Nutrient Loss 251
enzyme activity, and growth and proliferation of microorganisms. Fresh fruits and
vegetables are usually packaged in permeable polyethylene bags to minimize evap-
oration and control relative humidity. Refrigerated produce also may have received
other treatments such as blanching, irradiation, waxing, and disinfection by chem-
icals and controlled-atmosphere packaging. Low temperature treatments have been
shown to be effective in retaining the thermolabile nutrients in fruits and vegetables.
For example, ascorbic acid, folate, and carotene in broccoli, cabbage, cauliflower,
kale, collard and turnip greens, radishes, and kohlrabi were destroyed rapidly at
room temperature but significantly retained under refrigeration (Salunkhe, 1974). A
sample of brussels sprouts stored at 0°C for 72 d showed lower respiration rate and
maintained a satisfactory organoleptic quality compared to that stored at 14 to 20°C,
which decolorized and decayed (Salunkhe, 1974).
Minimally processed fruits and vegetables might be more perishable than
unprocessed produce during refrigeration because of the potential for subjection to
some physical stress. They also have a higher rate of respiration than unprocessed,
intact produce (Watada et al., 1996). Consequently, minimally processed fresh fruits
and vegetables should be held at lower temperatures than those recommended for
intact commodities. Although 0°C generally is the desirable temperature for most
minimally processed fruits and vegetables, many are prepared, shipped, and stored
at 5°C, and sometimes as high as 10°C. This is mainly due to concerns about chilling
injury, which occurs around 0°C. The deterioration of quality caused by enzyme
activity and infection by pathogens that occurs when a produce is stored outside the
chilling temperature is higher than it would have been if the produce had been stored
at chilling temperatures. It is strongly recommended, therefore, that chilling-sensi-
tive, minimally processed products be held at chilling temperatures at which injury
from chilling will be of less consequence than the deterioration that occurs at
nonchilling temperatures (Watada et al., 1996).
Freezing is a cooling process that involves storing the produce at temperatures
below 0°C. Fruits and vegetables are usually frozen after blanching. When frozen
without blanching, significant losses of ascorbic acid occur during storage (Salunkhe,
1974). Freezing generally does not cause significant nutrient losses if proper pack-
aging and processing conditions are observed. However, during thawing, losses may
occur. In strawberries and other berries, freezing without pretreatment practically
causes no change in the nutritive value (Beattle and Wade, 1996). A change in the
form of ascorbic acid has been reported during freeze storage of some produce.
Freeze storage of broccoli and cauliflower resulted in a slight oxidation of ascorbic
acid to dehydro-ascorbic and 2, 3-diketogulonic acids (Salunkhe, 1974).
8.10.3 DEHYDRATION
Dehydration is also an old processing method for fruits and vegetables that dates
back to ancient times. It involves exposing the produce to elevated temperatures to
reduce its water content, and hence its water activity. This helps to slow down the
rate of proliferation of lethal microorganisms and degradation by biochemical pro-
cesses (e.g., enzymic browning). Dehydration can be achieved by conventional or
unconventional methods. Conventional methods include sun drying, oven drying,