lecture about Precooling, Sorting and Grading for post harvested filed crops
- 1. LEC. 2 PRECOOLING, SORTING, GRADING PROF. DR. NANIS GOMAH
- 3. • The most important goals of post-harvest handling are keeping the product cool, to: 1. avoid moisture loss and slow down undesirable chemical changes 2. avoiding physical damage such as bruising 3. delay spoilage. • Sanitation is also an important factor, to reduce the possibility of pathogens that could be carried by fresh produce, for example, as residue from contaminated washing water.
- 4. • Precooling: Precooling (prompt cooling after harvest) is important for most of the fruits and vegetables because: 1. they may deteriorate as much in 1 hr at 32°C. In addition to removal of field heat from commodities 2. precooling also reduces bruise damage from vibration during transit. • Cooling requirement for a crop vary with the 1. air temperature during harvesting 2. stage of maturity 3. nature of crop. • There are many methods of precooling : 1. cold air (room cooling, forced air cooling) 2. cold water (hydrocooling) 3. direct contact with ice (contact icing) 4. evaporation of water • from the produce (evaporative cooling, vacuum cooling) and combination of vacuum and hydrocooling.
- 5. • Some chemicals (nutrients/growth regulators/ fungicides) can also be mixed with the water used in hydrocooling to prolong the shelf life by improving nutrient status of crop and preventing the spread of post-harvest diseases. • Pre-cooling is the key component in the preservation of quality for perishable fresh produce in post-harvest systems. It is likely the most important of all the operations used in the maintenance of desirable, fresh and salable produce. Precooling is defined as the removal of field heat from freshly harvested produce in order to : 1. slow down metabolism 2. reduce deterioration prior to transport or storage. One of the most important factors affecting the postharvest life and quality of fruits and vegetables is temperature. Quality loss after harvest occurs as a result of physiological and biological processes, the rates of which are influenced primarily by product temperature. As the maintenance of market quality is of vital importance to the success of the horticultural industry, it is necessary not only to cool the product but to cool it as quickly as possible after harvest
- 6. • Proper pre-cooling preserves product quality by 1. • inhibiting the growth of decay producing microorganisms 2. • restricting enzymatic and respiratory activity 3. • inhibiting water loss 4. • reducing ethylene production • Importance of Precooling • (i) Importance of lag time between harvest and cooling:.When it comes to produce quality, every minute counts and that precooling is among the most cost-effective and efficient quality preservation methods available to commercial crop produces. For example, strawberries experience increasing deterioration losses as delays between harvesting and cooling exceeds 1 h and the effects of the delay on cooling of strawberries is shown in the coming Fig.
- 7. Effects of the delay on cooling of strawberries From this it can be seen that even after a short time of 2 h at 30C, only 80% of the strawberries are considered marketable fruit, which represents an apparent loss of approximately 10% by not cooling the produce immediately after picking. Furthermore, precooling slows down the deterioration and the rotting process by retarding the growth of decay organisms, and it reduces wilting since transpiration and evaporation occurs more slowly at low temperatures.
- 8. • )ii) Influence of precooling on the respiration rate:The rate of deterioration after harvest is closely related to the respiration rate of the harvested product, therefore the reduction of respiration rate is essential to preserving market quality. Since the rate of respiration is influenced by temperature, precooling to remove the field heat before storage will reduce the respiration rate • (iii) Influence on metabolism:Within the plants temperature range, the rate of deterioration increases logarithmically with increasing temperature. Metabolic rates double for each 10 degree C rise in temperature. it can be seen that the quicker the temperature is reduced the less losses that can occur. Hence, precooling is essential in order to reduce metabolic changes such as enzyme activity, and to slow the maturation of perishable produce. • (iv) Effects of rapid cooing on ethylene:The reduction in temperature has the added advantage of reducing the production and sensitivity of the produce to ethylene that accelerates ripening and senescence.
- 9. • Method of Precooling • There are seven principal methods of pre-cooling fresh produce: • a) Room cooling • b) Forced-air cooling • c) Hydro cooling • d) Ice cooling • e)Vacuum cooling • f) Cryogenic cooling • g) Evaporative cooling • Considerable loss in quality and shelf life can occur as a result of holding harvested produce in the field before pre-cooling.All methods require sufficient refrigeration capacity to reduce the temperature of the produce within the required time plus the ability to remove the normal heat gain in the facility.
- 10. ROOM COOLING
- 11. • Forced air cooling: Forced air cooling was developed to accommodate products requiring relatively rapid removal of field heat immediately after harvest. Forced air or pressure cooling is a modification of room cooling and is accomplished by exposing packages of produce to higher air pressure • For successful forced air-cooling operations, it is required that containers with vent holes be placed in the direction of the moving air and packaging materials that would interfere with free movement of air through the containers should be minimized. A relatively small pressure difference between the two sides of the containers exists, resulting in good air movement and excellent heat transfer and hence faster cooling.
- 13. • Hydrocooling: Hydrocooling essentially is the utilization of chilled or cold water for lowering the temperature of a product in bulk or smaller containers before further packing. Hydrocooling is achieved by: 1. flooding 2. Spraying 3. immersing the product in/with chilled water. • There are several different hydrocooler designs in operation commercially.
- 14. Cut-away side view of a continuous-flow shower-type hydrocooler Cut-away side view of a continuous-flow immersion hydrocooler
- 15. • Ice cooling: In ice cooling, crushed or fine granular ice is used to cool the produce.The ice is either packed around produce in cartons or sacks, or it is made into a slurry with water and injected into waxed cartons packed with produce.The ice then fills the voids around the produce. Before the advent of comparatively modern precooling techniques, contact or package icing was used extensively for precooling produce and maintaining temperature during transit.Although, unlike other cooling methods ice not only removes heat rapidly when first applied, it continues to absorb heat as it melts.There are a variety of different methods in which ice is applied to the produce so as to achieve the desired cooling effect.
- 16. • Vacuum cooling: Rapid cooling. Vacuum cooling is achieved by the evaporation of moisture from the produce.The evaporation is encouraged and made more efficient by reducing the pressure to the point where boiling of water takes place at a low temperature. • In the vacuum cooling process the pressure in the vacuum chamber is reduced from atmospheric to about 20 mbar and, during this time, evaporation is slow and relatively little cooling takes place, i.e., temperature of the produce remains constant until saturation pressure at this temperature is reached.At approximately this pressure the `flash point' occurs; this is the point where the water in the produce begins to vaporize, i.e., produce begins to lose moisture and cool rapidly.
- 17. Key components of a 20-pallet capacity vacuum cooler
- 18. • Cryogenic cooling: The use of the latent heat of evaporation of liquid nitrogen or solid CO2 (dry ice) can produce `boiling' temperatures of -196- and -78-degree C, respectively.This is the basis of cryogenic precooling. In cryogenic cooling, the produce is cooled by conveying it through a tunnel in which the liquid nitrogen or solid CO2 evaporates. However, at the above temperatures the produce will freeze and thus be ruined as a fresh market product. This problem is prevented by : 1. careful control of the evaporation rate a 2. conveyor speed. • Cryogenic cooling is relatively cheap to install but expensive to run. Its main application is in cooling crops such as soft fruits, which have a seasonal production period. • The high cost of liquid nitrogen, dry ice and other suitable non-toxic refrigerants make this process most suitable for relatively expensive products.
- 19. • Evaporative Cooling: Evaporative cooling is an inexpensive and effective method of lowering produce temperature. It is most effective in areas where humidity is low. Dry air is drawn through moist padding or a fine mist of water, then through vented containers of produce.As water changes from liquid to vapor, it absorbs heat from the air, thereby lowering the produce temperature.The incoming air should be less than 65 percent relative humidity for effective evaporative cooling. It will only reduce temperature, 10-15°F.This method would be suitable for warm-season crops requiring warmer storage temperatures (45-55°F), such as tomatoes, peppers, cucumbers, or eggplant.
- 21. Sorting, Grading and Sizing • Sorting is done by hand to remove the fruits which are unsuitable to market or storage due to damage by insects, diseases, or mechanical injuries.The remainder crop product is separated into two or more grades on the basis of the surface color, shape, or visible defects. For e.g., in an apple packing house in India 3 grades viz: 1. Extra Fancy 2. Fancy 3. standard may be packed for marketing. 4. The fourth “cull” grade is meant for processing. • After sorting and grading, sizing is done either by hand or machine. • Machine sizers work on two basic principles: weight and diameter.
- 22. • Sorting • Sorting is done by hand to remove the fruits and vegetables which are unsuitable to market or storage due to damage by: 1. mechanical 2. Injuries 3. Insects 4. diseases, 5. Immature 6. over-mature 7. misshapen etc. • This is usually done before washing. it reduces losses by preventing secondary contamination. Sorting is done either at farm level or in the pack-houses.
- 23. • The simplest is belt conveyor, where the sorter must handle the produce manually in order to see all sides and inspect for damage. • Push bar conveyor causes the produce to rotate forward as it is pushed past the sorters. Roller conveyor rotates the produce backward as it moves past the sorter.
- 24. • Sizing • Before or after sorting, sizing is done either by hand or machine. Machine sizers work on two basic principles: weight and diameter. • Mechanisms/Types of sizing 1. Diverging belts:The different speed of belts makes produce rotate besides moving forward to a point where produce diameter equals belt/rope separation. E.g., Cucumbers, Pineapples, and large root vegetables 2. Sizing rollers: with increased spaces between rollers E.g., Citrus. 3. Handheld template: Sizing can be performed manually using rings of known diameter. 4. Sizing by weight: sorting by weight is carried out in many crops with weight sensitive trays.These automatically move fruit into another belt aggregating all units of the same mass. Individual trays deposit fruit on the corresponding conveyor belt Eg. Citrus, apples and pear and irregular fruits 5. Mesh screens: e.g., potato, onion etc.
- 25. Sizing onion bulbs by diverging belts/rope Sizing by rollers of increasing distance between them Sizing by weight.
- 26. Sizing with rings of known diameters
- 27. • Grading • The produce is separated into two or more grades on the basis of: 1. the surface color 2. shape 3. Size 4. weight 5. soundness 6. firmness 7. cleanliness 8. maturity & free from foreign matter /diseases insect 9. damage /mechanical injury. • For e.g.,Apple - I. Extra Fancy; II. Fancy; III. Standard; IV. Cull (for processing). Grading may be done manually or mechanically. It consists of sorting product in grades or categories based on weight/size. • Systems of Grading: Static and Dynamic • 1. Static systems:These are common in tender and/or high value crops. Here the product is placed on an inspection table where sorters remove units which do not meet the requirements for the grade or quality category.
- 28. 2.The dynamic system: Here product moves along a belt in front of the sorters who remove units with defects . Main flow is the highest quality grade. Often second and third grade quality units are removed and placed onto other belts. It is much more efficient in terms of volume sorted per unit of time. However, personnel should be well trained.This is because every unit remains only a few seconds in the worker’s area of vision. e.g., Onion grading There are two types of common mistakes: 1. removing good quality units from the main flow and more frequently 2. not removing produce of doubtful quality.
- 29. • New Innovation in Grading Systems • a) Computerized weight grader - operate on the basis of tipping buckets that drops to release the pre weighed item at a particular position. - Apples, citrus • b)Video image capture & analysis-used for size, color & external defect grading-coffee bean, apple • c) X-ray imaging and Computer aided tomography • d) MRI - Magnetic Resonances Imaging • e) Spectroscopy • f) Acoustic methods • g)Volatile emission analysis