The quality of frozen products and consumer acceptance can be enhanced by optimizing process conditions such as rate of freezing, quality of raw materials, and storage conditions details (see previous chapters for more detail). However, important factors can be grouped in a less confusing way based on conclusions drawn from the aspects related to quality deterioration. These groups are sensory quality, including the physical and chemical aspects of quality deterioration, microbiological quality, and nutritive quality of frozen products.
The main components of the overall sensation of flavor are taste and aroma (Salunkhe, 1991). The receptors on the tongue are responsible of perceiving flavors, while aroma generally contributes to total flavor. The analysis used to determine the effects of freezing process, frozen storage, and thawing on product flavor is largely based on the changes produced in chemical compounds. Sensory quality of frozen products is commonly determined based on texture, which includes both the properties perceived by sensation in mouth and appearance (Skrede, 1996). Therefore, a good understanding of the physical aspects of freezing will help improve the product’s quality retention during the freezing process.
Moisture migration is the principal physical change occurring in frozen foods, affecting the physical, chemical, and biochemical properties, including texture and palatability of the food (Pham and Mawson, 1997).
Texture
Most fruits and vegetables are over 90 percent water of total weight. The water and dissolved solutes inside the rigid plant cell walls give support to the plant structure, and texture to the fruit or vegetable tissue. In the process of freezing, when water in the cells freezes, an expansion occurs and ice crystals cause the cell walls to rupture. Consequently, the texture of the produce is generally much softer after thawing when compared to non-frozen produce. This textural difference is especially noticeable in products normally consumed raw, as in the case of fruits. It is usually recommended that frozen fruits be served before they are completely thawed, since in the partially thawed state the effect of freezing on the fruit tissue is less noticeable. On the other hand, due to the fact cooking also softens cell walls, textural changes caused by freezing are not significantly noticeable in products cooked before eating, as in the case of most vegetables (Schafer and Munson, 1990).
Freezer burn
One of the most common forms of quality degradation due to moisture migration in frozen foods is freezer burn, a condition defined as the glassy appearance in some frozen products produced by ice crystals evaporating on the surface area of a product (Kaess, 1961). The grainy, brownish spots occurring on the product cause the tissue to become dry and tough and to develop off-flavors. This quality defect can be prevented by using heavyweight, moisture proof packaging during the freezing process (Pham and Mawson, 1997).
Chemical changes that can cause spoilage and deterioration of fresh fruits and vegetables will continue after harvesting. This is the main reason for eliminating any delays during pre-freezing operations. As mentioned earlier, several enzymes that cause the loss of color, loss of nutrients, flavor changes, and color changes in frozen fruits and vegetables, should be inactivated by means of thermal treatments prior to freezing. In most cases, blanching is essential for producing quality frozen vegetables, since it also helps destroy microorganisms on the surface of the produce. However, in processing fruits, heat treatment may cause more degradation in quality. In this case, enzymes in frozen fruits can be controlled by using chemical compounds, which interfere with deteriorative chemical reactions. Ascorbic acid is an example that may be used in its pure form or in commercial mixtures with sugars for inhibition of enzymes in fruits.
Development of rancid oxidative flavors through contact of the frozen product with air is another group of chemical changes that can take place in frozen products. This problem can be controlled by excluding oxygen through proper packaging as mentioned earlier. It is also advisable to remove as much air as possible from the freezer bag or container to reduce the amount of air in contact with the product (Schafer and Munson, 1990).
Despite a general increase in microbiological outbreaks in processed foods, frozen foods have demonstrated a good food safety record. The confidence in frozen foods largely depends on the microbiological quality of the raw materials prior to usage, the efficiency of processing, and finally the efficiency of consumers in following specified product instructions (Mayes and Telling, 1993).
Microbiological aspects of freezing
Microorganisms differ significantly in their sensitivity to freezing, thus the main concern about the microbiological aspects of freezing is the growth of organisms during thawing rather than during freezing (Archer et al., 1995). In general, the freezing process does not significantly destroy the microorganisms that may be present in fruits and vegetables. The blanching process prior to freezing destroys some microorganisms and there is a gradual decline in the number of microorganisms during freezer storage. However, a sufficient number of survivors are still present that can multiply and cause spoilage of the product during thawing. Fluctuation in storage temperature is one of the main reasons for microbial deterioration of frozen products during storage. Thus, a careful inspection of frozen products is essential to ensure proper freezing storage with constant temperatures (Schafer and Munson, 1990).
Legal standards
Several agencies exist that establish regulatory standards for frozen fruits and vegetables based on import-export regulations of countries around the world. Some of the general regulations for consideration in the freezing of fruits and vegetables are summarized by the Canadian Food Inspection Agency Liaison (Preparedness and Policy Coordination).
Frozen fruit products
a. Shall be prepared from fresh or previously frozen fruit that is preserved by freezing.
b. Shall be packed(i) with or without a sweetening ingredient in dry form, or
(ii) in a packing medium consisting ofa. water, with or without a sweetening ingredient, or
b. one or more fruit juices, concentrated fruit juices, reconstituted fruit juices, fruit purees or fruit nectars, with or without a sweetening ingredient.
c. May contain citric acid or ascorbic acid, in accordance with good manufacturing practice.
d. May contain any other substance, the addition of which to frozen fruit is in accordance with good manufacturing practice, and is generally recognized as safe.
Frozen vegetable products
a. Shall be prepared from fresh vegetables, or as a mixture of frozen vegetables, that are preserved by freezing.
b. May contain salt.
c. May contain any other substance, the addition of which to frozen vegetables is in accordance with good manufacturing practice, and is generally recognized as safe.
For any type of food preservation method, the retention of nutritional components is a concern, but freezing is probably the least destructive when properly done (Sebranek, 1996). To maintain top nutritional quality in frozen fruits and vegetables, it is essential to follow directions contained in this manual, for each and every step of the freezing process (Schafer and Munson, 1990).
Fruits - Most frozen fruits will maintain a high quality for 8 to 12 months. Unsweetened fruits will lose quality faster than those packed in sugar or sugar syrups.
Vegetables - Most vegetables will maintain a high quality for 12 to 18 months at -18 °C or lower.
Fruits and vegetables are important sources of vitamin C, folate, and minerals, with colored fruits and vegetables also a source of carotenoids. The freezing process itself has no effect on nutrients, but during blanching (prior to freezing) water-soluble nutrients may be leached-out during the process (Bender, 1981).
Marketing of the product clearly is of great importance considering the increasing competition in the food industry. The ever-increasing number of brands and the technological improvements in the frozen food division require that improved marketing strategies be followed. The presentation of the product, which includes labeling, is one of the most appealing characteristics of a product in terms of consumer selection.
Suitable package for the product
A good quality freezer container should be used to maintain the quality of frozen fruits and vegetables. Many moisture- and vapor-resistant wraps, such as heavyweight aluminum foil, plastic coated freezer paper, and other plastic films are effective at excluding oxygen. These wraps are not as convenient for fruits and vegetables as plastic bags or rigid freezer containers, therefore polyethylene plastic bag containers can be used for frozen product formulations. Plastic film bags made especially for freezing are readily available. They can be sealed with twist and tie tops. Collapsible cardboard freezer boxes are frequently used as an outer covering for plastic bags to protect them against tearing, and for easy stacking in the freezer (Schafer and Munson, 1990).
Labeling
Most regulatory agencies, depending on the location of production, require that some specific information be included on the label of the frozen food package. Several basic requirements are recommended for labeling of frozen products:
The common or usual name of the ingredients
The form (or style) of vegetables or fruits, such as whole, slices, or halves. If the form is visible through the package, it need not be stated.
For some vegetables and fruits, the variety or color
Liquid in which a vegetable is packaged must be listed near the name of the product.
The total contents (net weight) must be stated in grams for containers holding 1 kilogram or less.
Other information required on the label, although not on the front panel:
Ingredients, such as spices, flavoring, coloring, or special sweetener, if used
Any special type of treatment
The packer’s or distributor’s name and place of business
Nutritional information
Expiration date
Storage requirements
Heating instructions
Labels may also give the quality or grade, count, size, and maturity of the vegetables, cooking directions, and recipes or serving ideas. If the label lists the number of servings per container, the law requires that the size of the serving be given in common measures, such as ounces or cups.
For specified vegetable mixture formulation, some of the items a package contains:
Nutrition Facts
Serving Size
3/4 cup (85 g)
Serving Per Container 5
Amount per
serving
|
Calories 25 |
Calories from fat 0 |
|
Total Fat 0 g |
0 % |
|
Saturated fat 0 g |
0% |
|
Sodium 20 mg |
1 % |
|
Total Carbohydrate 5 g 2 % |
|
|
Dietary Fiber 2 g |
8 % |
|
Sugars 2 g |
|
|
Protein 2 g |
|
|
Vitamin A 60 % |
Vitamin C 25 % |
|
Calcium 2 % |
Iron 2 % |
* Percent Daily Values are based on a 2 000-calorie diet. Your daily values may be higher or lower depending on your calorie needs.
|
|
Calories: |
2 000 |
2 500 |
|
Total fat |
Less then |
65 g |
80 g |
|
Saturated fat |
Less then |
20 g |
25 g |
|
Cholesterol |
Less then |
300 mg |
300 mg |
|
Sodium |
Less then |
2 400 mg |
2 400 mg |
|
Total Carbohydrate |
|
300 g |
375 g |
|
Dietary fiber |
|
25 g |
30 g |
Cooking Instructions
STIR FRY: Heat oiled wok or frying pan to 380 °F. Cook for 5-7 minutes, while stirring, until vegetables are done to taste.
MICROWAVE: Pour contents of bag into a microwaveable dish. Add 2 table spoons of water; cover and cook for an additional 4 minutes on high power. Stir and cook for additional 4-5 minutes.
