Can anyone recommend me research on food preservation by freezing?

Dear Saifeldin,

The following publications cover the answer to your question:

1- The Science of Freezing Foods

William Schafer, Food technologist — Department of Food Science and Nutrition

Reviewed 2014 by Suzanne Driessen, Extension Educator — Food Safety.

Freezing is a quick and convenient way to preserve fruits and vegetables at home. It is a popular method of home food preservation throughout Minnesota. Home frozen fruits and vegetables of high quality and maximum nutritional value can be produced done correctly (see freezing vegetables and herbs or freezing fruits). Our directions are based on:

the chemical and physical reactions which take place during the freezing process;

scientific knowledge of the effect of freezing on the tissues of fruits and vegetables; and food microbiology.

Chemical Changes During Freezing

Fresh fruits and vegetables, when harvested, continue to undergo chemical changes which can cause spoilage and deterioration of the product. This is why these products should be frozen as soon after harvest as possible and at their peak degree of ripeness.

Fresh produce contains chemical compounds called enzymes which cause the loss of color, loss of nutrients, flavor changes, and color changes in frozen fruits and vegetables. These enzymes must be inactivated to prevent such reactions from taking place.

Enzymes in vegetables are inactivated by the blanching process. Blanching is the exposure of the vegetables to boiling water or steam for a brief period of time. The vegetable must then be rapidly cooled in ice water to prevent it from cooking. Contrary to statements in some publications on home freezing, in most cases blanching is absolutely essential for producing quality frozen vegetables. Blanching also helps to destroy microorganisms on the surface of the vegetable and to make some vegetables, such as broccoli and spinach, more compact.

The major problem associated with enzymes in fruits is the development of brown colors and loss of vitamin C. Because fruits are usually served raw, they are not blanched like vegetables. Instead, enzymes in frozen fruit are controlled by using chemical compounds which interfere with deteriorative chemical reactions. The most common control chemical is ascorbic acid (vitamin C). Ascorbic acid may be used in its pure form or in commercial mixtures with sugars.

Some directions for freezing fruits also include temporary measures to control enzyme-activated browning. Such temporary measures include soaking the fruit in dilute vinegar solutions or coating the fruit with sugar and lemon juice. However, these latter methods do not prevent browning as effectively as treatment with ascorbic acid.

Another group of chemical changes that can take place in frozen products is the development of rancid oxidative flavors through contact of the frozen product with air. This problem can be controlled by using a wrapping material which does not permit air to pass into the product. 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.

Textural Changes During Freezing

Water makes up over 90 percent of the weight of most fruits and vegetables. This water and other chemical substances are held within the fairly rigid cell walls which give support structure, and texture to the fruit or vegetable. Freezing fruits and vegetables actually consists of freezing the water contained in the plant cells.

When the water freezes, it expands and the ice crystals cause the cell walls to rupture. Consequently, the texture of the produce, when thawed, will be much softer than it was when raw. This textural difference is especially noticeable in products which are usually consumed raw. For example, when a frozen tomato is thawed, it becomes mushy and watery. This explains why celery and lettuce are not usually frozen and is the reason for the suggestion that frozen fruits, usually consumed raw, be served before they have completely thawed. In the partially thawed state, the effect of freezing on the fruit tissue is less noticeable.

Textural changes due to freezing are not as apparent in products which are cooked before eating because cooking also softens cell walls. These changes are also less noticeable in high starch vegetables, such as peas, corn, and lima beans.

Rate of Freezing

Overloading the freezer with

unfrozen products will result

in a long, slow freeze and a

poor quality product.

The extent of cell wall rupture can be controlled by freezing produce as quickly as possible. In rapid freezing, a large number of small ice crystals are formed. These small ice crystals produce less cell wall rupture than slow freezing which produces only a few large ice crystals. This is why some home freezer manuals recommend that the temperature of the freezer be set at the coldest setting several hours before foods will be placed in the freezer. Some freezer manuals tell the location of the coldest shelves in the freezer and suggest placing unfrozen products on these shelves.

All freezer manuals give guidelines for the maximum number of cubic feet of unfrozen product which can be frozen at one time. This is usually 2 to 3 pounds of vegetable to each cubic foot of freezer space per 24 hours. Overloading the freezer with unfrozen products will result in a long, slow freeze and a poor quality product.

Changes Caused by Fluctuating Temperature

To maintain top quality, frozen fruits and vegetables should be stored at 0� F or lower. This temperature is attainable in separate freezer units and in some combination refrigerator-freezers. A freezer thermometer can help you determine the actual temperature of your freezer. If your freezer has number temperature settings, such as from 1 to 9, check the manual to see what settings are recommended for different uses.

Storing frozen foods at temperatures higher than 0� F increases the rate at which deteriorative reactions can take place and can shorten the shelf life of frozen foods. Do not attempt to save energy in your home by raising the temperature of frozen food storage above 0� F.

Fluctuating temperatures in the freezer can cause the migration of water vapor from the product to the surface of the container. This defect is sometimes found in commercially frozen foods which have been improperly handled.

Moisture Loss

Moisture loss, or ice crystals evaporating from the surface area of a product, produces freezer burn-a grainy, brownish spot where the tissues become dry and tough. This surface freeze-dried area is very likely to develop off flavors. Packaging designed specifically for freezing foods will prevent freezer burn.

Microbial Growth in the Freezer

The freezing process does not actually destroy the microorganisms which may be present on fruits and vegetables. While blanching destroys some microorganisms and there is a gradual decline in the number of these microorganisms during freezer storage, sufficient populations are still present to multiply in numbers and cause spoilage of the product when it thaws. For this reason it is necessary to carefully inspect any frozen products which have accidentally thawed by the freezer going off or the freezer door being left open.

Nutrient Value of Frozen Foods

Freezing, when properly done, is the method of food preservation which may potentially preserve the greatest quantity of nutrients. To maintain top nutritional quality in frozen fruits and vegetables, it is essential to follow directions contained in this bulletin for pretreatment of the vegetables, to store the frozen product at 0� F and to use it within suggested storage times.

Ready to start freezing? See freezing vegetables and herbs or freezing fruits.

Related Resources

Freezing Food: Questions and Answers

Freezing Tomatoes

Freezing Fruits

Freezing Vegetables and Herbs

Mini Module: Freezing Fruit for Sweet Success (narrated slideshow)

Mini module: Freezing Vegetables for Tasty Results (narrated slideshow).

http://www.extension.umn.edu/food/food-safety/preserving/freezing/the-science-of-freezing-foods/

2- Freezing, in food processing, method of preserving food by lowering the temperature to inhibit microorganism growth. The method has been used for centuries in cold regions, and a patent was issued in Britain as early as 1842 for freezing food by immersion in an ice and salt brine. It was not, however, until the advent of mechanical refrigeration that the process became widely applicable commercially. In 1880 a cargo of meat shipped from Australia to Britain under refrigeration accidentally froze, with such good results that the process was at once adopted for long-distance shipments and other storage. In the 20th century quick, or flash, freezing was found to be especially effective with certain types of food.

Except for beef and venison, which benefit from an aging process, meat is frozen as promptly as possible after slaughter, with best results at temperatures of 0 °F (−18 °C) or lower. Fruits are frozen in a syrup or dry sugar pack to exclude air and prevent both oxidation and desiccation.

Most commercial freezing is done either in cold air kept in motion by fans (blast freezing) or by placing the foodstuffs in packages or metal trays on refrigerated surfaces (contact freezing).

For freeze-drying, see dehydration.

http://www.britannica.com/topic/freezing-food-preservation

3- Chapter 1

Introduction to freezing

Freezing is one of the oldest and most widely used methods of food preservation, which allows preservation of taste, texture, and nutritional value in foods better than any other method. The freezing process is a combination of the beneficial effects of low temperatures at which microorganisms cannot grow, chemical reactions are reduced, and cellular metabolic reactions are delayed (Delgado and Sun, 2000).

1.1 The importance of freezing as a preservation method

Freezing preservation retains the quality of agricultural products over long storage periods. As a method of long-term preservation for fruits and vegetables, freezing is generally regarded as superior to canning and dehydration, with respect to retention in sensory attributes and nutritive properties (Fennema, 1977). The safety and nutrition quality of frozen products are emphasized when high quality raw materials are used, good manufacturing practices are employed in the preservation process, and the products are kept in accordance with specified temperatures.

The need for freezing and frozen storage

Freezing has been successfully employed for the long-term preservation of many foods, providing a significantly extended shelf life. The process involves lowering the product temperature generally to -18 °C or below (Fennema et al., 1973). The physical state of food material is changed when energy is removed by cooling below freezing temperature. The extreme cold simply retards the growth of microorganisms and slows down the chemical changes that affect quality or cause food to spoil (George, 1993).

Competing with new technologies of minimal processing of foods, industrial freezing is the most satisfactory method for preserving quality during long storage periods (Arthey, 1993). When compared in terms of energy use, cost, and product quality, freezing requires the shortest processing time. Any other conventional method of preservation focused on fruits and vegetables, including dehydration and canning, requires less energy when compared with energy consumption in the freezing process and storage. However, when the overall cost is estimated, freezing costs can be kept as low (or lower) as any other method of food preservation (Harris and Kramer, 1975).

Current status of frozen food industry in U.S. and other countries

The frozen food market is one of the largest and most dynamic sectors of the food industry. In spite of considerable competition between the frozen food industry and other sectors, extensive quantities of frozen foods are being consumed all over the world. The industry has recently grown to a value of over US$ 75 billion in the U.S. and Europe combined. This number has reached US$ 27.3 billion in 2001 for total retail sales of frozen foods in the U.S. alone (AFFI, 2003). In Europe, based on U.S. currency, frozen food consumption also reached 11.1 million tons in 13 countries in the year 2000 (Quick Frozen Foods International, 2000). Table 1 represents the division of frozen food industry in terms of annual sales in 2001.

4-Foods for packed lunches or elaborate dinners can be kept in your freezer ready for busy days, parties or unexpected company. By planning a steady flow of casseroles, main dishes, baked goods and desserts in and out of your freezer, you can make good use of your freezer and good use of your time.

ADVANTAGES OF FREEZING PREPARED FOODS

• You prepare food at your convenience.

• Your oven is used more efficiently by baking more than one dish at a time.

• You avoid waste by freezing leftovers and using them as "planned overs."

• Special diet foods and baby foods can be prepared in quantity and frozen in single portions.

• You save time by doubling or tripling recipes and freezing the extra food.

• If you cook for one or two, individual portions of an ordinary recipe can be frozen f

http://www.fao.org/docrep/008/y5979e/y5979e03.htm

http://nchfp.uga.edu/publications/uga/FreezingPreparedFoods.pdf

5-Freezing is a quick and convenient way to extend the storage time of many foods. The quality of frozen foods will vary depending on how they are prepared for freezing, on packaging materials and storage time. Some foods do not freeze well. These links will help you successfully freeze foods for future use.

Canning & Freezing Safely & Successfully: A Quiz (Video)

Quick Links to Freezing Topics on This Page

Freezing Foods - general information

Freezing Vegetables

Tomatoes and Tomato Products

Freezing Fruits

Freezing Meats / Animal Products

Freezing Meals / Advanced Preparation of Foods

Other Food Products

Other Freezing Questions - includes foods that don't freeze well

http://food.unl.edu/freezing

6- What nutrients are lost or destroyed by freezing?

You do lose some flavor when food is frozen but freezing can be a very good way to preserve the nutritional value, texture and flavor of many foods. The initial quality of the food and the length of time between harvest and freezing are important factors. As long as the food was grown in a high-quality way (for example, organically grown) and was fairly fresh at the time of freezing, the overall nutrient retention in a frozen food can be quite high. In other words, many of the vitamins and minerals will keep fairly well in frozen foods.

Some of the phytonutrients found in food may also keep fairly well. For example, we have seen research showing that anthocyanin flavonoids can be well preserved during freezing. Even though they are rather delicate, a recent study found no significant reduction in the levels of anthocyanins in blueberries after three months of freezing.

One of the main concerns for nutrient loss associated with freezing seems to be related to the blanching process that oftentimes occurs prior to freezing. About 25% of the vitamin C and a greater percentage of folate are lost during the blanching process that occurs before foods are frozen. About 10% of thiamin (vitamin B1) is also lost during blanching. It's important to remember that these percentages of nutrient loss are very general and can be different with different foods. Nevertheless, as you can see from the numbers above, the freezing-connected loss of these water-soluble vitamins never comes close to outweighing the amount of vitamins retained.

If you are freezing foods at home, we recommend that you visit the website for the National Center for Home Food Preservation at the following address: http://www.uga.edu/nchfp/how/freeze.html. This website provides some very detailed information about foods that do not freeze well, containers to use for freezing, the role of blanching, and other issues related to the freezing of specific foods. We would disagree with this website, however, in its recommendation of aluminum foil for freezing, since we believe that aluminum poses an unnecessary risk both to our health and to our environment.

Since we have seen some research that suggests that thawing degrades part of the vitamin C content, you may want to avoid this step when cooking frozen vegetables. Storing frozen foods properly (0°F) for no more than 6 months will also help maintain the nutritional value of frozen foods.

References:

Lohachoompol V, Srzednicki G, Craske J. The Change of Total Anthocyanins in Blueberries and Their Antioxidant Effect After Drying and Freezing. Biomed Biotechnol. 2004;2004(5):248-252.

Nursal B, Yucecan S. Vitamin C losses in some frozen vegetables due to various cooking methods. Nahrung. 2000;44(6):451-3.

Severi S, Bedogni G, Manzieri AM, et al. Effects of cooking and storage methods on the micronutrient content of foods. Eur J Cancer Prev. 1997;6 (Suppl 1):S21-4.

http://www.whfoods.com/genpage.php?tname=george&dbid=215

7- Home Freezing and Food Preservation Ideas: Fruits and Veggies

By Elaine Magee, MPH, RD

WebMD Feature Archive

The beginner's guide to preserving fresh produce.

Want to save money and boost nutrition? Try preserving fresh fruits and egetablesfrom your garden or the farmers market to use year-round -- no water bath or pressure cooker required! The trick: Let your freezer do the work.

And don't worry; we won't get too complicated here. This is a beginner's course to preserving food. Only the absolutely easiest ways to freeze and preserve fruits, vegetables, and herbs will be discussed! If you have a handful of freezer plastic bags, a mixing spoon, a refrigerator, and microwave or stove, you have everything you need to get started.

Here are some tips, techniques, and recipes to help you get started freezing fruits, vegetables, and herbs.

Dry Pack Freezing Technique for Fruit

The dry pack freezing method involves freezing individual slices or pieces of fruit on a cookie sheet. Just spread out the pieces of fruit on a cookie sheet or jellyroll plan (line the pan with wax paper if you like) and place in the freezer. When the pieces are solidly frozen, remove them with a spatula or large spoon and pack in plastic freezer bags or freezer containers.

http://www.webmd.com/food-recipes/home-freezing-and-food-preservation-ideas-fruits-and-veggies

8- Freezing food preserves it from the time it is prepared to the time it is eaten. Since early times, farmers, fishermen, and trappers have preserved their game and produce in unheated buildings during the winter season.[1] Freezing food slows down decomposition by turning residual moisture into ice, inhibiting the growth of most bacterial species. In the food commodity industry, there are two processes: mechanical and cryogenic (or flash freezing). The freezing kinetics is important to preserve the food quality and texture. Quicker freezing generates smaller ice crystals and maintains cellular structure. Cryogenic freezing is the quickest freezing technology available due to the ultra low liquid nitrogen temperature −196 °C (−320 °F).[2]

Preserving food in domestic kitchens during the 20th and 21st centuries is achieved using household freezers. Accepted advice to householders was to freeze food on the day of purchase. An initiative by a supermarket group in 2012 (backed by the UK's Waste & Resources Action Programme) promotes advising the freezing of food "as soon as possible up to the product's 'use by' date". TheFood Standards Agency was reported as supporting the change, providing the food had been stored correctly up to that time.[3]

https://en.wikipedia.org/wiki/Frozen_food

9- Food preservation involves preventing the growth of bacteria, fungi (such as yeasts), or other micro-organisms (although some methods work by introducing benign bacteria orfungi to the food), as well as retarding the oxidation of fats that cause rancidity. Food preservation may also include processes that inhibit visual deterioration, such as theenzymatic browning reaction in apples after they are cut during food preparation.

Many processes designed to preserve food will involve a number of food preservation methods. Preserving fruit by turning it into jam, for example, involves boiling (to reduce the fruit’s moisture content and to kill bacteria, etc.), sugaring (to prevent their re-growth) and sealing within an airtight jar (to prevent recontamination). Some traditional methods of preserving food have been shown to have a lower energy input and carbon footprint, when compared to modern methods.[1] However, some methods of food preservation are known to create carcinogens, and in 2015, the International Agency for Research on Cancer of the World Health Organization classified processed meat, i.e. meat that has undergone salting, curing, fermenting, and smoking, as "carcinogenic to humans".[2][3][4]

Maintaining or creating nutritional value, texture and flavor is an important aspect of food preservation, although, historically, some methods drastically altered the character of the food being preserved. In many cases these changes have come to be seen as desirable qualities – cheese, yogurt and pickled onions being common examples.

https://en.wikipedia.org/wiki/Food_preservation

10- Preserving Food: Freezing Vegetables1

Judy A. Harrison and Elizabeth Andress; adapted for use in Florida by Amy Simonne2

Freezing is an excellent way to preserve fresh vegetables at home. Freezing does not sterilize food; the extreme cold simply retards growth of microorganisms and slows down changes that affect quality or cause spoilage in food.

The quality of frozen vegetables depends on the quality of the fresh products and how they are handled from the time they are picked until they are ready to eat. It is important, also, to start with high-quality vegetables because freezing will not improve the product's quality.

Selecting Freezer Containers

Before preparing vegetables for freezing, assemble the containers you will use. The selection of containers depends on the vegetable being frozen, personal preference and the types that are readily available. Containers should be moisture-vapor resistant, durable, easy to seal and should not become brittle at low temperatures.

Containers suitable for freezing vegetables include plastic freezer containers, flexible freezer bags and protective cardboard cartons, or glass canning jars. Foods packed in wide-mouth jars are easier to remove than those packed in narrow-mouth jars.

Some household containers are not recommended for freezing. The cardboard cartons that milk, ice cream or cottage cheese come in are not moisture-vapor resistant enough. Regular (not canning) jars break too easily at freezer temperatures.

https://edis.ifas.ufl.edu/fy719

Hoping this will be helpful,

Rafik

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