Welcome to the solutions page

By Grant Laidlaw

Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. I will endeavour to enlighten. We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in industry is not in place.

GRANT LAIDLAW Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale. He holds a Bachelor of Business Administration and an associate degree in educational administration. He has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018. Currently he is the SAIRAC national treasurer.

Hi Joshua, thanks for the question. When dealing with refrigerated storage, for the purposes of food preservation, we find that food products are generally grouped into two categories:

1. Products in which the living processes continue and on which low temperature exposure has only a retarding effect. The degree of retardation depends on the temperature level. Fruits and vegetables are examples of foods in this category.
2. Non-living substances such as meats, meat products, fish and dairy products. These products are highly susceptible to the activity of spoilage agents. They lack any means of self-preservation and deteriorate rapidly unless drastic preventative measures are taken.

The initial cold storage conditions are determined by firstly looking at the type of food to be stored and secondly, by the length of time such foods are to be kept.

Refrigerated storage may be divided into three general categories:

1. Short-term storage – 1 to 15 days
2. Long-term storage – 15 to 150 days
3. Frozen storage – in excess of 150 days

The storage period in all cases depends on the type of product stored and the condition of the product when it enters the cold store. Some fresh food, such as tomatoes, are damaged by the freezing process and cannot be successfully frozen. To preserve these types of products for longer periods, other methods of preservation must be used.

For short- and long-term storage, products are chilled and stored at a temperature above the product freezing point.

Frozen storage takes place at temperatures far below the product freezing point; usually temperatures between -12°C and -23°C are employed, -18°C is the temperature most frequently used. Optimum storage conditions depend on:

• the nature of the product
• the length of time it is stored
• whether it is packaged or unpackaged

Whether products are to be stored in a frozen or unfrozen state depends mainly on the length of time they are to be kept, which may be for either short or prolonged periods. There are exceptions to this rule as not all uncooked products can be frozen without irreparable damage – apples and potatoes being good examples.

Some fruits and vegetables are susceptible to cold storage diseases when stored at temperatures above or below their critical storage temperature. Prolonged storage periods necessitate freezing of most commodities in which state they can be held indefinitely. Temperatures in this case are maintained considerably below freezing (-12°C to -25°C or lower).

Meats and other non-living substances as a rule present few problems in their preparation for freezer storage, whereas fruits and vegetables often require considerable processing before they can be frozen in order to safeguard them against deterioration while in storage.

STORAGE CONDITIONS

Joshua, with regards to storage conditions, we find that the main cause of deterioration of products such as meat, poultry, fish, fruit, vegetables, cheese and eggs, is the loss of surface moisture by evaporation into the surrounding air. This is really important as the effects can destroy a product, sometimes called ‘freezer burn’. It causes shrivelling and wilting in fruit and vegetables and the product loses mass and vitamins. It causes discolouration and shrinkage in meats and cheeses. Eggs lose moisture through their shells.

Loss of moisture, or dehydration, will occur whenever the vapour pressure of the product is greater than the vapour pressure of the surrounding air. The vapour pressure of the product depends on its temperature – with the higher the temperature, the higher the vapour pressure. The vapour pressure of the air depends on its dew point temperature, which is a combination of dry bulb temperature and humidity. The lower the humidity at a particular dry bulb temperature, the lower the vapour pressure.

An air relative humidity of 100% could represent the ideal conditions for preventing evaporation losses. However, product decay is also at a maximum at these conditions. Thus, controlled air movement and a humidity between 80 and 90% are usually applied in cold rooms.

When products are stored in vapour-proof containers, the humidity and the air velocities are not critical. Hydroscopic products such as dried fruits absorb moisture and require storage at low relative humidities. The rate of evaporation depends on the vapour pressure difference, the amount of exposed product surface areas and the air velocity over the product.

The following tables give storage conditions for perishables. There are complete storage condition charts available, the following being examples to give some guidance. You will see that the relative humidity and maximum storage times are given, not just temperature.

STORAGE CONDITIONS FOR PERISHABLE PRODUCTS

Supplied by Grant Laidlaw

MIXED STORAGE

Joshua, with regards to units that are used for mixed storage we find that in small installations, separate facilities for each and every product are economically not justified, hence, a number of products are usually stored together. Storage conditions will then be a compromise, temperatures usually being above the optimum for some products to prevent damage to the more sensitive products stored in the same cold room. Sealing products in plastic or glass prevents evaporation and thus serves to maintain the quality of certain products.

Higher storage temperatures tend to shorten the storage life of some of the products held in the mixed storage. When products are stored for short periods, however, this is not normally a serious problem. In larger installations, products are grouped for storage and only those requiring similar storage conditions are placed together.

Products which give off odour should never be stored with other products. Dairy products are highly sensitive to this.

PREPARATION

Joshua, some consideration to the preparation of the food stuffs prior to storage is advisable.

One of the most important factors that determine the maximum storage time of a product stored under refrigeration is the condition under which it enters the cold room.

Refrigeration only stops or slows down the natural process of deterioration and cannot restore an already deteriorated product to its original condition. Nor can refrigeration improve the quality of a product. For these reasons, only undamaged products of high quality are suitable for storage. Since deterioration of food products starts immediately after harvesting or slaughtering, it is important that products are cooled to the storage temperature as soon as possible. Products that need to be transported over long distances should be precooled and shipped in refrigerated transport.

Joshua, other aspects of preparation include, chilling, blanching and the ageing of meat.

Product chilling refers to the rapid cooling of a product from harvesting or slaughtering temperature to the storage temperature. Rapid chilling and high air velocities are desirable during this time so that the temperature and vapour pressure of the product are lowered as quickly as possible to avoid excessive moisture loss and shrinkage. High air velocities carry away evaporated moisture and prevent the recondensation of moisture on the surface of the product. High velocities also greatly increase the heat transfer rate. The humidity should be high when chilling to prevent excessive moisture loss. Blanching is accomplished by immersing the product in boiling water or steam for a short period, the length of which depends on the nature of the vegetable. It ranges from one minute for hulled peas to 10 minutes for corn on the cob.

Both vegetables and fruit must be cleaned and washed to remove dirt, insects, juices and any other foreign matters before chilling or freezing. Vegetables are then blanched. Most micro-organisms are destroyed by this process. However, some bacteria may survive and to prevent spoilage by these bacteria, the product should be chilled in cold water (about 10°C) immediately after blanching and then dried thoroughly before packing.

Blanching of fruit is not practiced. To do so would destroy its fresh quality, a characteristic that is highly valued.

Joshua, as a general rule, meat products do not require special processing prior to freezing. Pork and fish must be frozen immediately after chilling. This is because of the instability of the fatty tissues. Beef is often allowed to hang in a chiller room for a couple of days before freezing. During the ‘hanging’ period, the beef is tenderised by enzymic activity.

Beef carcasses must not be aged or ‘hung’ for excessive periods as this will affect their storage life. Poultry must be aged for 12–24 hours before freezing. Ageing makes its meat more tender than if it were frozen immediately after slaughtering.

FROZEN STORAGE

The exact temperature for frozen storage is not critical, provided it is low enough and does not fluctuate. About -18°C for short term and -21°C for long term is recommended. The temperature should remain as constant as possible during the whole period of storage. Some products are particularly sensitive to temperature changes. The accuracy of automatic equipment is generally in the range of 1 to 1.5°C, which is acceptable.

As many packaging materials do not offer complete protection against dehydration, the relative humidity should be kept at 85 to 90% in frozen storage rooms. This is particularly important for long-term storage.

Proper stacking of products is also important. Adequate air circulation must be maintained around the product. Space must also be left between the stored product and the walls of the storage room in order to eliminate the possibility of the product gaining heat directly from the walls.

Joshua, I hope that this increases your understanding of the basic aspects of food storage. As you can see it is not just the temperature that is important.

Thanks to everybody for the overwhelming response. I receive an average of over 60 questions a month and cannot answer all of them. But keep them coming, as I may answer you directly. Looking forward to hearing from you.

Register for free to gain access the digital library for RACA Journal publications