Food may be preserved by cooking in a material
that solidifies to form a gel. Such materials include gelatin,
flour and arrowroot flour. Fruit preserved by
jellying is known as jelly, marmalade, or fruit preserves.
In this case, the jellying agent is usually pectin,
either added during cooking or arising naturally from the fruit.
Irradiation of food is the exposure of food
to ionizing radiation; either high-energy electrons or X-rays
from accelerators, or by gamma rays. The treatment
has a range of effects, including killing bacteria, molds, insects, pests,
reducing the ripening and spoiling of fruits, and at higher doses inducing
sterility. The technology may be compared to pasteurization; it is sometimes
called 'cold pasteurization', as the product is not heated. Irradiation is not
effective against viruses; it cannot eliminate toxins already formed by
microorganisms, and is only useful for food of high initial quality.
atmosphere is a way to preserve food by operating on the atmosphere
around it. Salad crops which are notoriously difficult to preserve are now
being packaged in sealed bags with an atmosphere modified to reduce the oxygen
(O2) concentration and increase the carbon dioxide
(CO2) concentration. There is concern that although salad vegetables
retain their appearance and texture in such conditions, this method of
preservation may not retain nutrients, especially vitamins. Nitrogen gas (N2)
at concentrations of 98% or higher is also used effectively to kill insects. However,
carbon dioxide has an advantage in this respect as it kills organisms requiring
concentrations of only 80%.