Introduction

In today’s NutriKitchen article, I’d like to talk to you about food preservation using low temperatures: refrigeration and freezing.

The use of cold as a preservative dates back to early human history, when ice, snow, or cold river water were used to preserve food.

In 1840, Appert developed a method of preservation using cold.

Refrigeration and freezing are two types of food preservation techniques based on physical methods, whose principles are:

• Slowing down the rate of chemical and biochemical degradation reactions by lowering the temperature.
• Inhibiting microbial growth at low temperatures. Below -10ºC, they cannot develop.
• And below 3ºC, microorganisms stop producing the toxins responsible for foodborne illnesses.

Refrigeration or positive cold

This involves keeping the product at a stable, cold temperature (close to 0ºC), avoiding overcrowding and incorrect humidity levels. This helps to effectively control microbial growth. At these temperatures, only psychrophilic microorganisms such as Clostridium botulinum, Yersinia enterocolitica, or Listeria monocytogenes, which thrive at 2ºC, are able to grow. Chemical and enzymatic reactions are also slowed. This is why each food requires specific temperature and relative humidity conditions. For example, tomatoes and lemons require 10–12 ºC and 85% humidity, while meat requires 0–2 ºC and 85% humidity.

Preservation conditions and shelf life under refrigeration:

Freezing or negative cold

This is a long-term preservation method that involves turning the water in food into ice by storing it at temperatures below –18 ºC. The principle of freezing is to form ice crystals from water, thereby reducing water activity and consequently decreasing microbial and chemical degradation.

Food can be frozen at home following certain guidelines, but slow freezing tends to form large crystals.

Alternatively, food can be bought already frozen, resulting in smaller crystals. It’s important to ensure the packaging is fully intact and sealed, and the product is completely rigid.

Unpackaged items should be completely rigid, with no signs of softening or discolouration.

How ice crystals form

First, extracellular liquid crystallises. Then, cellular water moves outward to balance salinity and freezes in the extracellular space. Intracellular fluid becomes increasingly concentrated in sugars, salts, and proteins, leaving a small amount of unfrozen intracellular liquid. This liquid is rich in enzymes that can destroy vitamins and colour. To avoid this, vegetables should be blanched.

The freezing point of water depends on the concentration of dissolved solids. The more solids, the lower the freezing point.

Rapid freezing leads to the formation of a greater number of smaller, rounder crystals. This helps maintain the nutritional and organoleptic qualities of food. It is done using cryogenic freezers that perform ultra-fast freezing at -130ºC.

In slow freezing, crystals are larger and more elongated, often causing changes in texture and nutritional value. This typically happens in domestic freezers which reach -18ºC.

The freezing rate depends on the freezer’s power, the food’s thermal conductivity, packaging, and the mass and thickness of the product.

Fine crystallisation occurs with high freezing speeds, agitation (e.g., in ice cream), and in small-sized foods, resulting in uniform freezing.

Maximum crystallisation occurs at -80/-100 ºC, depending on the solute concentration in water.

At -18 ºC, between 5% and 15% of water remains unfrozen, increasing the concentration of salts in the unfrozen water, potentially leading to protein denaturation. Upon thawing, exudation occurs and chemical reactions continue.

Freezing is not a uniform process.

Stages in industrial food freezing

• Selection of high-quality raw materials.
• Product processing or preparation.
• Packing, which must meet certain criteria:
  • Be suitable for food use
  • Allow rapid freezing
  • Be liquid-proof
  • Be shock-resistant
  • Withstand low temperatures
  • Not stick to the contents
  • Be opaque to light
• Freezing process: transition from liquid to solid.
• Thawing and cooking.

According to legislation, frozen foods must be stored at –18 ºC. At this temperature, chemical reactions can still occur depending on the food type, so storage time varies, and frozen products do have an expiry date.

Before freezing, foods require handling or preparation to achieve optimal results.

Meat is aged for 3 to 4 days in cold storage, then carcasses and cuts are processed raw. Pre-freezing occurs at -10ºC and shelf life is 8–10 months.

Poultry and game undergo cleaning, singeing, and gutting. They’re usually frozen whole or in parts, raw. The temperature is -10ºC and they last 8–10 months.

Fish may be frozen whole or in parts, gutted, scaled, or skinned; frozen at -14ºC, with a shelf life of 4–6 months.

Seafood is frozen raw shortly after being caught or may be cooked and then frozen. Frozen at -14ºC, they last 2–3 months.

Vegetables must be harvested no more than 12 hours in advance. They are cleaned, peeled, cut, blanched, cooled, and hot-air dried. Frozen at -18ºC, shelf life is 6–9 months.

Fruit is also harvested no more than 12 hours prior, then cleaned and washed, frozen whole or in parts at -18ºC, lasting up to 12 months.

Cooked meals should have cooking interrupted 10 minutes before the end, frozen at -14ºC and last up to 12 months.

Effect of freezing on food quality

If frozen foods are stored too long, certain chemical changes may occur, such as fat oxidation—especially of unsaturated fats—leading to rancidity. Organoleptic qualities (flavour, texture, smell) are also affected. Vitamin oxidation and loss of activity occur. Protein denaturation can result in toughening and discolouration. Starch retrogradation can occur, affecting the binding of sauces (they appear split).

Temperature fluctuations in the freezer can cause physical changes in food, such as fusion and recrystallisation of ice crystals—i.e. thawing and refreezing—which happens when the freezer is frequently opened.

Defrosting

This is the process of returning frozen food to temperatures above 0 ºC. Heat must be applied for this to occur.

It involves 3 stages:

1. Specific heat: from storage temperature to melting point.
2. Latent heat: ice crystals turn to liquid water.
3. From melting point to final temperature.

Liquid water conducts heat worse than ice, so defrosting slows as the process advances.

Defrosting time depends on the product’s size, shape, and packaging thermal conductivity.

The defrosting process is crucial to final product quality.

Effect of defrosting on food quality

• Sugars are unaffected
• Proteins broken or denatured by crystals do not regain their original structure, leading to changes in organoleptic quality.
• Food exudes moisture, losing water retention.
• Broken cell walls lead to losses in minerals and water-soluble vitamins.

Tips for defrosting food

• Defrost at temperatures below 4 ºC or above 65 ºC to avoid microbial growth due to exudates (more exudate means higher microbial risk).
• Isolate food from the air.
• Only defrost outside the fridge if it will take less than an hour, if pathogens are not present, and if the food has little water and hardly exudes.

Recommendations for defrosting specific foods

Large pieces of meat or fish should be defrosted in the fridge, wrapped in cloths until flexible.

Small pieces of meat or fish can be cooked directly from frozen or defrosted under cold running water.

Do not defrost vegetables before cooking; if necessary, do so in their packaging.

How to cook defrosted food

Defrosted fish can be cooked by any method, but works best in stews and sauces.

Frozen fish can be boiled or cooked in sauce. Grilling, baking, and frying tend to yield poorer results.

Defrosted meat can be cooked using any method, preferably before it’s completely thawed, but stews and boiled dishes yield the best results.

Small frozen meat pieces work well in stews, braised dishes, and boiled recipes.

Frozen vegetables can be cooked by any method. As they’ve been blanched beforehand, they usually only need to be sautéed.