Blanching is a short heat treatment, in hot water or in steam, applied to a vegetable
immediately before it is frozen. It is followed by rapid cooling, and the cooling is
part of the step rather than an afterthought.
It exists mainly to inactivate enzymes. Vegetables carry enzymes – peroxidase,
catalase, lipoxygenase, polyphenol oxidase – that keep working slowly below zero.
Freezing slows them; it does not stop them. Left active, they produce the grassy,
hay-like off-flavours and the dull olive colour that develop in an unblanched frozen
vegetable over months of storage. The product does not spoil microbiologically, it
simply becomes unpleasant, and it does so uniformly across the whole batch.
Blanching also drives intercellular air out of the tissue, which brightens the colour
and reduces oxidation, and it knocks down the surface microbial load. It is not a
pasteurisation step and should never be specified as one.
The compromise is nutrient loss. Water-soluble vitamins leach into the blanch water,
and the loss rises with time, temperature and water-to-product ratio. Steam blanching
leaches less than water blanching and costs more to install and run.
Most fruit is not blanched, because it is eaten raw, has a different enzyme profile and
would be damaged texturally by the heat.