Lesson 24. STORAGE OF FROZEN FOODS

Module 2. Food freezing

Lesson 24

STORAGE OF FROZEN FOODS

24.1 Introduction

Food and dairy products are perishable in nature. They should be stored at low temperatures in order to increase their shelf life. Cold storages and deep freezers are used for this purpose. If the temperature in the room is up to 0oC it is generally known as cold store and if temperature is 0oC to – 22oC it is known as deep freeze.

Food, on being stored, may get spoilt by three mechanisms:

· Living organisms may feed on the food and contaminate and spoil it

· Biochemical activity within the food itself (e.g. respiration, staling, browning, rancidity development, etc.) may in time diminish its quality & usefulness

· Physical processes (e.g. bursting & spillage of the contents of the package) may have the same effect

The three main factors of the storage environment that influence the storage life of a particular commodity are the temperature, humidity & the composition of the store atmosphere. In addition, rough handling, careless packing or unsuitable packaging can reduce storage life.

24.2 Temperature

The rate at which biochemical reactions occur in food increases with increasing temperature. The logarithm of the reaction rate is a linear function of temperature. The lower the storage temperature, the more slowly do foods suffer degradation by those biochemical spoilage reactions listed above. In addition, the rate of growth of bacteria is reduced as the temperature falls and low temperature storage (frozen storage) has some bactericidal effect. Most insect activity is inhibited below about 4oC although some insect species and insect eggs are capable of surviving long exposures to those temperatures.

From all this, it might be inferred that a reduction in storage temperature inevitably results in an extension of the storage life. Foods containing water will freeze, if their temperature is lowered much below 0oC, the actual freezing temperature depending upon the nature of the aqueous solution in the food. The act of freezing and thawing alters the food, sometimes very extensively and the properties of the ‘fresh’ product such as fruits and vegetables are adversely affected should they freeze, the cans may burst and if such foods contain gels or emulsions, they may break down.

Even if held above the freezing temperature, deleterious physiological changes can result in the spoilage of fresh fruits and vegetables. Apples stored close to their freezing point can exhibit forms of injury not observed at slightly higher temperatures. Soggy or low temperature breakdown, internal browning, brown core or core flush result from storage at 0oC, but are not a problem at 3-4oC.

Chilling injury can be observed in a variety of tropical fruits & vegetables. There is evidence that ripe tomatoes can be stored at lower temperature than green tomatoes which can suffer cold injury below 7oC resulting in abnormal ripening and susceptibility to rots. Storage of potatoes below 3oC is undesirable as they are susceptible to chilling injury. However, even above this temperature there can be a change in the starch – sugar balance leading to the accumulation of sugar in the tissues making the tubers unsuitable for processing into chips or crisps. Storage at 10-13oC is recommended to avoid this.

Another factor to be taken into consideration is the cost of storage when storage temperature is lowered. To the prime cost of foods put into storage must be added the cost of maintaining them under the chosen storage conditions. Thus, stored foods have increased costs even if they do not increase in value. Storage at below ambient temperature is more expensive than unrefrigerated storage & lower the storage temperature, higher the cost. Hence, storage temperatures need to be no more elaborate than those required to maintain the stored commodity in good condition up to the time of use.

This concept has been applied particularly to the storage and distribution of frozen food products. Here, provided the products are suitably packaged, the deterioration is almost entirely a function of temperature.

Three factors influence the choice of the storage temperatures for a particular commodity:

· The temperature dependence of the rate at which spoilage processes occur

· The risk of cold injury to the commodity

· The economic balance between storage costs and the maintenance of product quality

24.3 Storage Humidity

If the humidity of a store atmosphere is below the equilibrium relative humidity of the food being stored that food will lose moisture to the atmosphere. Conversely, if it is above ERH of the food, the latter will absorb water. Thus, ideally the RH of the store atmosphere should be adjusted to the ERH of the stored product, for instance, granulated white sugar has a moisture content of about 0.02% & an ERH of about 60%. If the moisture content rises to 0.06%, the sugar is in danger of caking. Therefore, storage RH below 60% is recommended for this product. Conversely, brown sugar has a moisture content of 4% which must be maintained if the product is to retain a workable texture. So RH of 60-70% is recommended for long term storage. Again, peanuts become brittle and may split due to dehydration if the storage RH is below 70% and it is liable to mould spoilage above this RH. Thus, as with temperature, there is often an optimum condition for storage.

The ERH of a product has a considerable bearing on its vulnerability to microbial attack. When expressed as a fraction, instead of percentage, the ERH is known as the water activity of the material. Bacterial growth may be a major problem in foods with a high water activity, for instance, fresh fruits & vegetables, meat and fish. But in materials with lower water activity, it is the fungi that cause the most trouble. Insects can also flourish in foods with comparatively low water activity.

Packaging can be used to isolate the environment of the food from the store air and so allow foods to be held at ERH in storage atmospheres of undermined relative humidity. Occasionally, treatment of the surface of foods achieves the same purpose. Thus, shell eggs lose water freely and require high humidity environment if they are not to experience excessive weight loss. This may lend microbial growth on the egg shell and consequent tainting of the egg. Treating the shell with oil improves its water vapor barrier properties and allows the humidity of the store to be lowered, so avoiding more growth.

Recommended levels of store RH have been published for many products.

24.4 Composition of Cold Store Atmosphere

A variety of food materials are advantageously stored in atmosphere different from normal air. The most noteworthy example of this occurs in the refrigerated storage of fruit. Fresh fruits respire, taking up oxygen and evolving carbon dioxide. The rate of respiration can be reduced by cooling so extending storage life, but can be further reduced by storing the fruit in an atmosphere richer in carbon dioxide and poorer in oxygen than normal air. In the case of apples and pears, particularly worthwhile increase in storage life is possible and this technique which is known as controlled atmosphere or gas storage is in extensive commercial use. The oxygen and carbon dioxide levels used vary markedly between varieties and are controlled to optimum values, since too great a modification of the storage atmosphere can lead to secondary spoilage. This technique is used extensively for the storage of fruits such as apples, pears and oranges and is receiving some attention for improving the quality of vegetables in storage and transportation.

When strawberries and raspberries are cooled and transported in an atmosphere containing about 20% by volume of carbon dioxide in air, the development of fungal rots and ripening is delayed. Storage in carbon dioxide was at one time also used with meat. Chilled beef was shipped to England from Australia in an atmosphere of 10% carbon dioxide in air approximately doubled the life obtainable up to that time.

If grain is stored in an air tight silo, insects, mites and moulds can be controlled by the atmosphere they generate. These organisms use up the oxygen in the enclosed space and asphyxiate or suffocate themselves before becoming numerous enough to cause damage. Dry grain can be stored in this manner with little loss of its functional properties. However, wet grain will lose its power of germination, making it unfit for seed or malting and develop a taint which is transmitted to baked goods made from it – though it remains suitable for animal feed. Air tight storage is more satisfactory for non-viable food materials sufficiently dry to be protected against microbial growth, for example, ground roast coffee. In such cases, oxygen may be excluded from the container or package by replacing the air in it by inert gases like nitrogen or carbon dioxide or by a vacuum.

24.5 Odors and Taints

Stored produce may pick up foreign odors and flavors from other food stuffs stored with it or from inappropriate packing materials or from storage chamber and environment. Foods with strong odors, like spiced meat, smoked fish, citrus fruits, etc and spoiled meat and fish are likely to cause tainting. Packaging materials may either themselves contribute odors or be contaminated during production. Adhesives and printing materials may also cause trouble. Finally the constructional materials of the store may become contaminated or taints may be absorbed from vapors entering the store from outside, while such taints do not alter the nutritional status of the food, its commercial value can be seriously affected.

Fatty foods are particularly liable to absorb odors. Butter is very sensitive to tainting and meat shipped to England has been found to be tainted by diesel and fuel oil fumes, smoke and fruits, particularly oranges. Eggs will also pick up taints fairly easily.

The most satisfactory method of avoiding problems of tainting during storage is to avoid taint absorbing foods in the presence of odoriferous materials. Suitable packaging can help to preserve food untainted and activated charcoal and ozone have been used for removing odorous volatiles from food stores. Unfortunately, the concentrations of ozone for efficient action are toxic and so require special care, both in their use and in the purification of air in the chamber afterwards. To be efficient, after these treatments, the surfaces of the chamber and the refrigerator should be thoroughly cleaned treating all surfaces with a disinfectant. It is worth remembering that these processes are all the more effective when the chamber temperatures are high enough.

24.6 Light

The UV in the Sun Rays will quickly impart a taint to the butter and milk by oxidizing their fats, potatoes exposed to light turn green due to formation of chlorophyll. In storage buildings, the effect of light on produce is normally unimportant since daylight does not penetrate into them and a low level of artificial light is provided. Where high intensity lighting from fluorescent tubes is used to display foods, the UV rays from such lights are more intense than those from tungsten filament lighting and have been shown to be sufficient to oxidize fats, bleach colors and green the potatoes. Packaging in colored plastic film or the use of color filters on the light fittings will reduce the effects but are not satisfactory from commercial point of view.

24.7 Variability in Storage Conditions

Both spatial and temporal variations may be found in the conditions in a good food store. Temporal variations may either be transient, resulting say from recently introduced material coming to an equilibrium state in the store – or they may be periodic in nature and a permanent feature of the storage situation. The main causes of variability in storage conditions are:

· The equilibrium of the products to storage conditions

· Respiratory activity in fruits and vegetables

· Variation of climatic conditions external to the store

· Fluctuations in the performance of refrigeration & other equipment designed to maintain the storage conditions at the desired levels

· The activities of operating personnel

The effect of such variations is often complex, inter-related changes occurring in the temperature, humidity and atmosphere composition throughout the store. The magnitude of spatial variations in storage conditions are considerably influenced by the mode of transfer of heat and gases within the stack of stored material. If the store air does not move through the stack diffusional mechanisms and thermal conduction predominate and larger variations will be observed than when the store air moves and promotes additional convective transfer. Thus, in order to maintain uniform conditions throughout the store, such air movement is desirable. The produce must therefore be stacked in the store that air flow through the stack is facilitated.

Air circulation through the stack may either be fan assisted or due solely to the thermally generated density gradients in the air. The latter system is more frequently used today because the cooling systems are less bulky and cheaper. The most frequently used forced convection systems use a cooling system and fan located in separate enclosures connected to the store by delivery and return air ducts located in a floor mounted vertical duct or built into a compact unit which may be suspended from the ceiling of the store. These units use fin tube heat exchangers.

It is sometimes advantageous of the fan and duct systems in forced ventilation stores are so constructed that the direction of air flow can be reversed. Periodic air reversal can reduce the spatial temperature variations during the initial cooling of a room full of, say, fruit to strange temperature.

24.8 Maintenance and Control of Storage Conditions

Temperature

Refrigerated storage rooms are thermally insulated to reduce heat leakage. The insulation is normally fixed to the structural walls, ceiling and the floor of the room, though pre-fabricated panels have recently become more commonly used in conjunction with a steel or concrete load bearing framework. Insulation is sometimes desirable in unrefrigerated storage. For instance, root vegetables and potatoes can be stored in clamps or barns using either earth or straw as an insulating material. The purpose of the insulation is to minimize the effects of sudden ambient changes on the produce and so prevent frost damage. Canned foods may be transported in insulated vehicles for the same reason, while too little insulation would lead to high refrigeration costs and over insulation would initially be more expensive and will reduce the available storage space in the building, cabinet or container.

Another important source of heat leakage into a refrigerated store is air exchange at the door. This exchange is reduced by a variety of methods. The time the door is open may be minimized by automatic opening and closing mechanisms, double doors may be installed forming an air lock or an air curtain fixed above the outside of the doorway. This latter consists of a fan and a duct system that directs a stream of the external ambient air downwards and slightly outwards in a sheet across the door opening.

If the refrigeration requirements for a store are to be estimated, heat generation within the store and the cooling of the stored produce must be taken into account as well as heat leakage into or out of the store.

The magnitude of the heat load from all causes, except wall leakage and heat generation by the stored produce, during conventional store operation is approximately given by 0.003 V0.6oC difference between the inside and outside of the room and V is the volume of the room in m3. kJ/s for every

Important Features of Construction of Cold Stores

Air lock room

At entrance we have air lock room. This is lightly insulated room having two insulated doors with automatic door closer arrangement. One door opens to outside and the other opens inside the cold store/deep freeze.

Insulated doors

Cold store and deep freeze have special doors. These have sturdy frame grouted in RCC. Door has rubber gasket all around to make it air tight. It is made of teak wood battens and frame with two layers of thermocole of 5 to 15 cm thickness. Externally it is cladded with aluminum sheet. The door handle is catch operated from both inside and outside. The deepfreeze door has heater in the frame. Cold store doors in the dairy plant are having special features like, long handle, soft wood, long hinges, insulation, special locking system, defrosting mechanism, air curtains etc. These doors are known as patch type of door.

Air curtain

With a suitable blower, we form a curtain of air between hot outside air and cold inside air. These are installed generally at main door where chances of infiltration of atmospheric air are more.

Lighting and emergency bell

Cold store should be provided with an emergency bell switch. There should be proper lighting inside. Emergency may arise at a time when there is no power supply. Power inverters of suitable range are available to connect lighting and bell circuits of cold store.

Insulation

Expanded polysterene (thermocole) is used for insulation. For cold store 10 cm thick and for deep freeze 15 cm thick insulation is provided. For floor heavy density and for walls and ceiling normal density thermocole is used.

Syphon lock for air in defrosting drains

The defrosting drains should have water siphon seal (U shape) for barring outside air to form convectional current with cold inside air. The drain pipe from diffuser drain section should slope outside.

Protection to insulated walls and door

Guard railing is provided all along the walls for protection of wall insulation. Kota stone dado for better cleanliness may also be provided to avoid maintenance problems of painting up to 2 m height of wall.

Flooring

For can cold stores cast iron tiles floor is provided. Alternate panels should be cast with flexible joint filler to withstand thermal shocks due to rise and fall of temperature.

Diffuser unit

In dairy cold rooms we use high velocity, low temperature air for producing refrigeration effect. For this purpose packaged blast freezer i.e. diffuser unit is used. This unit has three main sections viz. fan section, coil section and drain section. For fan section, self aligning dry lubricated sealed bearings are used. Bearings are packed with soap stone or Teflon as dry lubricator. In some cases low temperature servogem-2 (N.P.2) grease may also be used.

Defrosting

The product moisture and cold store’s air moisture condenses and freezes at diffuser coil, it being the coldest point. This frosting ultimately restricts the air circulation through the coils there by increasing the cold room temperature. Frosting will also restrict proper heat transfer from refrigerant to air, there by reducing the plant efficiency. This may also lead to wet compression which is dangerous. Therefore defrosting should be done periodically.

  • When diffuser motor ampereage reduces to 75% of the actual running current, the unit should be defrosted.
  • Defrosting can be done by hot water spreader pipe arrangement after shutting down the plant. Alternatively hot gas defrosting can be done by running the compressor on reverse cycle by manifold or crossover system

Receiver

· Horizantal cylindrical tank with level indicator, oil air purging valves, safety valve etc.

When the plant is in run - 1/3 full

When the plant is idel - ½ full

Expansion valves

· Direct expansion valve/ TEV is mostly used in small capacity plants employing freon group of refrigerants.

· Low side float valve/ Hand expansion valves are used in conjuction with accumulators in large industrial plants employing ammonia

Controls

· H.P and L.P cut out

· Oil failure switch

· Thermostat or ice thickness controller

· `Solenoid valve

· Back pressure regulating valve

· Safety valves

Note:

  • L.P cutout is set at 1.19 kg/cm2 pressure for NH3 system (IBT, cold store etc).
  • H.P cutout for condensing temperature of 30 oC is set at 11.2 kg / cm2 and for 40oC condensing temperature is set at 15.4 kg/cm2.
  • Thermostat bulb is kept 5 cm (2”) away from coil surface and set at a temperature of –1 to –2oC.

Recent Developments in cold stores

· PUF panels.

· Walk in coolers

Recent Developments in insulation techniques

· Mass insulation: By replacing a shorter path of heat flow of lower resistance with a larger heat flow path of higher resistance.

· Reflection insulation.

· Vacuum insulation

· PUF- excellent vapour barrier so does not require any other vapour barrier.

The space required for storage of different dairy products

  1. Milk cold store (cans) - 360 lit/m2
  2. Milk cold store (pouches) - 400 lit/m2
  3. Milk cold store (bottles) - 250 lit/m2
  4. Butter cold store/ deep freeze - 100 kg/m2
  5. Cheese ripening in cold store - 250 kg/m2

24.9 Problem on Cold Stores

Example 20.9.1

Determine the inside wall temperature and rate of heat gain through the wall of a cold store maintained at -15oC. The walls are of 15cm cork with 2 cm plaster on each side and outside temperature is 30oC. Inside and outside heat transfer coefficients are 9,3 W/m2 K. Thermal conductivity of cork is 1.16 W/m K and that of plaster is 11.6 W/m K.

1

Example 24.9.2

Calculate the size of the compressor required for a 6m x 3m x 6m (outside dimensions) of refrigerated room, having 10cm of cork insulation. The room is to be maintained at 0oC. The outside air is at 32oC dry bulb temperature and relative humidity is 60%. The product is 1000 kg milk at 31oC to be cooled to 1oC in 24 hrs/ The electric load in the room is 200 watts.

Solution

The surface area of the room is 90m2.

From the table we take that for a temperature difference 32oC, the heat gain for 10cm cork insulation is 1838 kJ/m2 day. The wall heat gain is 90 x 1838 kJ/day = 165420 kJ/day. Assuming that the walls are 0.3 m thick, the internal volume of the room is 5.4 x 5.4 x 2.4 = 69.98 m3(approx. 70 m3).

Also from the table, number of air changes per day for 70 m3 is found to be 14 and heat required to cool the outside air is 93.4 kJ/m3. The air change load is 70 x 14 x 93.4 = 91532 kJ/day.

The specific heat of milk is 3.89 kJ/kg K.

The product load is 1000 x 3.89 x (31-1) = 116700 kJ/day

The load due to electrical equipment in the room is 200 x 3600 x 24/1000 = 17280 kJ/day

Parameter

kJ per 24 hr

Wall gain

165420

Air change

91532

Product

116700

Electrical

17280

Calculated load

390932

10% safety

39093

Total load

430025

For 16 hr operation of the compressor, the load becomes 430025/16 = 26877kJ/h

The size of compressor needed will be 26877/12560 = 2.14 TR (1 TR = 12560 kJ/h)

Last modified: Friday, 12 October 2012, 8:45 AM