Parameters for Internal Food Conditioning

The processes of food spoilage depend largely on the temperature at which it is stored. The growth of microorganisms decreases at low temperatures, but it should be noted that many microorganisms do not die even at the lowest temperatures used for cold preservation, so they begin to multiply again as soon as food reaches higher temperatures again.

In order to slow down the food decomposition process as much as possible, optimal storage conditions such as temperature, relative humidity, air circulation and composition of the chamber atmosphere are sought.

Alfredo Alvarez Cardenas

The processes of food spoilage depend largely on the temperature at which it is stored. The growth of microorganisms decreases at low temperatures, but it should be noted that many microorganisms do not die even at the lowest temperatures used for cold preservation, so they begin to multiply again as soon as food reaches higher temperatures again.

The processes that cause the deterioration of food are of character:

• Physical • Chemical

• Biochemical • Microbiological

Among the physical processes, the most outstanding is that of weight loss due to water evaporation, which can also cause losses of volatile components that greatly condition the aroma and flavor of the products.

Chemical processes that are given by reactions such as the oxidation of fats, cause rancidity in the products.

The action of enzymes are examples of biochemical processes. A typical test is the action of the enzyme polyphenoloxidase, which causes darkening in certain foods.

Microbiological processes are given by the action of certain pathogenic microorganisms that cause irreversible deterioration in food products.

To stop the action of these processes, storage conditions are sought that delay the deterioration of the products, among them are temperature, relative humidity, air circulation and composition of the atmosphere of the chamber.

Of these, temperature is the factor with the highest incidence. As the temperature decreases, all the processes causing the deterioration are diminished, which results in the prolongation of the shelf life of the stored products.

As relative humidity increases, water evaporation decreases as the gradient for transfer decreases; however, this benefits the development of microorganisms.

The relative humidity may be higher to the extent that the temperature is lower. However, this storage temperature has limits based on the possible influence on the product, as well as on an economic analysis, so it exerts a strong influence on the preservation of cold stored food.

Air movement also influences quality and preservation in refrigeration, freezing and storage; and as far as weight loss is concerned, water evaporation takes place rapidly with air circulation.

Similarly, in the maintenance of internal conditions have great influence the intensity and duration of the rotation of the products, the changes in concentration of the gaseous compounds inside, and above all, the criteria of storage compatibility, which depending on their nature, can be applied for the internal management of different species or variety of food.

Temperature control is the main means of minimizing microbiological activity

Importance of cold storage systems

The International Cold Institute (IIF) defines a cold store as a space intended for the storage of certain products (those foodstuffs especially of a perishable nature), to defined regimes of temperature and hydrometry, sometimes even in artificial atmosphere, which are not obtained in ordinary warehouses without insulation or refrigeration facilities.

The refrigerated storage of food, or other perishable products, has the following purposes:

• Minimize losses from these products

• Preserve its initial quality and health status

• Extend its distribution and consumption period without altering its initial state (reach more distant markets, availability at any time of the year)

• Consequently, to promote trade

Contrary to what happens with most other conservation procedures, cold does not transform food substances, but delays, more or less energetically, the biological phenomena of degradation and in the case of living products, the physiological evolutions that take place in these.

If refrigeration protection is lacking, evaporation and biological alteration losses frequently range from 20% to 50% of the initial total volume, according to the nature and alterability of the product, time and environmental conditions. By dramatically reducing the rate of degradation and drying of products and preserving their health status, cold is a means of increasing the availability of food, both for domestic consumption and for export to foreign markets.

In a cold store, considered as a system, there are simultaneous interactions of thermal and mass transfer phenomena, whose intensities are dependent on the effectiveness in the regulation and control of the internal conditions of the cold environment, manual or mechanical handling in the accommodation of the products, the constructive characteristics (with emphasis on the insulating material), among others. (see Figure 1):

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Among the phenomena of thermal transfer are: heat flow from the product to the cold medium, heat flow from the cold medium to the surface of the evaporator, heat flow through the walls, floor and ceiling, heat flow through door opening, etc.

Examples of mass transfer are: weight loss of the product due to evaporation or sublimation phenomena into cold air (due to inadequate regulation of relative humidity), flow of water vapor through the walls, floor and ceiling (hence the need to place screens or barriers to water vapor) that can cause irreversible damage by condensation and / or freezing of water in construction materials, etc.

Internal conditioning parameters

The main conditioning parameters that influence the shelf life of perishable products in cold storage systems are (see Figure 2):

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1. The storage or storage temperature

2. The speed and amount of air moving inside the warehouse

3. The relative humidity of cold air

4. The way in which the products are accommodated or distributed internally (type of stowage, use of converters, etc.)

5. The frequency of air replacements by door openings (product rotation)

6. How to clean or wash indoor air to maintain proper gaseous conditions

7. The incompatibility of preservation of different food species in the same cold space

Temperature

Temperature control is the primary means of minimizing microbiological or enzymatic activity in food. The temperature influences differently on each particular reaction, hence the need to establish a timely scale of values to be adopted depending on the characteristics of the product.

Indeed, the selection of the optimum temperature cannot be the same for different species, just as it cannot be the same for all varieties of the same species, and in that area it can also vary depending on various factors, with the state of maturity being the most appreciable.

For the control of the optimum storage temperature, the effect of:

• The minimum temperature

• Differentiated or alternating temperature

• The rate of temperature decrease

Both the air and product temperatures must be permanently regulated and controlled. For temperature control, the thermostat sensor element must be placed somewhere near the center of the warehouse or on the return path of the air to the evaporator.

It is also convenient to maintain a continuous review of the work of the evaporator, regularly measuring the evaporation temperatures and both the inlet and outlet side of the air.

Speed and amount of air in motion

These parameters have a direct influence on the speed of removal of the thermal load and on the speed of abatement or maintenance of the temperature.

The storage of fruit and vegetable products is of particular importance because of their continuous production of heat, carbon dioxide and water vapour, because the phenomena of respiration and transpiration cause continuous changes in the conditions of temperature and relative humidity, and in the composition of the gas in the atmosphere in the room where they are preserved.

To keep the ambient conditions stable and uniform at any point in the warehouse and to subtract the heat emitted by the product, active air circulation is necessary.

The speed and amount of air in motion depends on the following factors:

• Amount of heat emitted by the preserved product

• Internal dimensions of the warehouse

• Modality or pattern of stinging or other form of product accommodation

• How quickly you want to cool the product

• Air distribution method

An important decision is to define whether to size small or large capacity warehouses. Large warehouses are more economical, as soon as the construction costs per unit volume are lower, but in these it is easy to find irregular ventilation and non-uniform temperatures, they are difficult to fill in a short time and there are irregularities in the accommodation of the product. On the other hand, small warehouses are inconvenient both from a constructive and administrative point of view.

In addition, the shape and dimensions of the refrigerator are related to the rate of dehydration of the product. In proportion to the amount of products they contain, in a small warehouse more heat penetrates than in a large one, because its volume increases more than the area of its surface. This is why a large warehouse is more likely to offer better storage conditions than two small ones of the same capacity.

The following figure (see Figure 3) shows examples of air distribution in relation to the accommodation of products inside the cold warehouse.

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To minimize the passage of heat through the insulation, the ideal shape would be a cube, which combines the largest storage space with the smallest surface area. Products can be stowed up to heights greater than 8m, using appropriate forklifts; the limits to the heights of the warehouses are determined by the means of internal handling of the product (rotation of products) and by the use of containers, packaging, pallets, etc.

It must be taken into account, on the other hand, that the height of a warehouse is equal to the height of the stowage of the products plus a free distance of 0.8 to 1.2 m. This free space is essential for good circulation and distribution of air at the top of the stowage.

The length and width of the warehouse shall depend on the expected operating conditions and the type of internal cooling equipment adopted.

In order to maintain optimal air circulation with the consequent uniformity of internal conditions and, to facilitate the internal sizing of warehouses, the following free spaces should normally be taken into account:

• Between wall and stowage: from 0.20 to 0.40 m.

• Between stowage and stowage: from 0.10 to 0.20 m.

• Internal aisle: depends on the internal handling mode of products.

In relation to the air distribution method, the following operating conditions are recommended:

to. Small and medium cameras:

• Distribution by DIFFUSION

• Air velocity: 1-2 m/s

• Air mass equivalent to 10 times the volume of the chamber per hour.

b. Large cameras:

• Distribution by PIPELINE

• Air velocity: 2-4 m/s

• Air mass equivalent to 15-20 times the volume of the chamber per hour.

Convection heat exchanges can be reduced by using a wide strip curtain of flexible and thick plastic

Hygrometric degree

The control of the hygrometric grade of a cold store is a problem of particular importance because of the technical and economic effects it implies, but in most cases, this control is neglected by the technicians perhaps for a double reason: because it is complex to carry out the control of the facilities or because they ignore the importance and negative effects on the preserved product.

The relative humidity is inversely proportional to the temperature, because decreasing the temperature increases the relative humidity and the saturation or dew point (condensation of water vapor) can be reached, so it is enough to have any point in the environment that has a sufficiently low temperature to cause the phenomenon of condensation. In a cold store the evaporator represents the cold spot.

During storage, the relative wetness must be considered in relation to the nature of the product. With the condensation of water vapor in the evaporator, an imbalance is caused between the vapor pressures of the cold air and the vapor pressure of the product, the latter being the one that yields water vapor to reach the equilibrium point again, the phenomenon of weight loss by evaporation or sublimation is carried out.

Weight loss in perishable food products is due to factors such as:

• Physical: by variations in temperature, relative humidity and air circulation, individually or jointly.

• Physiological, in the case of fruits and vegetables, by breathing and by transpiration.

To avoid or minimize the above effects, the regulation of the hygrometric degree can be carried out by two methods:

1. By direct regulation using humidifiers.

2. By indirect regulation, considering the difference in temperatures between evaporation and storage.

To obtain a high relative humidity in a cold store, the evaporation temperature of the refrigerant must be as close as possible to the temperature of the air in the warehouse, because the greater the temperature difference between air and evaporation, the air will lose humidity and the hr will decrease.

This effect is shown in the following table:

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Internal product accommodation

The way products are accommodated inside the chamber is one of the most important aspects when handling any packaged or packaged product. So the height of accommodation or stinging is defined by the load limit, depending on the following considerations:

• Resistance of packaging. It should be noted that refrigerated products are more fragile than frozen products; if the packaging is on pallets, the maximum height of the loaded pallet must not exceed 1.70 m, if there is a danger of crushing, the pallets are equipped with converters (metal frames that can be removed, located around the load and that transmit the pressure of the upper pallets to the lower ones)

• Driving mode. If it is manual, loading and unloading are all the slower and more dangerous for personnel the higher the stowage, the maximum height is usually limited to about 2.5 m; on the other hand, if it is mechanical (forklifts) the operations are much faster and the height that the stowage can reach is that of the load limit of the forklift.

• Of the stable thermal regime (mode of conservation). In refrigerated products it will be necessary to leave enough space for air circulation, however, for frozen products the stowage can be more compact.

• Of the cooling or freezing regime.

• The use of large metal structures "racks".

To keep the ambient conditions stable and uniform at any point in the warehouse, active air circulation is necessary.

Air changes by opening doors

It is common to leave the door open during the operations of entry and exit of products. It should be borne in mind, however, that the heat exchange by convection increases with the duration of the opening, the dimensions of the door, the internal volume, the difference in exterior and interior temperature, etc., which causes an additional increase in the thermal load that can hardly be compensated by the cold production equipment, still in continuous operation, and consequently causes an uncontrollable rise in storage temperature.

Convection heat exchanges can be reduced by using a wide curtain of flexible and thick plastic strips, placed on the inner side, covering the door span; also placing on the outside of the door, a descending air curtain.

In practice, it is appropriate to:

• Organize the handling of products in such a way that the inputs and outputs are made by important items

• Clearly inform manipulators of the need to close doors, even for short periods

• Check door gaskets and threshold weatherstripping

• Change the curtain strips as soon as they are torn

• Ensure that the resistance of the heating mechanism of the door joint is continuously in service (only in freezing)

• Never delay maintenance operations.

Influence of gaseous compounds on conservation

The gaseous compounds that have a negative influence on the conservation of fruit and vegetable products at low temperatures are, among others:

• Carbon dioxide (combined with low oxygen concentration, modifies the internal atmosphere)

• Water vapor (weight loss in the product itself and excess in the air favors the development of microorganisms)

• Volatile compounds: ethylene (accelerates maturation)

• Odorous compounds: aromas (has an influence on the ripening of the fruit and the appearance of physiological alterations)

The degree of concentration of these compounds in the atmosphere can be regulated by refills and washes of the internal air.

Also the presence of these compounds suggests alternatives or possibilities of storing different species in the same environment, and what inconvenience such a mixture of species could cause (see Figure 2).

Peach, apples, pears, grapes.

Some are ethylene producers

Susceptible to moisture loss

Producers or sensitive to ethylene

Produce or absorb odors

In all perishable foods, the compatibility between temperature, relative humidity, odors, special atmosphere will have to be analyzed. Some cases of compatibility of storing different food species in the same cold space are exemplified below (see Figure 3).

Pig-egg

Lagosta-fish

Grapes-vegetables

Apple-orange

Egg-vegetable

Fish-apple

Cheese-grape

Fish-egg

Orange-pork

Egg-cheese

Fish-cheese

Cabbage-butter

Egg-orange

Potato-lobster

With all these elements we can summarize that the processes of deterioration of food depend to a large extent on the temperature at which they are stored. The growth of microorganisms decreases at low temperatures. It should be noted, however, that many microorganisms do not die even at the lowest temperatures used practically for cold preservation, so they begin to multiply again as soon as the food returns to higher temperatures. Relative humidity exerts a strong influence on the preservation of cold stored food. The weight loss by evaporation decreases with increasing relative humidity of the storage air, being proportional to the difference between the partial pressures of water vapor in the air and on the surface of the stored product. Weight loss can be substantially decreased by packaging the products. On the other hand, high relative humidity favors the multiplication of microorganisms, especially at high storage temperatures. Air movement also influences quality and preservation in refrigeration, freezing and storage. As far as weight loss is concerned, water evaporation takes place quickly with air circulation. Similarly, in the maintenance of internal conditions have a great influence the intensity and duration of the rotation of products, changes in the concentration of gaseous compounds inside and above all, the compatibility criteria of storage, which depending on their nature, can be applied for the internal management of different species or varieties of food.

Authors:

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