Temperature management

Cooling the harvested product

• Temperature- the most important factor in maintaining the quality of the harvested product.

Product temperature response:• Temperatures in the interim.• Low temperatures damage.• High temperatures damage.

Low temperature affects • The harvested product is transported in low temperature.

The ideal temperature:• For chilling insensitive product - just above freezing point.• For chilling sensitive product - slightly above the chilling

injury temperature.• The activity rate of various enzymes involved in the

metabolism of the harvested products, usually rises exponentially with the increasing temperature.

• By the Dutch chemist Van Hoff Chemical Reaction rate multiplied any 10oC.

The value of Q10 decreases with increasing temperatures.

10( / t2-t1)

Q10 = )R2/R1(

• Lowering the temperature inhibits the exhaustion of the harvested product

• Low temperature can delay the beginning of ripening in climacteric fruits (Onset of ripening.(Low temperatures slow the rate of ethylene production and the response to ethylene.

• Therefore, a prolonged exposure to ethylene is necessary to start ripening processes at low-temperature.

Low temperature affects

• Ripening temperatures- 10-30oC, usually.• Some pear varieties will ripe at temperatures lower than

10oC, but prolong storage in low temperature might inhibit adequate ripening.

• Maintaining products at low temperatures can inhibit developmental processes in various products, such as opening inflorescence of cut flowers, asparagus lignification and loss of sweetness of peas.

• Exposure to low temperatures may occur during transport, storage ect.

Low temperature affects

Low temperatures injuries

• Freezing injury• Freezing injury is not due to disruption of metabolism similar to chilling injury.• Caused irreversible damage due to ice crystals created in the product.• Brief freezing of fleshy tissue affects the product.• Freezing temperature is different between the products, due to the content of

different solutes.• The products’ moisture content influence its resistance to freezing.• water are not available to create ice crystals in products with very low humidity

content.• Freezing: lettuce - 0.2oC, grapes - 2.0oC.• When defrosting – in most products, the tissue is thaw, the texture changes

and water soaked areas appear.• Less sensitive products: a slow thaw of cabbage, onions and several varieties

of pear.• Acclimatization of different products to low temperature may reduce their

sensitivity to freezing injuries.

Low temperature injuries

Freezing injuries

• Chilling injury is the result of temperatures higher than freezing temperatures.

• The injury is due unbalanced metabolism and damage to the cellular compartments

Low temperature damages are divided into two categories:Chilling injury and physiological disorders.• tropical and subtropical fruits are particularly susceptible to chilling injuries.• Chilling injury is the combination of temperature and duration of exposure.• Low temperature for a short time - may not develop any injury.• Low temperature for a long time - irreversible chilling injury may develop.

Chilling injury

• Different sensitivity to Chilling injury may occur between varieties of the same fruit, or a result of different growth region.

• Sometimes the damage is visible after transition of the product to higher temperature (shelf life) .

Typical symptoms of chilling injury in some products:

Chilling injury

• Pitting- Collapse of cells beneath the surface, leading to pitting and change of color. Might increase water loss.

Chilling injury

Chilling injury• Browning- Usually appears around the fruit transport

organs ( e.g. xylem). Browning ,might be the result of the action of the PPO enzyme oxidizing phenols released from the vacuole as a result of the chilling damages.

Chilling injury

• uneven ripening- early harvested fruit might not ripen properly after prolonged cold storage.

Chilling injury

• De-greening- Slowing the loss of green color even by slight cooling.

Chilling injury

• water soaking- exposure to cold of leafy vegetables and some fruits like papaya.

Chilling injury• Increasing the sensitivity to pathogens- chilling injury

damage the cellular compartments and results the release of amino acids, sugars, minerals and other factors that serves as excellent substrates to pathogens.

Chilling injury

Development of off flavors, aftertaste and undesirable odors.

Preventing Chilling injury• Cooling the product above its critical temperature.• Conditioning- exposure of the product to relatively low

temperature for a short time following by storage at higher temperature may prevent chilling injury.

• This method is effective in preventing browning of pineapple, peach wooliness and plum internal browning.

• Successful treatment to prevent chilling injury in nectarines and peaches is a combination of intermediate heating and controlled atmosphere.

• Immersion in CaCl2 reduced low temperature breakdown (LTB) in Jonathan apples.

Chilling injury

Lipid hypothesis of chilling• Some lipids undergo changes at low temperature that affects

the physical properties of cell membranes.• As a result, membrane properties are affected, including:

membrane integrity, ions and metabolites permeability, and the activity of membrane-anchored enzymes.

• Later, there may be effects on the metabolism and cellular compartment, leading to cell death and the known symptoms of chilling injury.

The mechanism of chilling injury development

• Physiological disorders during cold storage are mainly deciduous (apples, pears), stone fruits (peaches, plums) and in most citrus.

• The damage might affect only the surface of the product or the flesh and core sections as well.

• The metabolic process that results the damage is usually unknown, and perhaps different symptoms caused by different malfunctioning of the metabolic pathway.

• Physiological response to cold damage have been studied mainly on apples.Most damages are developing during exposure to temperatures lower than 5oC.

Physiological disorders during cold storage

Physiological disorders during cold storage

Factors affecting the susceptibility to the damage: ripening stage at harvest, orchard treatments, the climate during fruit development, fruit size, method of harvest.

storage methods for chilling sensitive fruit:• short storage period.• Gradual cooling at the first stages of storage.• Heating intermediate 20oC for several days during storage

reduced this disorder in apples and stone fruits, but problematic in commercial terms:It is difficult to quickly elevate the temperature in the storage room with large amount of fruit.

Physiological disorders during cold storage

Prevention of chilling injuries:

Brief exposure to high temperature prevented:

• Superficial scald in apples

• Pitting in avocado, citrus and cucumber.

• Brown spots in prickly pear.

• Grapefruit - after harvest:48 hours at 27-29oC, or 7 days in 21oC,followed by storage at 10-16oC.

Physiological disorders during cold storage

Controlled atmosphere conditions:

• Jonathan spots prevention, reduction of core browning and internal breakdown of apples.

• On the other hand, an increase of internal breakdown was reported in controlled atmosphere storage due to: High humidity, lack of air movement, accumulation of fruit volatiles.

• Low oxygen levels and high carbon dioxide levels: core browning in apples and pears.off flavors- anaerobic metabolism.

Physiological disorders during cold storage

• Fruit peel blemishes might affect its appearance and reduce its marketability.

• Internal blemishes - more commercially " tolerable".

• It is possible to reduce these blemishes by chemical and physical treatments, or by choosing less sensitive varieties.

Physiological disorders during cold storage

High temperatures damagesExposure to high temperatures:• Exposure to direct sunlight• Hot wind.

Heat treatment to prevent pathogens:• Immersion in hot water, evaporation,

and dry heat.• Enzymatic activity decreases in most

harvested products above 30oC, and above 40oC are inactive due to denaturation.

High temperatures:• Prolonged exposure to heat of climacteric fruits

advanced fruit ripening, but delayed the color change. E.g. yellowing of banana peel or redness of tomato.

• The metabolism is disrupted over 35oC resulting in impaired membrane integrity leading to cellular compartment damages expressed as loss of pigment and transparency of the product.

Additional use of different temperature exposures

Prevention of pests:• Essential to prevent their spreading to new areas.• Temperature treatments can prevent various insects in

fruits, vegetables, nuts, flowers and more.• For example, 125 minutes at 51.5oC prevents Caribbean

fruit fly in mangoes.

Additional use of different temperature exposures

Prevention of pests:• Low temperature - delay the development of pathogens including

bacteria and fungi and also delay the development of insects.• High temperature - short-term exposure is effective for pathogen

inhibition, for example:3 seconds of steam (100oC) reduced decay in carrots. Short exposure of mangoes to 55oC reduced decay. Immersion of papaya in water at 49oC for 20 minutes reduced decay.

Additional use of different temperature exposures

Conditioning• Conditioning before storage or marketing can affect a

variety of fruit traits such as increased firmness of apples, delay of asparagus gravitropism, delay of sprouting of potatoes.

• Conditioning after storage can speed up germination and flowering of gladioli bulbs or lily bulbs.

Additional use of different temperature exposures

Curing• Exposure to conditions that allow healing of wounds or the

development of a protective layer.

• Brief exposure to high temperature after harvest: potato - Exposure to 29oC for 5-7 days (80-90% R.H.) accelerating the creation of periderm layer at the site of injury which reduce decay.Kiwi - stay in the shed for 24-48 hours after immersion before storage.

Additional use of different temperature exposures

the affects of temperature on starch and sugar balance

• Storage at low temperatures can affect the balance of sugar and starch in various vegetables such as potatoes, sweet potatoes, green peas, corn and more.

starch↔sugar→CO2

• Ambient temperatures: starch←sugar• Respiration decrease in low temperatures:

starch↔sugar→CO2

• Sugar accumulation begins at typical temperature critical for the product: potato 10oC, sweet potato 15oC.

the affects of temperature on starch and sugar balance

Due to the accumulation of sugar:• Inferior texture and sweetness in cooking.• Browning due to caramelization during frying.• Maillard reactions as a result of interaction between

amino acids and sugars.• Typically, raising the temperature to15-20oC returns the

sugar to its previous level.

• high sugar content is desirable in corn and peas.

• Those are picked at early stage of maturity (immature) when sugar content is highest.

• Fast storage at low temperature is needed to prevent acceleration of starch sugar conversion.

the affects of storage temperature on starch and sugar balance

The benefits of cold storage

• Decreases the rate of respiration.• Low respiration rate → long shelf life• Reduction of 10oC may decrease the

respiration rate by 2-3 folds.• shelf life increases by 2-3 folds.

• Reducing water loss.• hot product has the largest water loss.• Fast cooling → less water loss.• In some products the water loss during 1 hour in hot and dry

air is similar to 1 week in cold storage with high humidity.

The benefits of cold storage

• Reducing decays.• Harvest temperature is optimal for many pathogens.• Low temperatures significantly reduces the development

of pathogens.in the product.

The benefits of cold storage

What are the heat sources?

• Field heat (sensible heat).• Metabolic heat generated during the respiration

process (vital heat).• Passing heat conduction through walls, floors etc.• Heat exchange of air or leaks.• Other sources: lights, motors and more.

• 47%- Field.• 37%- The fans in the storage facility. • 8%- Forklifts. • 7%- Conduction through walls, roof, and air. • 1%- lights, labor, and more.

What are the heat sources?

Half cooling timeor 7/8 cooling time

• Half cooling time - time required to reduce by half the temperature difference between the product and the environment.

• cooling rate is higher as the temperature difference between the product and the environment is larger, although half-cooling time remains constant.

• Three cooling cycles of the product will allow cooling in 7/8 of the temperature difference between the product and the surrounding environment (1/2 ← 1/4 ← 1/8).

Heat removal• Conduction

• Convection

• Evaporation

• Radiation

Cooling methods Cold air• air is simple and accessible medium easy to move.• Low thermal capacity.

Ways of cooling with cold air:• Cold room.• Forced air cooling.

Cold room• Cooling method which requires less cooling capacity

than other cooling methods because heat removal lasts a relatively long time (e.g. overnight)

• The product can be stored where it is cooled.• The process is slow.

Cooling methods

Forced air cooling

• Much faster compared with passive cooling (4 - 10 fold times faster).

• This method allows rapid mobilization of the product to the markets without extensive use of the storage rooms.

• High cooling capacity required to deal with heat peaks.

Cooling methods

Factors that affect the speed and efficiency of cooling

• Cooling capacity:Improper cooling - air temperature may rise by adding warm product.The initial product temperature:As product temperature is higher → more time will take to cool it.

• Air temperature in the storage room:If the air temperature will increase especially towards the end of cooling, the whole process would be prolong.

• Air velocity over the product:The air takes the heat from the product, therefore if there is no movement of cooling air the process will be very slow.

• Fan speed:Cooling units are designed to cool efficiently.doubling the flow velocity can cause cooling to be 40% faster.

• Product near cooling diffusers will cool quickly than remote product.

• product at the top of the packaging cools faster than the product at the bottom.

• space between the containers is important for air flow and effective cooling.

• The first product in contact with cold air would cool faster.

Uneven cooling

Importance of packaging design • cooling holes at the packaging are required for proper cooling rate.• 5% - recommended ventilation area.• Large ventilation holes are better than many small ones.• There is high importance for hole punching method and locatin in

the cardboard for achieving effective ventilation and to maintain its strength.

• Other packaging materials in the carton (plastic containers, for example) will significantly reduce air flow and cooling efficiency.

Cooling sensing • temperature reading is recommended after a few thrusts

of the thermometer in the fruit depth to Equilibrate temperatures with fruit.

• Temperature measurement should be made in the fruit which is expected to cool the slowest.

• Typically, the warmest fruit is far from the fan and at the bottom of the box.

Hydro coolingThe advantages of using water for cooling:• water has high cooling capacity (1kcal/kg/oC), therefore

water absorb more energy before the temperature rise.• water has high heat conductivity (5.2 cal/g/h/oC), therefore

there is faster heat loss to the water.• Use of water for cooling the product prevents loss of water

during the cooling process.

• A wide variety of fruits and vegetables are suitable to cool in cold water.

• This is one of the fastest methods for cooling.

products which are not suitable for this cooling method:• products sensitive to free water : grapes, flowers and most

berries.• Products that can be cooled more efficiently with other

methods: for example, vacuum cooling of leafy vegetables.

Hydro cooling

Drenching• drip or splash of water through small holes.• The product passes under the stream and washed by water.• Applied directly on the product which in waterproof package.

Hydro cooling

Dipping• The product is dipped in cold water canal.

• The product moves through the water stream or by conveyor belt.

• Products with lower density than water should keep under water.

• Some products are not suitable due to high risk of infections.

Hydro cooling

Factors affecting the cooling rate

Product Size• Larger product requires more time for cooling.

Barriers for direct contact with the product• For example, leaves that cover the corn slow the cooling

rate, due to air trapped between the leaves and kernels. Waste of Leaves in the bin may reduce water movement.

Water flow around the product – • Sufficient water flow should match the refrigeration capacity.• Active circulation of water around the product will allow efficient

cooling.• Holes in the bin should provide efficient movement of cooling water.Water temperature – • Large difference between cold water and the product allow faster

heat transfer from the product to the water.

Factors affecting the cooling rate

Cooling control Do not assume a proper refrigeration!• Check the initial and final temperature in the fruit pulp.A decrease the cooling efficiency:• Check the temperatures in the water tank and water stream.• If the temperatures are proper, increase the water flow or the

duration of contact between the product and the cold water

Factors that affect the cooling of the product

• Maintaining the water without decay - a daily change of water.

• Primary wash of dirt from the product before dipping.• Prevent large debris from entering the pumps.• Use approved material to maintain sanitation.

Using ice for cooling

Advantages• Water has high heat capacity

(80 kcal/kg).• 1kg of ice will cool about 3 kg of

product by 28oC.• by injecting a slurry (mixture of

ice and water) into the top of a pellet of product it may take 3 minutes for cooling.

Disadvantages:• Water resist packaging is needed (expensive cardboard).• Increase of weight for transportation.• The melted ice water may affect other produce at the

same shipment.

Using ice for cooling

Vacuum cooling • The method is economical compared to other cooling methods

because only the product is cooled.• This method rely on tissue water loss.• The sealed container and should be filled to reduce free air.• Removing air from the container to create the vacuum.• The decrease of pressure drops the boiling temperature of

water.

Vacuum cooling principles• Evaporation of water requires a lot of energy (540kcal/kg),

therefore the transition of water from liquid to gas is cooling the plant tissue.

• water evaporate from product at field temperature when the pressure decreases from 760 mmHg to around 40 mmHg (atmospheric pressure is 760 mmHg).

• At pressure of 4.6mmHg the water evaporate at 0oC.• Cooling of lettuce should maintain this pressure for 3 to 6 minutes.• Evaporation of water removes evenly a considerable amount of

heat from the product.

Products suitable for vacuum cooling:• Leafy vegetables with large surface (e.g.

lettuce but not cabbage).• A large and bulky products may cool but for

a long time (even 2-4 hours).Water loss:• For reduction of 6oC there is about 1% of

water loss. This may affect the quality of the product and its value.

• Reduce of water loss by adding water to the product before vacuum cooling.

• The water that evaporate first are those added and are in contact with the product.

Vacuum release:• After cooling, it is important to release the

vacuum slowly in a controlled manner to prevent damage to the product.

Vacuum cooling

High quality product!