Food Microbiology & Public Health

Food packaging and further processing????? You might ask how is THIS related to being a vet. However, whilst there isn't a big role played by vets on this topic in the UK industry, its actually part of the vets responsibilities in other countries. With our RCVS degree we can practice in Europe, and it's important that we understand that the role of vets in other countries is aimed a lot more towards the food industry and food safety.

So on with trot...

Food processing started as far back as 12,000BC with the drying of food in the middle east. Some ancient methods of meat preservation are still used today. 

Meat preservation:

• Cooking, drying, smoking, salting, fermentation (salami and continental sausage), curing & freezing. 
• Food Irradiation- treating food with a certain dose of radiation to prevent spoilage. 
• Use of a modified atmosphere in the packaging- usually carbon dioxide or nitrogen which also lowers PH and inhibits the growth of bacteria. Carbon monoxide is used for keeping the red colour of the meat. 
• Special packing or canning. 
• A combination of the above. 

Public health approach includes the characteristics of the original product and aspects of the product that may represent hazards to health. This may include biological agents and their toxins, environmental effects and human interventions. Establishing standards that and procedures that safeguard health and create a level playing field in the trade. 

The Microbial Growth Curve

Meat is frozen or chilled when organisms are in the lag phase. As soon as the material is exposed to air the organisms enter exponential growth phase. 

Organisms That Affect Food

• Spoilage microorganisms- Clostridia, bacilli, pseudomonas etc... 
• Food Poisoning Microorganisms- Salmonella, Campylobacter, E.coli, certain viruses, fungi etc.. 
• Probiotics and fermentation agents. 

Principles of Food Microbiology

Factors which affect microbial growth in food can be split into the following categories:

• Intrinsic Factors- the physiochemical properties of food.
• Extrinsic Factors- the storage environment.
• Implicit Factors- properties and interactions of micro-organisms. 

Processing factors can modify all or some of the above factors. 

Intrinsic Factors

1. Nutrient Content- are the available nutrients suitable for growth of the micro-organism. A high sugar content e.g. the addition of fruit in yoghurt increases the risk of fungal growth. Different micro-organisms have different enzymes which allow them to grow on different media therefore knowledge of food composition can help predict the micro-organisms to control. 
2. PH & Buffer Capacity- PH is defined as 'the negative logarithm of the hydrogen ion capacity.' PH affects the transmembrane transport of nutrients & ATP synthesis. It also affects the stability of enzymes and consequently the growth and metabolism of micro-organism growth. Different micro-organisms favour different PH- bacteria favours PH 6-8, yeast favours PH 4.5-6 and filamentous fungi favours PH 3.5-4. Different food commodities has different PH e.g. muscle is PH 5.6 whilst egg white is PH 9.2. 
3. Redox Potential- ' the tendency of a medium to accept or donate electrons to oxidise or reduce.' If the sample gives electrons to the electrode then the redox potential is positive, if electrode gives to the the sample then the redox potential is negative. Different kinds of micro-organisms favour different redox potentials. Obligate aerobes need a high redox potential- positive. Obligate anaerobes grow in a low or negative redox potential but depends on the presence of oxygen and the ability to destroy superoxides. Different food commodities have a different redox potential. Meat @ PH5.6= -200. 
4. Water Activity- ' the ratio of the partial pressure of water in the atmosphere in equilibrium with the substrate.' Different food commodities have different water activities e.g. fresh meat is 1. Honey is 0.5. Different micro-organisms like different water potentials depending on how they can adjust to different pressures. Most gram +ve bacteria= 0.9. Most gram -ve= 0.97. Most yeasts= 0.88. 



Cheese rind= outer coating of cheese. 

5. Antimicrobial Barriers- egg white proteins (ovotransferrin), milk proteins (lactoferrin & lysozyme), egg shell and cuticle, and processing can introduce factors such as cheese rind, preservatives and packaging. 

Environmental Factors

• Relative humidity- affects water activity. Affects surface of food. White spots by sporotrichum carnis which affects the surface of meat. Black spots by cladosporium herbarum on imported chilled carcasses often accompanied by decomposition. Penicillium gives blue-green mould. 
• Temperature- membrane integrity affected by high temperatures. Nutrient availability affected in low temperatures. Psychrophiles bacteria is the most important for fresh meat with an 12 to 15 degree optimum temperature. Mesophiles (30-40 degree optimum) and psychrotrophs (25 to 30 degree optimum) the most important in food preservation. 
• Gaseous Atmosphere- oxygen concentration affects the redox potential. Carbon dioxide and nitrogen used for modified atmosphere packaging. Affects PH by producing carbonic acid with water which reduces surface contamination. This also acts as a weak organic acid which penetrates membranes affecting nutrient transport and enzymes availability. 

Implicit Factors

• Specific growth rate- bacteria grow faster than mould in fresh meat. 
• Mutualism- growth of one organism helps another. 
• Antagonism- lactic acid fermentation restricts bacterial growth of pathogens.
• Commensalism- some micro-organisms are ubiquitous. 

Meat Processing

Meat is defined as all parts of the animal suitable for human consumption. Meat is 75% water, 19% protein, 2.5% lipids and so forth. In post mortem meat changes the PH declines due to anaerobic metabolism which converts glycogen to lactic acid. Tenderising meat involves enzymes, mechanical or electrical methods. Temperature control of post mortem changes is most common with 4 degrees for white meat and 7 degrees for red meat. Some accidents include cold shortening where in tough meat muscle proteins have frozen in a shortened position. Depends on the rate of temperature reduction and which muscles are stretched. 

Raw Meat Processing 

• Cutting increases surface exposure whereas mincing causes a change in redox potential of the meat. Aerobes need a high redox potential which minced meat possesses (+225) compared to the negative redox potential seen for fresh meat earlier. Mince has <1% salt. 
• Licensed premises must apply for the health mark for wholesale production. Local Authority supervision for retail.
• For meat that has the health mark you would check for initial contamination & temperature control. When in storage check the relative humidity, PH, temperature and contamination. 

Further Processing 

Curing- 2-6% salt. Nitrite <200mg/Kg. Salt decreases water potential. May interact with cell components responsible for electron transport. Generates N-nitrosamines which are carcinogenic. Gives a nice red pink colour when cooked. Nitrosmyoglobin + T= nitrosylhaemochrome. Happens by either dry curing or curing by injection. 

Fermentation- lactic acid producing bacteria. Works by PH control and antagonism. Cheese, yoghurt, salami and fermented sausages. Salt and drying decreases water potential, PH, antagonism, and possible pasteurisation which uses a whole combination of factors. 

Smoking- cold @ 30 degrees, warm @ 60 degrees or hot @ 80 degrees. Can be hardwood smoke or liquid. Drying, phenols & formaldehyde and temperature. 

Appertisation- heat treatment is a hermetically sealed container. Inactivates certain micro-organisms for long enough to increase shelf life. The treatment depends on other characteristics of the contents. D= time at a given temperature which reduces the population by 90% or 1 log. C.botulinum is a major concern. Minimum accepted treatment is 12 days. 

Other packing methods:

• Vac pac- exclusion of air from packaging. Shelf life of 3 months for beef. Requirements= low bacterial level, hygenic handling, PH <5.8 and temperature below 2 degrees for clostridium botulinum.
• Modified atmosphere. Red meat= 60-80% oxygen and 20 to 40% carbon dioxide which is bacteriostatic and fungiostatic. Cured meat & poultry- 75-80% N2. 

And that's it for the wonderful world of food packaging and preparation...