Freezing Foods

Freezing is a popular and effective method of food preservation that has been used for centuries. The process also extends storage time and thus the marketing season. It helps to even out production time so that gluts of produce are managed more effectively. It also brings a range of foods to people who would have to rely on a supply chain that can only deliver perishables. It involves lowering the temperature of food to a point where microorganisms, enzymes, and chemical reactions are significantly slowed down, allowing the food to remain safe to eat and retain its quality for an extended period. Freezing is a versatile technique that can be applied to a wide variety of foods, and it is particularly valuable for preserving seasonal produce, extending the shelf life of perishable items, and reducing food waste. In this article, we will explore the basics of freezing food.

How Freezing Works

The freezing process primarily relies on the principle that lowering the temperature of food inhibits the activity of microorganisms and enzymes that cause spoilage. When food is exposed to freezing temperatures, water in the food forms ice crystals. These ice crystals help preserve the texture, flavor, and nutritional value of the food. The methods of freezing range from placing in a cold room at -18 to-21C, to immersion in ice/salt mixtures and even liquid nitrogen. The latter is the most rapid method commericially available.

There is a process in the freezing of food which is:-

1. Preparation

Before freezing, it’s essential to prepare the food properly. This may involve washing, peeling, cutting, and blanching (briefly boiling) certain foods to preserve their quality.

2. Packaging

Proper packaging is crucial to prevent freezer burn and maintain the quality of frozen food. Vacuum-sealed bags, airtight containers, or freezer-safe wrapping materials are commonly used for this purpose.

3. Freezing

The food is placed in a freezer or deep freezer, where the temperature is maintained at or below 0 degrees Fahrenheit (-18 degrees Celsius) for most home freezers. The lower the temperature, the better the food’s quality will be preserved.

Methods of Freezing

The main methods of freezing are the following:

1. Flash Freezing:
   • Flash freezing involves rapidly freezing individual food items or small portions at extremely low temperatures. This method is often used in the commercial food industry.
   • It’s excellent for preserving the quality of fruits, vegetables, and other delicate items.
   • Liquid nitrogen or specialized flash-freezing equipment is typically used.
2. Blast Freezing:
   • Blast freezing is a method used to quickly freeze larger quantities of food, typically in a commercial setting.
   • It involves using high-velocity cold air to lower the temperature of food rapidly.
   • This method is beneficial for preserving the quality of prepared meals, meat, and seafood.
3. Plate Freezing:
   • Plate freezing involves placing food items between metal plates or on a chilled surface to rapidly freeze them.
   • It’s often used for seafood and meat products and can be done on a smaller scale for home use.
4. Tray Freezing:
   • Tray freezing is a simple method where food items are spread out on a tray or baking sheet and placed in the freezer.
   • This method is suitable for freezing items like berries, sliced fruits, and vegetables before transferring them to storage containers.
5. Bag or Container Freezing:
   • Most commonly used for home freezing, food is placed in airtight bags or containers before being placed in the freezer.
   • It’s versatile and suitable for a wide range of food items, including soups, stews, and individually portioned items.
6. Vacuum Sealing:
   • Vacuum sealing involves removing the air from specially designed bags or containers and sealing them airtight.
   • This method is ideal for preventing freezer burn and preserving the quality of a wide variety of foods, including meats, fruits, and vegetables.
7. Ice Cube Trays:
   • Small items like herbs, broth, or pureed baby food can be frozen in ice cube trays and then transferred to a freezer bag or container.
8. Freezing in Water:
   • Some foods, like berries or sliced fruits, can be frozen in a single layer in a container of water. This prevents them from sticking together and can be convenient for later use in smoothies or recipes.
9. Dry Ice Freezing:
   • Dry ice (solid carbon dioxide) can be used for flash-freezing foods when a very low temperature is required. It’s not commonly used in home settings.

Benefits of Freezing Food

Freezing offers several benefits when it comes to food preservation and meal planning:

1. Long-Term Storage

Frozen food can be stored for an extended period, often months to years, without significant loss of quality or nutritional value.

2. Reduces Food Waste

Freezing allows you to save excess or leftover food for future use, reducing food waste.

3. Convenient Meal Preparation

Preparing meals in advance and freezing them can save time and make meal planning more convenient.

4. Seasonal Preservation

You can preserve seasonal fruits and vegetables when they are abundant and enjoy them throughout the year.

5. Cost-Effective

Buying food in bulk and freezing it can be cost-effective, as you can take advantage of sales and discounts.

6. Retains Nutritional Value

Freezing helps retain the nutritional value of food, especially when compared to canning or other preservation methods that involve high heat.

Types of Foods Suitable for Freezing

While many types of food can be successfully frozen, some are better candidates than others. Foods suitable for freezing include:

1. Fruits and Vegetables: Most fruits and vegetables can be frozen, but they often require blanching to maintain their quality. Examples include berries, peas, corn, and green beans.

One of the measures of an effective freezing process of all fresh produce is whether the texture is the same before and after freezing then thawing. Unfortunately it is not often the case! With green beans for example, only very rapid immersion in liquid nitrogen prevents damage and reduces loss of texture on thawing but it is still not perfect.

2. Meats and Poultry: Beef, chicken, pork, and other meats can be frozen, either as whole cuts or in portions.

3. Seafood: Many types of seafood, such as fish and shrimp, freeze well and retain their quality. Even the shelf-life of a highly susceptible food such as oysters is extended by freezing especially when deeply-frozen say at −20 °C (Cook & Ruple, 1992).

4. Dairy Products: Milk, cheese, and butter can be frozen, though the texture of some dairy products may change upon thawing.

5. Baked Goods: Breads, muffins, and cookies can be frozen and reheated later.

6. Soups and Stews: These dishes are excellent for batch cooking and freezing in portions for later consumption.

7. Prepared Meals: Cooked dishes like casseroles, lasagnas, and stir-fries can be frozen in portions for quick and convenient meals.

To Avoid Freezing

Not all foods are suitable for freezing. Foods to avoid freezing include:

1. High-Water Content Fruits and Vegetables: Foods like watermelon, lettuce, and cucumbers have high water content and do not freeze well.

2. Foods with High Fat Content: Foods with high-fat content, like mayonnaise, cream-based sauces, or may separate and become grainy when thawed.

3. Cooked Pasta and Rice: These can become mushy when frozen and then thawed.

4. Eggs in Shells: Eggs will expand and may crack when frozen in their shells.

5. Foods in Cans: Foods in cans should not be frozen in the can, as it may expand and burst.

Thawing and Using Frozen Food

When you’re ready to use frozen food, it’s important to thaw and prepare it properly. Theoretically, thawing and freezing are the reverse of each other but they are different processes. Thawing is actually more complex than freezing.  For all intents and purposes, the rate of thawing is slower than freezing. The direction of the phase change is different in both cases because one is a cooling and the other is a heating up process. There are also differences in freezing and within the food differences in the internal temperature.

The thawing process turns the freezing ice melt into water inside the food. This is absorbed by the food to restore the freshness which is hopefully similar to that before frozen. During thawing foods undergo a range of physical and chemical changes which damages the food. One of the challenges faced by food technologists is ensuring food is thawed appropriately, even optimised and at the appropriate rate.

The guidelines are:

1. Slow Thawing

For best results, thaw frozen food slowly in the refrigerator. This reduces the risk of bacterial growth and helps preserve the food’s texture and flavor. Slow thawing is better for fish and meat because it allows reabsorption of much of the moiture from melting ice crystals so the level of ‘drip out’ is reduced.

2. Quick Thawing

If you need to thaw food quickly, you can use cold water or the microwave. Be cautious with the microwave, as it can partially cook the food, making it essential to use it immediately. Quick thawing is recommended for frozen vegetables, bread and pastries however.

3. Safety

Always ensure that the food reaches a safe internal temperature when reheating, especially for meat, poultry, and seafood, to prevent foodborne illness.

There are novel thawing methods being assessed all the time including microwave thawing, acoustic thawing, and ohmic thawing.

The Physical Process of Freezing

There are 3 stages in the freezing process: first cooling or prefreezing, phase change or freezing, and second cooling or reduction to storage temperature. 

The three main stages in the freezing process are:

1. Cooling of Food:
   • In this initial stage, the food is gradually cooled down from its initial temperature to just above freezing. This step is important to prepare the food for the subsequent freezing phase.
   • The purpose of this cooling stage is to remove the initial heat from the food, which helps prevent large ice crystals from forming, as these can damage the cell structure of the food and affect its texture and flavor.
   • The cooling can be done in a refrigerator or a controlled cooling environment to ensure a uniform temperature throughout the food product.
2. Phase Change of Freezing:
   • The phase change of freezing is the critical stage where the food undergoes a change from a liquid state to a solid state as it freezes. This is typically initiated by lowering the temperature to the freezing point.
   • During this phase change, water molecules within the food form ice crystals. The rate at which this occurs can affect the size of the ice crystals and, consequently, the texture of the frozen food.
   • Slower freezing allows for smaller ice crystals to form, which is generally preferred as it helps maintain the food’s quality. Rapid freezing can lead to the formation of larger ice crystals, which may cause cell damage and result in a less desirable texture.
3. Second Cooling or Reduction in Storage Temperature:
   • After the food has undergone the phase change of freezing and most of the water has turned to ice, the temperature is lowered further to the desired storage temperature.
   • The goal is to reach and maintain the storage temperature, typically around -18°C (0°F) for long-term frozen storage.
   • The reduction in temperature helps keep the food in a stable, frozen state, preventing further degradation of texture, flavor, and nutritional quality. It also inhibits the growth of microorganisms, extending the shelf life of the frozen food.

Properly controlling the cooling and freezing process is essential to preserve the quality of frozen foods. It’s also important to package the food appropriately to prevent freezer burn and to label containers with the freezing date. The entire process, from cooling to storage, is designed to slow down the growth of ice crystals and maintain the integrity of the food as much as possible.

Freezing Time Prediction

Back in 1984, Clelland who worked in Massey University New Zealand and  who was a pioneer on freezing times was surprised that given the amount of research data on predicting freezing times, there was no definitive prediction method. He set out to gather the data to produce calculation methods on freezing times.

The methods were classed into two groups – numerical methods (finite difference and finite element) and simple formulae. The formulae are methods based on adaptation of analytically derived formulae, and those based on curve fitting experimental data.

Freezing is a valuable method for preserving food that extends the shelf life of a wide range of products, reduces food waste, and allows for convenient meal planning. By understanding the principles of freezing, the types of foods that freeze well, and the proper techniques for thawing and using frozen food, you can make the most of this effective food preservation method. With proper preparation and packaging, you can enjoy the benefits of frozen food for months and even years while maintaining its quality and safety.

Effects of Freezing On Microbiological Stability

Freezing generally improves the microbiological stability and thus the overall safety of food. In vegetables, the blanching step is highly appropriate in destroying most microorganisms except for bacterial spores. Freezing then puts a halt to spore germination in the stored produce.

The Effect of Storage on pH

Freezing vegetables causes inorganic and organic salts to concentrate in the aqueous phase of the tissue. Such salts are able to precipitate during cooling and in doing so cause the pH to change in any unfrozen tissue water.

The pH of frozen vegetables such as minced cauliflower for example rises from pH 4.8, drops slightly to pH 4.6 after 10 days and then rises to a plateau of pH6 at 35 to 40 days storage when stored at -18ºC. At -10ºC, the change in pH changes much sooner arriving at pH 6 by 30 days storage. A more alkaline pH makes the food more prone to microbial spoilage and thus potentially making it less safe (Van der Berg, 1968). When pH changes too, the rates of various biochemical reactions also alters although these may or may not be detrimental.

Effects of Freezing On Food Colour

The colour of food is one of the most important visual clues as to the quality of any food. It is a sensory property that has greater influence in some foods than others – vegetables are some of the most critically affected. The main chemical changes occurring in all foods is due mainly to enzymatic browning and non-enzymatic browning reactions.

Vegetables and Fruits

The major pigments in vegetables are carotenoids and chlorophylls, and anthocyanins. Carotenoids and chlorophylls are the main compounds involved in photosynthesis. The former are precursors of vitamin A. Anthocyanins and carotenoids too may have benefit in reducing the development of cancers and ulcers in people through their antioxidant. Magnesium in the diet is often supplied via ingestion of chlorophylls.

The main carotenoids are lutein and β-carotene and the predominant chlorophylls are chlorophylls a and b. These chlorophylls generally occur at a ratio of about 3 : 1 in higher plants.

The carotenoids are easily oxidized and they are much more prone to damage during any processing including freezing and frozen storage. The β-carotene is known to decrease during frozen storage itself as seen in mango and tomato.

Effect of Freezing on Cell Structure

Ripe fruits and vegetables should preferably be frozen compared to unripe produce. There are also significant differences in freezing performance between cultivars and varieties in terms of sensory and chemical assessed quality. All varieties need to be tested for freezing performance, storage and thawing.

Fruits and vegetables suffer intracellular damage during freezing. Fibrous tissues including vascular cells and and other thick-walled tissues are much more resistant to the damage caused by freezing. 

Fruit cell walls, especially the middle lamella between the cells are usually the source of pectins. When fruit ripens, pectin is deesterified and the tissue becomes softer. When calcium ions are added before freezing, the fruit becomes firmer especially when it is thawed. 

References

Bakal A., Hayakawa K. I. (1973): Heat transfer during freezing and thawing of foods. In: Advances in Food Research (eds. C. O. Chichester, E. M. Mrak and G. F. Stewart), Academic, New York, Vol. 20, pp. 218–256

Cleland A. C. (1990): Food refrigeration processes — Analysis, design and simulation. Elsevier Applied Science, London, England, pp. 284.

Cook, D.W., Ruple, A.D. (1992). Cold storage and mild heat treatment as processing aids to reduce the numbers of Vibrio vulnificus in raw oysters. J Food Prot. 55(12) pp. 985–9.

Oruña-Concha, M. J., González-Castro, M. J., López-Hernández, J., & Simal-Lozano, J. (1997). Effects of freezing on the pigment content in green beans and padrón peppers. Zeitschrift für Lebensmitteluntersuchung und-Forschung A, 205, pp. 148-152.

Petzold, G., and J. M. Aguilera. (2009). Ice morphology: fundamentals and technological applications in foods. Food Biophys. 4 pp. 378–396 (Article)

Van der Berg, L. (1968) Physiochemical changes in foods during freezing and subsequent storage. In: Low Temperature Biology of Foodstuffs (J. Hawthorne and E. Rolfe, eds.), Pergamon Press, Oxford. pp. 205