2 Principles of low temperature storage of fruits and vegetables

Mr. Wridhama Prasad

1.0 INTRODUCTION:

Post harvest handling of produces has a major role to play in determining their quality and life. About 20-25% of the total agricultural production of India is lost as post harvest losses. Various attempts, processes have been devised to reduce these losses to minimum level; further newer technological interventions are harnessed to help in achieving the same. Of all the treatments intended to reduce these losses, low temperature treatment (including cold chain) appears the most important and also is commonly employed but, due to large geographical area it has not been able to reach all the harvest sites.

2.0 HARVEST TIME:

Heat within the produce comes from convection (from the air surrounding it), radiation (mainly from the sun) or as metabolic heat (from metabolic reactions within them). Of the three sources, radiation and convection being external in origin, thus occurs during day time, while metabolic heat being inherent to the produce, heats the produce throughout its life. Thus, produce harvesting should be performed early in the morning, since the sun has not been able to warm the air or the produce, also the produce will have a lesser heat due to losing its heat to the surrounding air in the night.

3.0 TEMPERATURE DEPENDENCE OF CHEMICAL REACTIONS (Q10 value):

Rate of reaction varies with the temperature at which it occurs. Q10 value at T + 10oC is defined as the ratio of rate of reaction at T + 10oC to the rate of reaction at ToC. Thus, rate of reactions can be altered with variation in its surrounding temperatures.

For example, enzyme catalyzed reactions have Q10 value 2 i.e., the rate at 30oC is twice the rate at 20oC, and thus, lowering the temperature from 30 to 10oC reduces enzyme catalyzed reaction (such as respiration, micro-organism growth) rates by a factor of four.

4.0 LOW TEMPERATURE STORAGE (CHILLING TEMPERATURES):

Storage at temperatures above freezing point (of produce) and below 15oC is often referred to as refrigerated or chilling storage. This is widely used for short-term preservation of the produce.

4.1  Desirable Consequences of Chilling Temperatures:

4.1.1 Growth of micro-organisms:

Temperature has a profound effect on growth of micro-organisms. Below 3.3oC pathogenic micro-organism can not grow. Psychotropic micro-organisms have the ability to grow in the range of 0-15oC, but their growth at these temperatures is very slow than at temperatures 15-45oC. Chilling temperatures therefore substantially retard spoilage caused by micro-organisms.

4.1.2 Metabolic activities:

The storage temperature has a major effect on the ripening rate of the produce. With the decrease in storage temperatures, the rate of ripening also decreases and at temperature below 4oC ripening almost ceases.

4.1.3 Deteriorative chemical reactions:

Loss of quality resulting from physiological activity and other chemical reactions (such as enzyme catalyzed browning or oxidation of lipids and color degradation) are effectively reduced at chilling temperatures.

4.1.4 Thermal disinfestations:

Exposing produces to low temperature inactivates many and kill some insect-pests, and thus this can be used to control infestations of insects such as fruit flies, etc.

The above changes ultimately delays senescence and decay of plant tissues and enables ripening at controlled (low) rates.

4.2 Undesirable consequences of chilling temperatures:

Reduction in storage temperature results in lowering reaction rates and thus increases the storage life of produce. However, certain produces are subject to physiological disorders at these low temperature exposures, called as chilling injury.

4.2.1 Chilling injury:

a substantial number of fruits and vegetables, especially those of tropical or sub-tropical origin, develop physiological disorders when exposed to temperatures below their optimum storage temperatures, but above freezing points. This type of disorder is known as chilling injury / low temperature injury/ cold injury.

The exact mechanism by which chilling injury affects the produce has still not been fully understood but it has been accompanied with loss of membrane integrity, ion leakage from cells and changes in enzyme activity.

The symptoms of chilling injury differ depending upon the produce but, common symptoms include browning (external or internal or both), pitting or other skin blemishes, excessive rotting and fruits failure to ripen.

4.2.1.1 Factors influencing onset of chilling injury:

•  Time at critical temperature (lowest temperature at which chilling injury does not occur): The produce must be exposed to critical temperatures for a minimum period of time for the injury symptoms to develop. This minimum exposure time varies with produce, for example a minimum of 12h for banana and 2-3 months for citrus fruits is required for chilling injury to develop, respectively. The minimum time needed for symptoms of chilling injury to develop also depends on the critical temperature exposed, for example symptoms often develop more rapidly at high critical temperatures; but, given sufficient time, maximum injury occurs at the lowest non-freezing temperatures.\
• Metabolic state of tissues: Susceptibility of produce to chilling injury frequently depends on their maturity, composition and growth environment.
• Composition of the atmosphere: Alterations in atmospheric composition also affect the incidence of chilling injury in them.
• Rate of water loss: rate of water loss from the produce can influence susceptibility to chilling injury, but results vary depending upon the product.

4.3 Considerations For Fruits And Vegetables Chilling Storage:

Fruits and vegetables being living in nature, thus metabolic and other life associated reactions continues to occur even after their harvest from plant during which, besides of synthesis of volatile and other desirable compounds, heat is also liberated, which needs to be removed constantly from their storage environment. In order to have maximum storage life of produce at chilling temperatures, it needs to be considered that during their chilling storage:

• Aerobic respiration is allowed at a slower rate so that the maintenance process associated with produce continues to remain active.
• Refrigeration calculations must include vital and sensible heat of produce, besides of other considerations (as heat leakage from the storage facility, etc).
• Storage at temperatures above critical temperature for chilling injury.

Thus, the selected temperature should be suitably low so that the major deteriorative reactions are slowed down without causing any undesirable changes to the produce.

4.4 Zero-Energy Cold Chambers:

Evaporation of liquid results in lowering its temperature, due to the fact that the energy required for phase conversion is taken from the sensible heat of the liquid (Evaporative cooling). This principle is used to cool stores or chambers by passing air through them after its passage through a stream of water. Degree of cooling depends on air humidity and efficiency of evaporating surface.

These can be easily constructed at small and marginal farmers’ field with the capacity to lower 15-30oC temperature than that of atmosphere and can be efficiently used for short-term storage of fruits and vegetables at field.

4.5 Pre-Cooling:

Pre-cooling could be defined as process of storage produces which yields their maximum life. This is often done at temperatures below which physiological disorders (chilling, freezing injuries) are exhibited by the produce or usually just above the temperature that which will cause chilling injury or freezing injury. To have maximum storage life, the produce should be brought to these temperatures as quickly possible after harvest. The rate of precooling depends upon:

1. Difference in temperature between the produce and cooling medium.

2. Accessibility of cooling medium to produce.

3. Nature of the cooling medium.

    4. Velocity of the cooling medium (air, water).

5.  Rate of heat transfer from produce to the cooling medium.

4.5.1 Methods for precooling: Several commercial methods for precooling the produces are available, but every produce require requires a particular precooling method and the commonly employed method is based upon the produce, its marketable or storage life required and economics. The methods commonly used to achieve precooling are briefed below:

4.5.1.1 Icing: Oldest precooling method, also called contact or top icing. Ice is applied to produce by placing a layer of crushed ice directly on them. Ice melts and resultant cold water runs down thereby cooling them. For further lower temperatures, ice slurry can be used instead of ice. The major application of top icing is for early precooling of harvest at the field and for low temperature transport.

4.5.1.2 Room cooling: This method involves placing the produce in a cold store. The type of room used may vary, but generally they are equipped with a refrigeration unit through which cooling is achieved. Air circulation by fan increases the cooling efficiency by blowing air from the refrigeration unit to the produce. The main advantage associated with it is its cost, while the drawback of this method is the length of time it takes to lower the temperature.

4.5.1.3 Forced air cooling: The principle of this type of precooling remains same as that of room cooling, but the provision of an efficient forced air circulation system increases its cooling efficiency by having very high air velocity, which also leads to desiccation of the produce due to uptake of moisture by the air from produce. To reduce produces desiccation various methods of humidifying the cooling air have been devised.

4.5.1.4 Hydro-cooling: Transfer of heat from solid to liquid is faster than from a solid to gas. Therefore cooling produced by cooled water can be very rapid and result in no loss of weight. Cooling of produce is achieved by submerging them in cold water which is continuously being circulated through a heat exchanger.

4.5.1.5 Vacuum cooling: Cooling is achieved by removal of latent heat (of vaporization) from the produce, by vaporization of moisture. This also is often accompanied with about 5% reductions in produce weight (moisture loss due to evaporation). This weight loss can be prevented by spraying water on the produce before loading them in vacuum cooler. Advantages of this type of cooling includes even and quick cooling of produce, but it has the limitation for produce which have an outer coating (effective barrier to water loss from the surface) for example, tomatoes, which have a relatively thick wax cuticle and thus are not suitable for vacuum cooling.

4.6 Handling of Produce Following Their Removal From Chilling Storage: When the produce is moved from chilling storage, its temperature is below the dew point of atmospheric air, thus, condensation occurs and growth of micro-organisms is encouraged on its surface. Condensation can be avoided by warming the product gradually, but this is often not practical. If condensation does occur, ventilation should be adequate to warm and dry the produce.

5.0 FREEZING: Freezing is the unit operation in which temperature of food is reduced below its freezing point and a proportion of the water undergoes a change in state to form ice crystals.

Preservation is achieved by a combination of low temperature and reduced water activity.

5.1 Freezing Process: The freezing points of produce is just below the freezing point of water; for example, apple freezes at about -1.5 0C. The actual freezing point will vary between the cultivars or even the conditions in which the produce was grown.

If temperature is monitored at the thermal centre of food (the point that cools most slowly) as heat is removed from it, a characteristic curve is obtained (Fig. 5.1), often called as freezing curve. The freezing curve has the following important points in it:

AS: Sensible heat is first removed from the produce and temperature is lowered to below its freezing point (always below 0oC), even at point S, water remains in liquid state, although the temperature is below the freezing point (super-cooling).

SB: Temperature rises to freezing point, as ice crystals begin to form and latent heat of crystallization is released.

BC: Latent heat of ice-crystallization and sensible heat from produce is removed. The freezing point is gradually depressed by the increase in solute concentration in the unfrozen liquor, and the temperature therefore falls slightly. It is during this stage that the major part of the ice is formed.

CD: One of the solutes becomes supersaturated and crystallizes out with evolution of latent heat of crystallization

DE: Crystallization of water and solutes continues.

EF: Temperature of ice–water mixture falls to the temperature of the freezer.

5.2 Equipment For Freezing:

5.2.1 Blast freezers: In theses freezers, air is re-circulated over the produce at -30oC to -40oC at a velocity of 1.5–6.0 m/s. In batch equipment, food is stacked on trays in rooms or cabinets, while in continuous equipments food on trays or conveyor belts are passed through an insulated tunnel, in which they are exposed to freezing air. Air flow is either parallel or perpendicular to the food.

Blast freezers are relatively economical and highly flexible as different shape and size of foods of can be frozen in them. But, their major limitation lies in regular defrosting of refrigeration coils and handling of large volumes of air, which causes up to 5% moisture losses, freezer burn and oxidative changes to unpackaged or individually quick frozen (IQF) foods.

5.2.2 Fluidized-bed freezers: During freezing, glaze formation results in clumping of foods together, this causes handling difficulties to consumer. Food in theses freezes are frozen in such a manner that they maintain their individual identity.

Air at -25oC to -35oC is applied from bottom of the foods (contained on a perforated tray or conveyor belt) in perpendicular direction to them. Velocity of air is so adjusted that the foods remains in fluidizing condition and are frozen at that stage. The shape and size of the pieces of food determines the air velocity needed for fluidization.

These equipments have high capacity, require less frequent defrosting and are highly suited for IQF foods production. However, they are restricted to particulate foods (for example peas).

5.2.3 Plate freezers:

Theses freezers comprise of a vertical or horizontal stack of hollow plates, through which refrigerant is pumped at around -40oC, on other hand flat (relatively thin) food pieces are placed in single layers between theses plates and a slight pressure is applied by moving the plates together. This improves the contact between surfaces of the food and plates and thus increases the rate of heat transfer.

Advantages of this type of equipment include effacement space utilization, little dehydration of product (therefore minimum defrosting of condensers) and high rates of heat transfer, while the main disadvantages are its high capital cost and restriction to flat and relatively thin shape of food.

5.2.4 Cryogenic freezers:

Freezers of this type are characterized by a change of state in the refrigerant as heat is absorbed from the freezing food. The cryogen is in intimate contact with

food and rapidly removes heat from all surfaces of food to produce high heat transfer coefficients and rapid freezing.

The choice of refrigerant is determined by its technical performance for a particular product, its cost and availability, environmental impact and safety. The two most common refrigerants are liquid nitrogen and solid or liquid carbon dioxide.

Major advantages of these freezers are:

• Simple continuous operation with relatively low capital costs.
• Smaller weight losses from dehydration of the product.
• Rapid freezing
• Exclusion of oxygen during freezing.
• Rapid startup and no defrost time.

Their major disadvantage is high cost of refrigerant and immersion of food in liquid nitrogen causes a high thermal shock which, in many foods, results in internal stresses thereby cracking or splitting.

5.3 Effect of Freezing of Foods: Freezing causes negligible changes to pigments, flavors or nutritionally important components. The main effect of freezing on food quality is the damage caused to cells by ice crystal growth and fruit-vegetable tissues are particularly susceptible to this. The extent of damage depends on the size of the crystals and the rate of heat transfer. Freeze injury: injuries occurring in the food due to improper rate of freezing. Rate of freezing could be of two types: Slow freezing (thermal arrest time < 30 mins) and fast freezing (thermal arrest time <30 min).

5.3.1 Slow freezing: During slow freezing, ice crystals grow in intercellular spaces between the plant tissues, and deform and rupture adjacent cell walls. Further, ice crystals have a lower water vapor pressure than within the cells and thus water moves from the cells to the growing crystals due to vapor pressure gradient, thereby resulting in dehydration of cells and permanently damaged upon increased solute concentration yielding a collapsed and deformed cell structure.

Upon thawing, the ice present intercellularly melts and the water flows back into the cell where hydration takes place. If no damage has occurred to the plasma membrane (punctured, ruptured) then the cell is alive and well. However, cell death is eminent if cell damage has occurred, cells do not regain their original shape and turgidity, the food is softened and cellular material leaks out from ruptured cells (drip loss).

5.3.2 Fast freezing: In fast freezing, smaller ice crystals form both outside and inside of the cells, due to this no vapor pressure gradient exists in the food, thus there is little physical damage to cells; and hence there is minimal dehydration of the cells. The texture of the food is retained to a greater extent.

However, very high freezing rates may cause stresses within some foods that result in splitting or cracking of the tissues.

The influence of freezing rate on plant tissues is shown in Fig. 5.2.

5.4 Effects of Frozen Storage: In general, the lower the temperature of frozen storage, the lower is the rate of microbiological and biochemical changes. However, freezing and frozen storage do not inactivate enzymes and have a variable effect on micro-organisms. Furthermore, reduction in water activity, changes in pH and redox potential of the frozen product also plays important role in affecting final product quality.

5.4.1 Recrystallisation: Physical changes to ice crystals (changes in their shape, size or orientation) are collectively known as recrystallisation and are an important cause of quality loss.

There are three types of recrystallisation in foods as follows:

• Isomass recrystallisation: This involves change in surface shape or internal structure, resulting in a lower surface area-to-volume ratio.
• Accretive recrystallisation: This involves joining of two adjacent ice crystals together to form a larger crystal, resulting in an overall reduction in the number of crystals.
• Migratory recrystallisation: This involves an increase in the average size and a reduction in the average number of crystals, due to the growth of larger crystals at the expense of smaller crystals.

Migratory recrystallisation is the most important in most foods and is largely caused by fluctuations in the storage temperature.

Since the moisture from the air is removed by the refrigeration coils, cold stores often have a lower humidity maintained in them. Due to this, fast moving air removes moisture (ice) from the surface of the food, producing areas of visible damage known as freezer burn. Such areas have light color due to microscopic cavities, previously occupied by ice crystals. Freezer burn is a particular problem in foods that have a large surface-area-to-volume ratio (for example IQF foods) but is minimized by packaging in moisture-proof materials.

5.5 Thawing: When food is thawed in air or water, surface ice melts to form a layer of water.

Water has lower thermal conductivity and diffusivity than ice, thus thawing is takes longer time than freezing under similar conditions.

During thawing (Fig. 5.3):

AB: initial rapid rise in temperature, due to absence of water around the food.

BC: temperature of the food is around 0oC (melting point of ice). During this period drip losses occur, which have occurred during the freezing process, resulting in loss of water-soluble nutrients.

Suggested Readings

• Kader AA (2002). Postharvest Technology of Horticultural Crops, 3rd Edn, University of California, Davis publication, USA.

• Mishra, V. K. and Gamage, T. V. (2007). Postharvest physiology of fruits and vegetables, In Rehamn, M. S. (ed.) Handbook of Food Preservation, 2nd edition, CRC Press, Boca Raton.

Web References

• ¨ http://ucanr.edu/datastoreFiles/234-214.pdf
• ¨ http://www.youtube.com/watch?v=THaYQyqwOP0