FIRST THEMATIC WORKSHOP PRESERVATION TECHNOLOGIES FOR FRESH FRUITS AND VEGETABLES: COLD STORAGE, MODIFIED ATMOSPHERE PACKAGING AND CONTROLLED ATMOSPHERE STORAGE”. ISTAMBUL. JANUARY 2002.
Introduction • Fresh fruits and vegetables are living dynamic systems. They respire and transpire. • Respiration is the process of O2 uptake and production of CO2 and H2O and energy. • Transpiration is the process of water loss. • Before harvesting losses due to respiration and transpiration are replaced by water, photosynthates and minerals from the plants. • After harvest, when they are removed from the plant, losses of respirable substrates and moisture are not replaced and deterioration occurs (causing quality decline and limit the shelf life of fruits and vegetables).
Primary causes of lossThose are directly affect the food Enzymic changes Examples include: 1- the post-harvest spoilage of fruit and vegetables 2- oxidation of phenolic substances in plant tissues by phenolase (leading to browning) 3- sugar - starch conversion in plant tissues by amylases 4- post-harvest demethylation of pectic substances in plant tissues (leading to softening of plant tissues during ripening
The major factors useful in controlling enzyme activity are 1-temperature 2- water activity 3- pH 4- chemicals which can inhibit enzyme action
Chemical changes 1- Sensory quality The two major chemical changes which occur during the processing and storage of foods and lead to a deterioration in sensory quality are lipid oxidation and non-enzymatic browning. Chemical reactions are also responsible for changes in the colour and flavour of foods during processing and storage.
2- Nutritional quality The four major factors which affect nutrient degradation and can be controlled to varying extents by packaging are 1- light 2- oxygen concentration 3- temperature 4- water activity.
Physical changes One major undesirable physical change in food is the absorption of moisture as a consequence of an inadequate barrier provided by the package; this results in caking. It can occur either as a result of a poor selection of packaging material in the first place, or failure of the package integrity during storage.
Biological changes • Microbiological • Micro-organisms can make both desirable and undesirable changes to the quality of foods • The two major groups of micro-organisms found in foods are bacteria and fungi, the latter consisting of yeasts and moulds. Bacteria are generally the fastest growing, so that in conditions favourable to both, bacteria will usually outgrow fungi.
Macrobiological • 1- Insect Pests • Warm humid environments promote insect growth, although most insects will not breed if the temperature exceeds about 35 °C or falls below 10 °C. Also many insects cannot reproduce satisfactorily unless the moisture content of their food is greater than about 11%. • 2- Rodents • Rats and mice carry disease-producing organisms on their feet and/or in their intestinal tracts and are known to harbour salmonella of serotypes frequently associated with food-borne infections in humans.
Secondary causes of loss: Those lead to conditions that encourage a primary cause of loss such as: 1- Inadequate harvesting, packaging and handling skills. 2- Lack of adequate containers for the transport and handling of perishable crops. 3- Storage facilities inadequate to protect the food. 4- Transportation inadequate to move the food to market before it spoils. 5- Inadequate refrigerated storage. 6- Inadequate drying equipment or poor drying season.
7- traditional processing and marketing systems can be responsible for high losses. 8- Legal standards can affect the retention or rejection of food for human use. 9- Bumper crops can overload the post-harvest handling system or exceed the consumption need and cause excessive wastage.
Temperature. • In general, the higher the temperature the shorter the storage life of horticultural products and the greater the amount of lose within a given time. For example, lettuce stored at 25 C becomes unsaleable within 7 days, while lettuce stored at 10 C will reach the unsaleable condition in approximately 18 days and lettuce stored at 0°C requires 35 days to reach the point of being unsaleable.
Humidity. • There is movement of water vapor between a food and its surrounding atmosphere in the direction towards equilibrium water activity in the food and the atmosphere, Fresh horticultural products have a high moisture content and need to be stored under conditions of high relative humidity in order to prevent moisture loss and wilting.
Solar Radiation. • The solar radiation that falls upon foods held in direct sunlight increases the temperature above the ambient temperature. The amount of increase in temperature depends on the intensity of the radiation, the size and shape of the food' and the duration of exposure to the direct rays of the sun. The intensity of solar radiation depends upon latitude, altitude, season of the year, time of day, and degree of cloud cover.
Altitude • Within a given latitude the prevailing temperature is dependent upon the elevation when other factors are equal. There is on the average a drop in temperature of 6.5°C for each Km increase in elevation above sea level. Storing food at high altitudes will therefore tend to increase the storage life and decrease the losses in food provided it is kept out of the direct rays of the sun.
Atmosphere. The normal atmosphere contains by volume, approximately 78% nitrogen, 21% oxygen, 1% argon, 0.03% carbon dioxide' various amounts of water vapor and traces of inert gases. Modifying the atmosphere can improve the shelf life and reduce wastage of certain foods.
Controlled atmosphere storage (CA) is refrigerated storage in which the level of oxygen is reduced to about 3% with the carbon dioxide content being raised to 1 to 5%, depending on the commodity. This CA storage may double the storage life over that of regular cold storage for certain varieties of apples and pears by slowing down the natural rate of respiration.
"Modified atmosphere storage" is another type of controlled atmosphere storage. This term denotes storage of horticultural products in a beneficial atmosphere other than air that is not under as close regulation as in CA storage. Modified atmosphere storage can be obtained in boxes of pears, apples, and cherries that are lined with polyethylene film which acts as a barrier to the escape of carbon dioxide and the ingress of oxygen.
Another method of obtaining a modified atmosphere storage is by the addition of dry ice which increases the carbon dioxide in the atmosphere to some extent,
Time The longer the time the food is stored the greater is the deterioration in quality and the greater is the chance of damage and loss.
Gentle handling • Because of their soft texture all horticultural products should be handled gently to minimize bruising and breaking of the skin.
Temperature control/ Refrigeration Optimum storage conditions for fresh fruits and vegetables
Relative humidity control The amount of moisture in the environment may influence transpiration and water loss, Proper relative humidity is 85% to 95% for most fruits and 90 to 98% for most vegetables except dry onions and pumpkins (70-75%). To minimize transpiration and water loss, produce should be held at low temperature, high relative humidity and minimum water vapor pressure deficit (WVPD).
Waxing of the surface • Waxing the surface of horticultural products is a treatment used on a number of commodities including citrus fruits, apples and cucumbers. It retards the rate of moisture loss, and maintains turgor and plumpness and may modify the internal atmosphere of the commodity.
Controlled and modified atmosphere storage Controlled or modified atmospheres ( CA or MA) are terms implying the addition or removal of gases from storage rooms, transpiration containers, or small-size food packages to manipulate the levels of oxygen, carbon dioxide, nitrogen, ethylene, etc., and achieve an atmospheric composition different than that of normal air around the food.
The term modified atmosphere is used when the composition of the storage atmosphere is not closely controlled (i.e. plastic packages), Controlled atmosphere generally indicates more precise monitoring and control of gaseous composition. In combination with refrigeration, CA and MA provide longer marketing periods, which allow farmers and producers to benefit from extended selling seasons; they also provide consumers with higher quality produce over longer period of time.
Wholesale Markets, Cold Storage and Packing Facilities for Plant Production In Palestine Wholesale markets in the WB are 12, while 2 major ones operate in GS. These municipal markets deal with fresh products (fruits and vegetables).
The inadequate service of cold storage and sorting capacities in wholesales markets have limited role in: • -adjusting the agricultural supply of perishable agricultural products • -extending the marketing period • -reducing export as well as local price fluctuations.
Support infrastructures (such as cold storage) are lacking in the Palestinian wholesale markets In fact, 75 percent of all sales of Palestinian fruits and vegetables are exchanged through wholesale markets. Particularly, at municipal markets the products can be either sold directly to wholesalers (40 percent) or to commission agents that are intermediaries to wholesalers (35 percent). Moreover, fresh vegetables and fruits are sold directly to retailers in a percentage that is estimated to be 5 percent and 20 percent of the production is directly exported.