Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.
Food can deteriorate as a result of two main factors:
1) growth of micro-organisms - usually from surface contamination - especially important in processed food - see below
2) action of enzymes - from within cells - part of normal life processes, (responsible for respiration, for instance). It is important to note that many plants - fresh vegetables and fruit - are still alive when bought and even when eaten raw, and meat from animals undergoes gradual chemical changes after slaughter.
Micro-organisms are sometimes also called microbes (or the old-fashioned word "germs"), and include the following groups:
bacteria and fungi (including yeasts)
Various members of these groups may cause changes in the character of food, which may be classed as "positive" or "negative".
Products of "positive" microbial transformations include cheese, yoghurt, and wine, which can be seen as increasing the nutritional value or keeping quality of products with a short shelf life (see later topics).
"Negative" aspects of microbial growth include food deterioration and spoilage by decay, and food poisoning, mainly caused by different and less widespread bacteria. As they grow, micro-organisms release their own enzymes into the liquid surrounding them, and absorb the products of external digestion. This is the main basis of microbial food spoilage, which lowers its nutritional value. Bacteria and moulds may also produce waste products which act as poisons or toxins, thus causing the renowned ill-effects.
It is perhaps worth noting that, viewed from another angle, bacteria and fungi have a major positive role in the recycling of all forms of biological waste in nature, and that yet other bacteria (together with viruses) are the main agents of infectious diseases in plants and animals, including Man.
If milk goes off in 1 day on a warm summer day (24 °C), how long will it last in a fridge?
> 4 days ( 10 °C warmer means 2x faster, 20 °C warmer means 2x2 faster )
In a freezer (about -15 °C) bacteria are completely inactive, but internal enzymes are still active. For this reason, frozen vegetables such as peas are blanched by treatment with boiling water before freezing.
Give 2 examples of food that is adversely affected by freezing. Think about the effects on cell structure!
> celery etc > lettuce and other 'crunchy' vegetables
The cooling process may also have the effect of removing water from food, so it is necessary to wrap it to prevent dehydration.
Where does the water go from unwrapped food, in a fridge or freezer?
> it condenses, solidifies and freezes to the side of the freezer or cooling panel
Increased temperatures can have a more permanent preservative effect, and only require a fairly brief treatment. They may also alter the flavour of food.
Gentle heating (about 60-70 °C, up to boiling, 100 °C) kills most bacterial cells in a few minutes, but does not affect some species which form spores. It also denatures proteins, so it deactivates enzymes. Most cooking does this, and pasteurisation of milk is carefully controlled (63-66 °C for 30 minutes) to prevent flavour changes, although it does not quite kill all contaminating bacteria.
After heat treatment, it is essential to ensure that foods cannot become contaminated by contact with raw food, because they are now more easily colonised by bacteria.
List some food handling precautions to achieve this.
> keep on separate shelf of fridge (cooked above raw)
food shops must have separate storage units
> use separate knives for raw and cooked food etc
Salted foods, and foods with added sugar are also effectively using the same technique, since the water they contain is unavailable for microbial growth. Indeed, cells of micro-organisms become plasmolysed when they come into contact with the surfaces of these foods.
What happens to the cytoplasm of any such micro-organisms? (Clue: think about osmosis!)
> loses water: so micro-organism dies
Smoking foods, as well as drying, covers the outside (most exposed to microbial contamination) with a thin film of antimicrobial chemicals. Some people even like the taste!
Give some examples of vacuum-packed foods, and say what other preservation methods are combined with it.
> bacon (salting/smoking) > cheese (salt/low pH :see below)
Canning is heat-treatment in an autoclave, together with sealing of the food in an air-tight container.
List and explain some disadvantages of the process.
> Heavy > Metal may contaminate > Needs can-opener!
Various labelling regulations have been put into effect to ensure that contents of (processed) foods are known to consumers, and to ensure that food is fresh - important in unprocessed foods and probably important even if preservatives are used.
E numbers are a short method of declaring some common food additives (not actual ingredients of food as such), which have been cleared as probably harmless to most people by EEC bureaucrats. However, some authorities disagree about their methods, and say that some individuals may be susceptible to ill effects. Others distrust the system, and believe it is used to mislead. It is not universal, and other names can be substituted. Terms like natural are certainly used in a cynical way by some manufacturers. The same system covers both artificial (synthetic) and natural substances, which are not by definition without risk!
Flavourings are not covered by the system, either.
Food additives fall into several categories:
Try to find a food item containing an example of each, with its name &/ E number, and any further information.
(E nos in range 200- )
(E nos in range 100- )
n.b. some are "natural", e.g.>
Many are synthetic dyes derived originally from coal tar - "azo dyes" (originally recommended because they are so stable)
Emulsifiers and anti-oxidants
(E 300- )
List some advantages, and some disadvantages
of additives in food.