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Food freezing and refrigeration

Food refrigeration and freezing are the modern methods of choice for preserving food. Most often refrigeration has positive results; the food keeps safe for long, nevertheless, sometimes the unexpected happens. For example, refrigerated bananas and a variety of vegetables develop black marks; tomatoes turn soggy, frozen meat turn brown and even some foods go bad.

These unexpected changes invite a flurry of questions such as: is refrigerated and frozen food safe? Why do some foods go bad instead of keeping longer in the fridge? Which foods should/should not be refrigerated and which should be frozen? Does refrigeration kill nutrients? Welcome to the answers to these and other burning questions about food refrigerators and freezers.

Why do foods go bad?

Food go bad because micro-sized unwanted animals called bacteria breed, multiply and grow in it. Not all bacteria cause food to go stale and some are even beneficial. The specific group of bacteria that spoil food are called spoilage (from the word spoil) bacteria.

Stale food is recognized either by bad smell, unusual colour, bad taste or slimy texture which render it unpalatable. These undesirable sensorial properties originate from waste matter expelled by the bacteria.

There is another category of relatively more harmful bacteria that can grow in food but rarely cause any undesirable flavour or discolouration. These bacteria are called pathogenic (from the word pathogen) bacteria. They either release poisonous chemicals into the food or they grow into large numbers that are toxic.

Although food contaminated by pathogenic bacteria does not show any sensorial noticeable changes, if consumed, it causes food borne illness which is usually referred to as food poisoning.

How does refrigeration preserve food?

Spoilage bacteria thrive well in high temperatures ranging from room temperature to 40 °C but, do poorly in cold temperatures.

Refrigeration provides low temperatures that slow reproduction, multiplication and growth of spoilage, pathogenic and poison producing bacteria.

Refrigeration does not kill the bacteria!

What is the origin of refrigeration and freezing?

The use of cold storage to preserve food dates back to pre-historic ages. Ancient man discovered that his hunted game meat could be stored in coolness of caves or covered with snow for the time of scarcity.

Before the first millennium, Chinese, Romans, Hebrews and Greek harvested ice and stored it in pits on the ground insulated with wood or straw for food preservation.

The first record of man who paved the way for modern refrigeration is William Cullen of Glasgow University, Scotland who produced cold by evaporating ether in 1748.

Jacob Perkins developed the first practical refrigerator machine in London in 1834 by using ether as a refrigerant.

In 1897, William Singer of New York patented the earliest automatic electric unit for small scale refrigeration.  From then on refrigeration evolved into what we have today, domestic, commercial and transport refrigeration systems.

How does refrigeration differ from freezing?

In refrigeration, food is stored between a temperature of 3-7 degrees Celsius which are slightly higher than the sub zero temperatures of the freezer.

Freezing causes the water in the food to turn into ice which is normally not the case with refrigeration. Therefore, frozen foods store longer (up to 6 months) compared to a maximum of 7 days in a fridge because freezing eliminates the free water available for bacterial growth, chemical and biochemical reactions.

How does the refrigerator work?

A refrigerator cools food and remains cool even under the blistering heat of Mombasa by employing two principles of gas laws. One is compressing air; the other is depressurizing the air. To do this, the refrigerator has a small unit called compressor at bottom rear that makes a refrigerator to hum so often.

Running from the compressor is a continuous loop of coils, one loop to the inside of the refrigerator, regarded as evaporator and the other to the outside (condenser). The coils to the inside are invisible while those to the outside are exposed.

Gas laws state that if you compress a gas into a small volume the temperature increases and when you expand a gas it temperatures drop. That is why when you pump a bicycle tyre, the tyre and the pump gets warm because you are trying to fit large amount of gas into a small volume.

On the other hand when you decompress a gas or spray an already pressurised gas such as deodorant you get a really cold sensation. This is because the released gas from deodorant nozzle expands and occupies a large volume, cooling down.

The refrigerator compressor pumps a type of special gas called refrigerant or coolant into the coil running to the inside of the refrigerator. As the coil enter the fridge the diameter gradually increases, the refrigerant passes through a valve, evaporates and expands dramatically cooling down. This is known as the Joule-Thomson (or Joule-Kelvin) effect after the physicists who discovered it, James Prescott Joule (1818–1889) and William Thomson (1824–1907).

The second law of thermodynamics states that when a substance at higher temperature is in contact with a cooler substance, the heat flows from the hotter material to the colder one. This is the magic the fridge uses to do the job, the heat from the food in the fridge moves to the colder refrigerant in the coils.

As the refrigerant is leaving the refrigerator it passes from wider tube to narrower rear coil (condenser), gets compressed and turns into liquid releasing the heat to the outside. That is why the rear of the refrigerator is normally warm.

The constant and continuous flow of the refrigerant picks up heat from the inside of the refrigerator and carries it to the outside like a heat conveyor belt. Voila, food is cooled.

Does refrigeration kill food nutrients?

Refrigeration does not kill nutrients. However some foods can change the colour, become soggy or develop dark marks. This is due to different injuries they suffer in the refrigerator but these does not in any way affect the nutrients unless food is stored for unsuitably long periods.

Why does pepper and tomatoes develop black marks in the fridge?

Some fruits and vegetables undergo chilling injury in the fridge. Signs of chilling injury include; surface lesions, discoloration and wilting. Fruits and vegetables susceptible to chilling injury which should not be refrigerated are apples, tomatoes, cucumber, oranges, pawpaw, pineapple, banana, mango, pepper and lemons.

Which food should be refrigerated?

The best guide on foods to be refrigerated is your local supermarket. Take note of foods that are stored at ambient temperature and those refrigerated and do the same at home. The exception arises to some products though. For example packed fruit juices and pasteurised milk should be refrigerated after opening. Check for such instructions on the package.

Common foods that are refrigerated are vegetables, some fruits, milk, yoghurt, meat and meat products and fresh fruit juices.

Can bacteria grow in a refrigerator?

Yes. This occurs if food is stored for longer than recommended time or if the refrigerator is dirty and contaminated with bacteria.

Both spoilage and pathogenic bacteria can grow. If spoilage bacteria grow, they cause food to have objectionable flavour and smell. However, pathogenic bacteria will not cause such unpleasant odour, flavour or texture to the food. Therefore it is not possible tell if these bacteria are present or not. But once that food is consumed it causes illness.

To avoid such instances, observe strict hygiene of the food and the fridge and store food only for the recommended periods. It is also a good practice to clean the fridge at least once a month.

Comments on this entry are closed.

  • JOSEPH MWANGI February 24, 2014, 7:44 pm

    Good notes now i get the meaning of food refrigeration.