PRESERVATIVES
Food for you is invariably also food for countless other life forms, from bacteria and yeasts, to fungi and insects to rodents and larger mammals. And even enzymes and oxidation continue the process of decay.
Unless you take steps to exclude competitors and slow down decomposition, you will find that not much food is left for you. Early attempts of slowing down decay included drying in the sun and wind, adding salt (both common salt and various rock salts), adding acids, sugar, smoke, and alcohol. Many traditional techniques were combinations of these methods, together with excluding air and further contamination.
The objective of course, was to be able to access nutrition long after the harvest or the hunt, and in the depths of winter or during a famine, or on a long sea voyage.
Canning combined heat-treatment with hermetically-sealing in a robust metal or glass container, while the relatively recent invention of the refrigerator/freezer enabled storage of most foods for longer periods. Modern developments (e.g. freeze-drying) enable storage without a power supply.
Most of these methods involve the principle of making water unavailable to the microbe: lowering the "water activity" (aW). Freezing makes water unavailable, as does adding salts and other solutes such as sugar. .
Why are nitrates bad in meat but appear to be good in fruits or vegetables?
Nitrites (in combination with nitrates that become reduced to nitrites) have been added to meat products such as bacon, hams, and salamis for ages. They inhibited some of the bacterial growth (long before anyone thought of refrigerators), and added a distinctive taste. In recent years we have found that the amino acids in the protein can combine with nitrites to create a family of nitroso compounds, mainly nitrosamines, many of which have an association with cancer. The nitrosamines can form in the cooking process (that’s why bacon has a safer formulation for than ham and salami that are not heated to the same extent), or in the stomach.
So what about plants? Plants have nitrates naturally occurring in their structure, and often the concentration of nitrates from plant foods far exceeds that in meats. Yet this source of nitrate does not appear to be associated with tumor formation. The reason now appears that in the presence of ascorbic acid, the formation of nitrosamines is inhibited. Lettuce, green leafy vegetables, tomatoes, and citrus fruits all contain ascorbic acid, and these have a protective action against nitrosamine formation.
The take-home message? … Always include some ascorbic acid when you eat nitrite-preserved meats; Orange juice with your breakfast bacon, lots of lettuce with your ham sandwich, tomato or sauerkraut with your salami on rye, or a vit-C tab at the same time as any of these. Oh, and it's best not to over-cook bacon.
As a postscript, you may have seen packages of cold-cuts with the declaration that no nitrates have been added. Look more carefully, and you will see "celery extract" among the ingredients. That's where the nitrates have been added, so as not to alarm anyone who prefers not to see anything with a name resembling a chemical.