Freezing and cooling by use of natural ice and snow is a method of food preservation traditionally available only in cold climates or in winter in temperate climates. Natural ice refrigeration on an industrial scale first developed in the late 19^th century, when refrigerated containers used in trains, ships, and then later trucks, greatly increased the production and consumption of red meat. Domestic freezing, chilling, and refrigeration on a mass scale is a phenomenon mostly of the second half of the 20^th century.

Today, much perishable food is solid frozen or chilled. Together with the growth of industrial refrigeration, domestic refrigerators began to be used in the USA, Australia, and New Zealand on the scale in the 1920s, and the Europe and Japan mostly since the 1950s. In Japan, for example, household processing refrigerators increased from 9 per cent in 1960 to 91 per cent in 1970, and 99 per cent in 2004. Supermarkets with freezers, chill cabinets, and domestic refrigerators are now commonable in the cities and towns of tropical countries; poorer rural communities still rely on drying, fermenting, salting, bottling, tinning, and other methods of food preservation, as well as their own gardens and farms. It is unlikely that refrigeration itself has any direct effect on the risk of cancer. Their effects are indirect.

• Refrigeration enables consumption of fresh perishable foods including seasonal vegetables and fruits all year round, as well as of fresh meat.
• Refrigeration reduces microbial and fungal contamination of perishable foods, notably cereals (grains) and pulses (legumes).
• Refrigeration reduces the need for and use of salting, smoking, curing, and pickling as methods of preserving vegetables, fruits and meat.

It can therefore be said that refrigeration (including freezing and chilling) indirectly influences risk of those cancers, the risk of which is affected by the above factors.

Evidence mounting to a judgement of ‘convincing’ or ‘probable’ for such factors relates to cancers of the mouth, pharynx, larynx, masopharynx, oesophagus, lung, stomach, pancreas, liver, and colon.

Reference: Reports from World Cancer Research Fund and American Institute for Cancer Research

  Mycotoxins are toxins produced by certain moulds or fungi. Although moulds that contaminate foods are usually destroyed by cooking temperatures, the toxins they produce may remain. Aflatoxins are one type of mycotoxin. All naturally occurring aflatoxins are classified as human carcinogens (group 1) by International Agency for Research on Cancer; other mycotoxins, such as fumonisins, are suspected carcinogens. It is common to find co-contamination by more than one species of mycotoxin-producing fungus. In Europe, the joint FAO/WHO expert committee on Food Additives and Contaminants recommends that aflatoxin concentrations in foods be kept as low as possible.

The main foods that may be contaminated by aflatoxins are all types of cereal (grain), including wheat, rice, maize (corn), barley, and oats; and pulses (legumes)-notably peanuts. Nuts and seeds may also be contaminated. Feedstuffs for farm animals may also be contaminated with aflatoxins, which can then be secreted in milk or accumulated in tissues.

Aflatoxins, which are produced by Aspergillus flavus and A. parasiticus, are most problematic in countries with hot, damp climates and poor storage facilities. Under these conditions, food may become contaminated with fungi and then accumulate such toxins. Such foods are marketed and consumed in the countries in which they are produced; they are also exported to neighboring countries and intercontinentally. Aflatoxin contamination is therefore an international issue.

Levels of aflatoxin contamination tend to be highest in countries where rates of liver cancer are high, such as some African countries and South-East Asia, including China. In general, rates are low in Europe, but relatively high rates of contamination have on occasion been found in the USA. Aflatoxin exposure levels are low in Europe and Australia, higher in the USA, and high in many low-income countries. This is particularly the case in tropical and subtropical regions where grains and nuts are stored for long periods under non-ideal conditions.

Rates are reduced by inspection, use of fungicides, and screening of imported foods. However, monitoring of levels of aflatoxin contamination in low-income countries is generally lacking.

Hepatitis B and hepatitis C viruses are causes of liver cancer. The former appears to act directly by damaging cells and their DNA. The latter shows an indirect effect, mediated by cirrhosis. For both, there is potential for nutrition status to have an effect at several stages: susceptibility to and duration of infection, liver damage, DNA damage, and cancer progression (Refer Figure 2 in Nutrition and Cancer).

Around 7-8 per cent of the world’s population is estimated to be infected with hepatitis B virus. It is mostly spread by blood and sexual transmission. In endemic areas, the carrier rate may be 10-20 per cent. It is often acquired at birth or in childhood, and is endemic in areas of Africa and Asia. Chronic hepatitis B virus carriers have a 100-fold greater chance of developing liver cancer than non-carriers. Those infected in adulthood have a lower risk of this cancer than those infected in childhood because there is less time for virus to cause inflammation. Vaccination against hepatitis B virus has been shown to reduce the prevalence of liver cancer by 60 per cent.

Liver cancer in hepatitis B virus carriers is not necessarily connected with cirrhosis: up to 40 per cent of associated liver cancer cases are non-cirrhotic. Hepatitis B virus carries its genetic code as DNA rather than RNA. Viral DNA can insert itself into liver cells and alter their DNA.

Around 3 per cent of the world’s population is estimated to be infected with hepatitis C virus. It is more prevalent in high income countries. Approximately 80 per cent of these infections become chronic, of which 15-20 per cent develops into cirrhosis. Of those, 1-4 per cent develops into liver cancer each year. Interruption of the sequence of chronic hepatitis developing into cirrhosis prevents liver cancer. Also, there is an interaction between hepatitis C virus infection, liver cancer risk, and consumption of alcoholic drinks. There is no vaccine against hepatitis C. It is mostly spread by blood.