Micelles

• The structure of casein micelles remains unknown, but they are thought to be made of many smaller “submicelles” held together by a calcium phosphate.
• Short negatively charged regions of casein proteins (“protruding chains”) are exposed all over the surface of the micelle.

Microflora of Raw Milk

• Milk is an excellent medium for microbial growth due to high water availability, moderate pH (6.4 to 6.6), and nutrients.
• High hygiene standards during production and processing are important.
• Milk has some antimicrobial properties but not enough to prevent spoilage

Milking and milking equipment

Microflora of Raw Milk

• Mastitis can lead to higher bacterial counts and is a major cause of economic loss in the dairy industry.
• 1-2% of cows have a clinical infection at any one time.
• In early acute stage of illness, bacterial count in mastitic milk can exceed 10^8 c.f.u./ml
• Mastitic milk will also contain high levels of leucocytes.
• Many organisms can cause mastitis – e.g. Staph.aureus, E.coli, Strep.agalactiae, Ps.aeruginosa.
• Other human pathogens also occasionally reported: Salmonella, Listeria monocytogenes, Mycobacterium bovis and Mycobacterium tuberculosis.
• Infected cows treated with antibiotics must have their milk withheld from sale for several days to prevent antibiotic residues.
• Attempts to control mastitis include good milking hygiene, disinfectant teat dips, and antibiotic infusion.

Hygienic Milk Production

• Use of disinfectant teat dip after milking helps reduce Staph and Strep mastitis (but not E.coli)
• Cows on open pasture have fewer infections than cows housed indoors
• Provide clean bedding and replace regularly
• Remove slurry from concrete areas at least twice daily
• Prevent muddy areas where possible
• Shave udders and trim tails
• Wash and dry teats
• Keep milking parlour floor clean during milking
• Clean teat cups if they fall off during milking and discard foremilk.
• Clean, sanitise and dry milk-handling equipment.
• Chill milk after issue from cow and store at low temp thereafter.

Hygienic Milk Production

• Neglect of cleaning can result in large numbers of organisms in a batch. However, these tend to be fast-growing and heat-sensitive and are killed during Pasteurisation.
• Persistent neglect of cleaning can result in build-up of ‘milkstone’ that protects organisms from sanitisers and allows slower growing organisms to grow that are thermoduric and not as easily killed by Pasteurisation.
• Education of Farmers important and Incentive schemes used to encourage good hygiene

Heat Treatment of Milk

• Heating or boiling milk was recognized in the early 1800s to reduce milk borne illness and mortality in infants.
• Increased milk production and distribution led to outbreaks of milk borne diseases.
• Common illnesses included typhoid fever, scarlet fever, septic sore throat, diphtheria, and diarrheal diseases.
• Pasteurization has virtually eliminated these illnesses.
• Pasteurisation is the process of heating a liquid to below the boiling point to destroy microorganisms.
• Commercial pasteurization of milk began in the late 1800s in Europe and in the early 1900s in the United States.
• Initial pasteurisation conditions, known as flash pasteurisation, to heat the milk to 155 to 178°F (68.3 to 81°C) for an instant followed by cooling
• Pasteurisation conditions were adjusted to 143°F (61.7°C) for 30 minutes or 160°F (71.1°C) for 15 seconds to inactivate Mycobacterium bovis
• In 1957 these conditions were found inadequate for the inactivation of Coxiella burnetii
• New pasteurisation conditions of 145°F (62.8°C) for 30 minutes / 161°F (71.7°C) for 15 sec were required to inactivate Coxiella burnetii

Testing Milk

• Phosphatase test: The alkaline phosphatase (ALP) is an enzyme normally present in raw milk and it is inactivated in conditions of heat treatment

• p-Nitrofenilfosfato + OH ALP yellow complex

• The temperature of inactivation of ALP is slightly higher than that required for the destruction of pathogenic bacteria.

• Used in pasteurised milk to verify if the heating process of pasteurisation is done correctly.

Testing Milk

• Microbiological Quality: EU regulations require Pasteurised milk to contain 105/ml), then rancidity and casein degradation may occur respectively.
• Spoilage of Pasteurised milk normally due to growth of psychotropic Gram-negative rods such as Pseudomonas, Alcaligenes and Acinetobacter, introduced as postPasteurisation contaminants. (0°-35°)
• Spoilage usually manifests itself as off odours and flavours. E.g. Fruity or putrid. May also get visual defects such as clotting due to proteolytic activity or souring caused by LAB.
• Bacillus cereus may cause ‘bitty cream’ due to lecithinase activity. This enzyme hydrolyses the phospholipids associated with milk fat globule membrane to produce small fat particles which float in the surface of hot drinks

Milk Products

• Include yoghurt, cheese, dried milk, sweetened condensed milk, evaporated milk, UHT & Pasteurised milk, cream and butter.
• Fat can be concentrated from milk by centrifugal separation to produce a number of different types of cream that differ mainly in fat content.
• Cream tends to have higher mo count than milk.