Protection of Foods by Drying
447
STORAGE STABILITY OF DRIED FOODS
In the absence of fungal growth, desiccated foods are subject to certain chemical changes that may
result in the food’s becoming undesirable upon holding. In dried foods that contain fats and oxygen,
oxidative rancidity is a common form of chemical spoilage. Foods that contain reducing sugars undergo
a color change known as Maillard reaction or nonenzymic browning. This process is brought about
when the carbonyl groups of reducing sugars react with amino groups of proteins and amino acids,
followed by a series of other more complicated reactions. Maillard-type browning is quite undesirable
in fruits and vegetables not only because of the unnatural color, but also because of the bitter taste
imparted to susceptible foods. Freeze-dried foods also undergo browning if the moisture content is
about 2%, thus the moisture content should be held below 2%.
With regard to a w , the maximal browning reaction rates in fruits and vegetable products occur in
the 0.65–0.75 range, whereas for nonfat dry milk browning, it seems to occur most readily at about
0.70
36 Other chemical changes that take place in dried foods include a loss of vitamin C in vegetables,
general discolorations, structural changes leading to the inability of the dried product to rehydrate
fully, and toughness in the rehydrated, cooked product.
Conditions that favor one or more of the above changes in dried foods generally tend to favor all,
so preventive measures against one are also effective against others to varying degrees. At least four
methods of minimizing chemical changes in dried foods have been offered:
1. Keep the moisture content as low as possible. Gooding
14 has pointed out that lowering the
moisture content of cabbage from 5% to 3% doubles its storage life at 37
◦ C.
2. Reduce the level of reducing sugars as low as possible. These compounds are directly involved
in nonenzymic browning, and their reduction has been shown to increase storage stability.
3. When blanching, use water in which the level of leached soluble solids is kept low. Gooding
14
has shown that the serial blanching of vegetables in the same water increases the chances of
browning. The explanation given is that the various extracted solutes (presumably reducing
sugars and amino acids) are impregnated on the surface of the treated products at relatively high
levels.
4. Use sulfur dioxide. The treatment of vegetables prior to dehydration with this gas protects vitamin
C and retards the browning reaction. The precise mechanism of this gas in retarding the browning
reaction is not well understood, but it apparently does not block reducing groups of hexoses. It
has been suggested that it may act as a free-radical acceptor.
One of the most important considerations in preventing fungal spoilage of dried foods is the RH of
the storage environment. If improperly packed and stored under conditions of high RH, dried foods
will pick up moisture from the atmosphere until some degree of equilibrium has been established.
Because the first part of the dried product to gain moisture is the surface, spoilage is inevitable; surface
growth tends to be characteristic of molds due to their oxygen requirements.
INTERMEDIATE-MOISTURE FOODS
Intermediate-moisture foods (IMF) are characterized by a moisture content of around 15–50% and
an a w between 0.60 and 0.85. These foods are shelf-stable at ambient temperatures for varying periods
of time. Although impetus was given to this class of foods during the early 1960s with the development
and marketing of intermediate-moisture dog food, foods for human consumption that meet the basic
447
STORAGE STABILITY OF DRIED FOODS
In the absence of fungal growth, desiccated foods are subject to certain chemical changes that may
result in the food’s becoming undesirable upon holding. In dried foods that contain fats and oxygen,
oxidative rancidity is a common form of chemical spoilage. Foods that contain reducing sugars undergo
a color change known as Maillard reaction or nonenzymic browning. This process is brought about
when the carbonyl groups of reducing sugars react with amino groups of proteins and amino acids,
followed by a series of other more complicated reactions. Maillard-type browning is quite undesirable
in fruits and vegetables not only because of the unnatural color, but also because of the bitter taste
imparted to susceptible foods. Freeze-dried foods also undergo browning if the moisture content is
about 2%, thus the moisture content should be held below 2%.
With regard to a w , the maximal browning reaction rates in fruits and vegetable products occur in
the 0.65–0.75 range, whereas for nonfat dry milk browning, it seems to occur most readily at about
0.70
36 Other chemical changes that take place in dried foods include a loss of vitamin C in vegetables,
general discolorations, structural changes leading to the inability of the dried product to rehydrate
fully, and toughness in the rehydrated, cooked product.
Conditions that favor one or more of the above changes in dried foods generally tend to favor all,
so preventive measures against one are also effective against others to varying degrees. At least four
methods of minimizing chemical changes in dried foods have been offered:
1. Keep the moisture content as low as possible. Gooding
14 has pointed out that lowering the
moisture content of cabbage from 5% to 3% doubles its storage life at 37
◦ C.
2. Reduce the level of reducing sugars as low as possible. These compounds are directly involved
in nonenzymic browning, and their reduction has been shown to increase storage stability.
3. When blanching, use water in which the level of leached soluble solids is kept low. Gooding
14
has shown that the serial blanching of vegetables in the same water increases the chances of
browning. The explanation given is that the various extracted solutes (presumably reducing
sugars and amino acids) are impregnated on the surface of the treated products at relatively high
levels.
4. Use sulfur dioxide. The treatment of vegetables prior to dehydration with this gas protects vitamin
C and retards the browning reaction. The precise mechanism of this gas in retarding the browning
reaction is not well understood, but it apparently does not block reducing groups of hexoses. It
has been suggested that it may act as a free-radical acceptor.
One of the most important considerations in preventing fungal spoilage of dried foods is the RH of
the storage environment. If improperly packed and stored under conditions of high RH, dried foods
will pick up moisture from the atmosphere until some degree of equilibrium has been established.
Because the first part of the dried product to gain moisture is the surface, spoilage is inevitable; surface
growth tends to be characteristic of molds due to their oxygen requirements.
INTERMEDIATE-MOISTURE FOODS
Intermediate-moisture foods (IMF) are characterized by a moisture content of around 15–50% and
an a w between 0.60 and 0.85. These foods are shelf-stable at ambient temperatures for varying periods
of time. Although impetus was given to this class of foods during the early 1960s with the development
and marketing of intermediate-moisture dog food, foods for human consumption that meet the basic