Microbes and food 1. Menu - you are what you eat
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Pigs on a farm
Picture 1.3a Ham comes from pigs.
1.3 Ham
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What is it?
Ham is a type of cured meat made from pork. The raw meat is treated with curing salts such as sodium nitrate, sodium nitrite and sodium chloride, which gives ham its characteristic flavour and pink colour and prolongs the storage life of the product. Examples of other cured meats are corned beef and bacon. Most cured ham is cooked, but products like the Italian Parma Ham are raw and made by a different process.
How is it produced?
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Traditionally when a pig was killed, the carcass had to last the family for many months. Raw meat quickly goes off as bacteria from the skin and guts break down the sugars in the meat before attacking the proteins, leading to slime production and a putrid smell. People discovered that by rubbing the carcass with saltpetre and keeping it in an airy place (once the gut, lungs and other soft tissues had been removed, of course) the meat stayed edible for a lot longer, as well as tasting good.

What happened during this process was that the sodium nitrate in the saltpetre became converted to nitrite by the natural flora of microbes present, mainly a type of bacterium called Micrococcus. Nitrite acts as a preservative and inhibits many different bacteria, including food poisoning organisms. Nowadays nitrates have been replaced by nitrites and dry cures are only used for specialist products.

Ham on a plate photo
Picture 1.3b A plate of sliced ham.

Modern cured ham is usually made by injecting curing brines into whole muscles, or by soaking cut-up meat in a vat of brine. The brines contain sodium nitrite, salt and sometimes other preservatives such as ascorbate. The salt reduces the water activity, which slows down the rate at which microbes grow. Other ingredients include sugar, spices and phosphate which contribute to the flavour and juiciness of the product.

Once the cure is complete the ham might be smoked before it is cooked. Smoking over burning wood chips not only adds flavour to the ham but also helps the product to keep longer due to the antimicrobial activity of components of the smoke. Hams are usually cooked by boiling or by steam under pressure, before being cooled in cold water or in the air and then packaged. Some hams are packed whole, others are sliced first. Such ham is kept refrigerated until the point of sale. Some hams are canned, which means they receive a thorough heat treatment in the container which destroys all microbes including any spores. These can be kept at room temperature provided the seal is unbroken.

Air-cured hams like Prosciutto di Parma from Italy and Serrano from Spain are made by traditional processes and are a speciality of mountain regions. The pigs are often specially bred and even their treatment before slaughter is carefully controlled. The hind legs of the carcass are removed, chilled and trimmed, and then covered in salt for 2-3 weeks. This draws off water to levels at which microbes cannot grow. The salt is then removed and the hams are rested before any remaining salt is washed off and they are hung up to dry in a carefully controlled atmosphere. This pre-curing stage takes about 3 months. For the final maturing or curing period of up to 18 months, the hams are moved to special drying rooms where the characteristic flavours and aromas develop. In Italy the whole process is regulated by law and only the product from a certain area can be sold as Parma ham.

Air-cured hams are either sold whole or are sliced and packaged in hygienically controlled conditions to prevent contamination by micro-organisms.

Mouldy bacon photo
Picture 1.3c This mouldy ham doesn't look very appetising.

How does it spoil?
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Curing does not eliminate all the micro-organisms in ham. The curing process is quite complex and the mixture of curing salts and other preservatives, coupled with factors such as pH, the amount of oxygen available, the type of packaging and storage temperature, affects which microbes survive. For example, the inhibitory effect of nitrite increases with decreasing pH. Another complication is that microbes are spread right through the product during cure injection and processing. More organisms can get in during slicing and packaging. Ham is a very perishable food. Most spoilage is due to lactic acid bacteria as nitrite is not very effective against them, leading to greening, sour odours and flavours and sometimes gas production. Dry cured hams can be spoiled by moulds. It is important to keep ham in the fridge and eat it quickly after opening the packet.


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Can it be harmful?
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Food poisoning bacteria can be in the raw pork, so it is important to get the curing mix and other processing conditions right. Cooking should destroy these microbes, other than spores, but the ham can also become contaminated during processing and packaging, both from the environment and food handlers. Strict attention to hygiene and good manufacturing practice is essential.

Nitrite is effective against most food poisoning bacteria, including Clostridium botulinum and its spores (which might survive the cooking stage), although Salmonella is more resistant. There have been many cases of Staphylococcus aureus food poisoning from eating cooked cured ham, usually due to contamination of the food by unhygienic handling during slicing and failure to keep it in the fridge e.g. at a picnic. Getting the right balance of nitrite, salt, pH, anaerobic conditions and low temperature is crucial in preventing the growth of this particular type of bacteria.