Freezing is one of the most cost-effective ways of extending the long-term shelf life of many foods. Though other emerging technologies, such as infrared irradiation, microwave, pulsed electric field, and ultrasound, are gaining more attention, freezing still remains a dominant food preservation method. That said, there are also new freezing processes that continue to emerge, alongside these other technologies.

Freezing preserves food through physically changing the state of a substance’s water content into ice by cooling it. However, even though freezing will slow down the physicochemical and biochemical reactions that influence the deterioration of foods, it will not completely stop them. The loss in quality of frozen foods is largely dependent on the freezing process, storage temperature, the length of freezer storage time, and the thawing procedure. Microbial growth will be completely stopped below zero degrees F.  However, enzymatic and nonenzymatic chemical changes will continue, albeit at a slower rate.  Therefore, maintaining safety and quality in frozen foods is a delicate process that involves the entire frozen food cold chain: pre-treatment, processing, packaging, frozen food storage, transport, and thawing food.

How The Freezing Rate Affects Quality

Controlling the entire freezing process, which includes the pre-freezing, actual freezing, and post-freezing storage, is critical to achieving a high-quality product. One important parameter is the freezing rate. In general, fast freezing produces better quality frozen food than slow freezing.

When freezing fruits and vegetables, the rate of cooling will determine the size of ice crystals formed and the amount of damage to the cell walls. Damaged cell walls cause food that is supposed to be crisp to be limp instead.  When freezing meat, fish, or poultry water migrates out of the cells.  Rapid freezing minimizes this dehydration and the texture degradation.

The faster the crystallization, the smaller the ice crystals will be, which will cause less damage during the freezing process. In terms of thawing, the reverse is true, as slow heating allows water to diffuse back into the cells.

Microbial Cultures

The detrimental effects of freezing on microorganisms may or may not be desired, depending on the type of product. When foods do not contain any beneficial cultures, microbial growth is not desirable. In foods that do contain beneficial microbial cultures, maintaining their activity is important. In frozen dough, for instance, rapid freezing will have a detrimental effect on yeast activity.

The maximum recommended storage temperature at which microbial spoilage ceases is between 16°F and 10°F. And although microbiological spoilage can be avoided, the enzymes in the frozen product will still play a part. Therefore, hygienic conditions and heat processing will help increase the product’s shelf life. Generally, freezing will kill between 10 and 60% of the viable microbe population, depending on the frozen storage time. However, in terms of microorganisms, there is a considerable difference between their sensitivity to freezing and how quickly they are able to grow once the product is thawed. Special attention needs to be given here as some microbes can not only lead to spoilage but also to various food borne illnesses.

Physical Changes and Frozen Food Quality

There are certain physical changes that take place when food is frozen. Among the most common, we can include:

  • Weight Loss – Unpacked foods will lose moisture during the freezing process. Meat will lose up to 3% of its weight during the freezing process and up to 6.15% during freezing and 20-hour-long refrigeration.
  • Recrystallization – Recrystallization is the process of changes in number, size, and shape of ice crystals during frozen storage. Small crystals fuse together, creating larger ones, affecting the overall quality and shelf life.
  • Retrogradation – Quality loss in baked goods occurs as a result of staling and starch retrogradation. This process happens most rapidly while the product’s temperature is above freezing. Rapid freezing preserves freshness.
  • Protein Denaturation – Protein denaturation and solubility changes happen as a result of freezing. Fish deterioration during storage, for example, is associated with a decrease of protein solubility, which diminishes the nutritional value. In frozen raw meat and poultry, the water-holding capacity, enzyme activity, and surface hydrophobicity are affected by freezing.
  • Freezer Burn – Moisture loss due to evaporation causes freezer burn. This type of dehydration during processing can be controlled with humidification, lower storage temperature, or better packaging.

Chemical Changes and Frozen Food Quality

Aside from the physical changes that happen during freezing, there are also chemical changes that can affect the quality of frozen food. Among these, we can include the following:

  • Rancidity – Oxygen found in the air will affect almost all frozen foods. Oxygen contact causes oxidative rancidity. It will also cause color loss and off-flavors. Freezing leads to a concentration of solutes, which dehydrate cell membranes, exposing them to oxidation.
  • Color, Flavor, and Aroma Loss – Color changes in fruits and vegetables are a result of changes in natural pigments of vegetable tissues, the development of enzymatic browning, and the breakdown of cellular chloroplasts and chromoplasts. A light surface color is preferable in poultry, This is best achieved with rapid surface freezing. The process of freezing will also affect the flavor and aroma of frozen foods.
  • Vitamin Loss – The retention of nutritional value can be challenging with any preservation method. Freezing is among the least destructive. The destruction of ascorbic acid (Vitamin C), for example, does occur during freezing, but the total loss is dependent on several factors, including the type of freezing, packaging, pre-treatment methods, and storage conditions. The main contributor to the loss of vitamins is oxidation.
  • Enzyme Release – The disruption of plant and animal tissues through freezing leads to the release of enzymes. At around 32°F, the enzymatic breakdown of protein is the main cause of quality loss.
  • Acetaldehyde Formation – This process occurs in frozen vegetables during storage and limits of shelf life. The amount of acetaldehyde formation depends on the blanching process and storage efficacy.

Freezing Pre-Treatments

It’s important to recognize that a successful freezing process will only retain the already existing food quality and will not improve it. Maintaining the quality and safety of frozen food starts with the pre-treatment. The most common pretreatments include the following:

  • Blanching – The majority of vegetables and some fruits undergo blanching before freezing. This is a process of scalding fruits or vegetables in boiling water or steam for a brief period of time. This process destroys the permeability of cell membranes, removes intercellular air, and fills those spaces with water. As a result, ice crystallization can form over the entire surface without interruption. It also preserves the texture, color, flavor, and nutrient content of foods by deactivating their enzymes.
  • Heat Treatments – Maintaining proper texture is important when freezing fruits and vegetables. Heat treatments help maintain their firmness while killing microorganisms that may be present. For example, carrots heated for 30 minutes at 140°F and then frozen at the optimum rate of 23°F/min will avoid both cell damage and excessive softening. Common foods that undergo heat treatments include cooked foods, some vegetables, meats, and fish.
  • Dipping Treatments – In some cases, foods are dipped in different solutions before freezing. Apple slices, for instance, are usually soaked in a 1% salt solution to remove intercellular air. Fruits are generally dipped in ascorbic acid and various sugar solutions to minimize browning. Glazing compounds are also used to add a protective layer of coating against dehydration and oxidation. Sodium acid phosphate, sodium carbonate, calcium lactate, ascorbic and citric acids, glutamic acid, and some corn syrup solids, among other solutions, can be used for glazing.
  • Cryoprotection – Cryoprotectants such as sugars, amino acids, polyols, methylamines, or carbohydrate polymers are compounds introduced during processing or product formulation, or they may occur naturally. These help prevent negative changes in foods during the freezing and thawing processes.

Storage, Packaging, and Thawing

The packaging, storage, and thawing processes also affect frozen food quality. The detrimental effects on quality and food safety are slow, gradual, cumulative, and irreversible. To maintain quality, all of these need to be addressed. It has been revealed that aluminum foil laminated packages perform best in limiting oxygen permeability, light transparency, and water vapor transmission.

Storage and distribution temperatures need to be kept at 0°F or lower to maintain quality. Every 9°F increase over 0°F, increases the rate of quality loss by about 200 to 250%. The amount of food in a storage unit can affect the temperature fluctuation.

Some food types, such as pork, fish, fried chicken, animal organs, and spinach, can be maintained at high-quality only for about 3 to 7 months at -4°F. In contrast, sugared fruits, beef, most vegetables, and bakery products can be maintained over one year under the same conditions.

Thawing is the final step of the freezing process and is essential to the quality and safety of frozen foods. Microbiologically safe methods of defrosting foods are to store foods in the refrigerator at temperatures below 41°F, microwave, or to cook.

Takeaway

All of the steps included in the frozen food cold chain need to be considered to provide high quality and safety.   Cablevey’s Frozen Food Conveyor Systems will facilitate both quality and safety throughout the entire food production and freezing process, going from mixing, pre-treatment, freezing, safe food handling, and packaging. While stainless steel equipment is seen as the most “clean” and easily cleanable, when it comes to most frozen food applications CVT tubing also has the added benefit of reducing the incidence of black specks occurring on the product.

Unfortunately, this is an all-too-common issue with other systems, particularly open belt conveyors or bucket elevators. Being enclosed systems, cable, disc and tube conveyors are also better equipped to eliminate product spillage, warming, and cross-contamination from the environment. Cablevey’s frozen food conveyor systems come in different sizes and offer a full range of different components to cover any facility layout and frozen food conveying needed.