252 Produce Degradation: Reaction Pathways and their Prevention
and controlled solar drying. Sun drying involves exposing produce to direct sunlight
and heat, while with controlled solar drying the produce is exposed to elevated
temperatures (e.g., 40 to 60°C) but is protected from direct sunlight. Light can cause
degradation of ultraviolet light-sensitive vitamins, including riboflavin, carotene, and
thiamin in foods that are dried in the open sun. Controlled solar drying has been
associated with a greater retention of vitamin C, riboflavin, thiamin, α-carotene,
β-carotene, and total carotenoids relative to sun drying. It also results in a product
of superior sensory quality (Mosha et al., 1995, 1997). The most common uncon-
ventional drying method is freeze-drying. This process involves freezing the
blanched product and removing the moisture rapidly under vacuum by sublimation.
Dehydration is achieved at a low temperature because of the vacuum condition, and
this reduces degradation of the thermolabile nutrients. Since the product is dried
without thawing, the losses of nutrients and flavor associated with thawing and
heating are minimized.
Oxidation is the primary cause of nutrient losses during dehydration and drying.
This can be reduced by drying fruits and vegetables in a vacuum, or by flushing the
product with nitrogen gas and shortening the dehydration time without increasing
the temperature. Freeze-drying of fruits and vegetables, which is carried out in the
absence of oxygen, does not result in appreciable loss of vitamin C (IFT, 1986).
Dehydration of broccoli resulted in a 40% loss of thiamin but insignificant losses
of riboflavin, niacin, and pantothenic acid (Salunkhe, 1974). Dehydration of fruits
and vegetables with high fat content yields more unstable products. The oil in the
produce, if present (e.g., in olives), may undergo oxidative rancidity, leading to off-
flavors during storage. Blanching of fruits and vegetables prior to dehydration is
important because it helps to inactivate the enzymes. During dehydration of cabbage,
ascorbic acid is oxidized to dihydroascorbic acid and 2,3-diketogulonic acid, which
in turn reacts nonenzymatically with free amino acids to form a red-brown compound
that causes discoloration in the dried product. During this process, carbonyl groups
are also released that impart an objectionable flavor. Treating the produce with some
chemicals prior to drying may enhance retention of some thermolabile nutrients. For
example, soaking cabbage in 1% sodium sulfite for 30 sec prior to drying signifi-
cantly improved retention of ascorbic acid. Similarly, soaking broccoli and brussels
sprouts in sodium chloride solution prior to dehydration enhanced the retention of
ascorbic acid and carotene considerably (Salunkhe, 1974).
8.10.4 FERMENTATION
From the Roman times, people knew how to preserve vegetables for long periods
without the use of freezers or canning machines. This was done through the process
of fermentation. There are three forms of fermentation: lactic acid, acetic acid, and
alkaline fermentation.
8.10.4.1 Lactic Acid Fermentation
Lactic acid fermentation is carried out by lactic acid bacteria, which are Gram-
positive, nonrespiring, non-spore-forming, and coccoid in structure (Teitelbaum and