Food spoilage

Microbial spoilage

Micro-organisms not only discolour food, rot it, and make it offensive to smell and eat - they can also present serious public health hazards.

Chemical and bio-chemical spoilage

When animal or vegetable material is removed from its natural source of energy and nutrient supply, chemical changes begin to occur which lead to deterioration in its structure. These changes can be slowed using MAP techniques. For example, unsaturated fats and oils tend to combine with oxygen in the atmosphere. In some fatty foods, this oxidation can lead to rancidity development - a process that can be slowed with good effect if the food is packaged in a low oxygen atmosphere.

Chemical and bio-chemical spoilage

When animal or vegetable material is removed from its natural source of energy and nutrient supply, chemical changes begin to occur which lead to deterioration in its structure. These changes can be slowed using MAP techniques. For example, unsaturated fats and oils tend to combine with oxygen in the atmosphere. In some fatty foods, this oxidation can lead to rancidity development - a process that can be slowed with good effect if the food is packaged in a low oxygen atmosphere.

Colour change (red to brown)

The colour cycle in fresh meats is reversible and dynamic, with the three pigments, oxymyoglobin (red), myoglobin (purple) and metmyoglobin (brown) being constantly interconverted. Brown metmyoglobin, the oxidised or ferric form of the pigment, cannot bind oxygen even though it is oxidised by the same oxygen which converts the purple myoglobin to red oxymyoglobin (oxygenation). Therefore in the presence of oxygen, the purple myoglobin may be oxygenated to the bright red oxygenated pigment oxymyoglobin, producing the familiar "bloom" of fresh meats, or it may be oxidised to metmyoglobin, producing the undesirable brown colour of less acceptable fresh meats. Under high oxygen MAP, the conversion of myoglobin to oxymyoglobin is favoured and the desirable red colour of fresh meats is maintained for a longer period. Under low oxygen MAP, the reduced myoglobin is oxidised to the undesirable brown metmyoglobin pigment.

Oxidative rancidity

Fats can become rancid as a consequence of oxidation and such oxidative rancidity is a major cause of food spoilage. The reaction of oxygen with unsaturated fatty acids in fatty foods constitutes the major means by which fats or fat-containing foods deteriorate. Oxidation of unsaturated fat is frequently alluded to as autoxidation since the rate of oxidation increases as the reaction proceeds. Hydroperoxides are the predominant initial reaction products of fatty acids with oxygen. Subsequent reactions control both the rate of reaction and the nature of the products formed. Some of these products, such as hydroxy acids, keto acids and aldehydes, are largely responsible for the off-flavours and off-odours characteristic of stale or rancid foods.

Colour change for cured meats (red/pink to brown/grey/green)

The red colour of raw cured meat products is due to the presence of nitrosylmyoglobin which is formed from the reaction of purple myoglobin with nitric oxide. The source of nitric oxide in meat curing is usually sodium nitrite. During heating, red nitrosylmyoglobin is converted to pink denatured nitrosylmyoglobin.

Green discoloration of cured meats can be caused by too high a concentration of nitrite, especially at low pH which can result in the formation of green nitrihaemin or "nitrite burn".

Oxidative warmed-over flavour

Oxidative warmed-over flavour is a characteristic off-flavour primarily associated with cooked meats and poultry in chilled ready meals and other cook-chill products. In cooked meats and poultry held at chilled storage temperatures, this stale, oxidised flavour becomes apparent within a short time (48 hours) particularly if the product is stored under air. MA packaging under low oxygen levels helps to delay the on-set of oxidative warmed-over flavour.

Colour change for green pasta (green to brown/grey)

The undesirable colour change of green pasta to brown/grey shades is due to the deterioration of the green chlorophyll pigments to dull olive/brown/grey pheophytin pigments. This deterioration of the chlorophyll pigments is an oxidative process which is accelerated by light. Hence, the photo oxidation of chlorophyll and loss of desirable green colour can be significantly reduced by MAP under low oxygen levels in opaque packages.

Enzymic browning or discoloration

In fruit and vegetables, enzymic browning or discoloration occurs due to damage such as bruising and preparation procedures such as cutting, peeling and slicing. The yellowish brown through to black pigments that are formed can appear very rapidly and are undesirable. In intact fruit and vegetables, the polyphenolic oxidase enzymes responsible for this discoloration are separated from their polyphenolic substrates. However, when they are brought into contact as a result of bruising or preparation procedures, naturally occurring polyphenolic substrates are enzymically oxidised to form colorless/yellowish compounds which subsequently polymerise to form brownish melanin-type compounds. The extent of browning or discoloration is dependent on the amount and activity of polyphenol oxidase present in the specific fruit or vegetable, the availability of polyphenolic substrates and the presence of oxygen. Hence, enzymic browning or discoloration can be inhibited by MAP under low oxygen levels, restricting oxygen diffusion into tissues by immersion in water, brine or syrup, addition of enzyme inhibitors such as sulphite or addition of acids such as citric acid, malic acid or ascorbic acid which also inhibit the activity of polyphenol oxidase.

Oxidative off-flavours

Oxidative off-flavours and off-odours can be caused by numerous oxidative reactions in food and drink products. Meats, fish, poultry, liquid food, beverage and dairy products for example are highly susceptible to oxidative processes which can initiate a sequence of reactions resulting in rapid flavour impairment. Oxidative enzymes such as lipoxygenase, proteases and lipases can cause a wide range of off-flavours and off-odours in many food products. Hence, MAP under low oxygen levels is beneficial in inhibiting undesirable oxidative off-flavours and off-odours.

Physical spoilage

• Staling of bakery products and components
• Moisture migration between different components
• Physical separation
• Moisture loss or gain

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