CHAPTER TWO
LITERATURE REVIEW
2.1 Development of Cold Room Storage for Fruits and Vegetables
Refrigeration and cooling of perishable food products is an important and fascinating area of application of heat transfer and thermodynamics. Refrigeration slows down the chemical and biological processes in foods, the accompanying deterioration and the loss of quality. Cooling is defined as decrease in temperature of a substance or medium below its temperature of its environment. Wilson et al, 1995 reported that most storage crops require low temperatures and high humidity. Most fruit and vegetable crops retain better quality were selected based at high relative humidity (80 to 95%) but at this humidity, disease growth is encouraged. The cool temperatures in storage rooms help to reduce growth, but sanitation and other preventative methods are also required. Maintaining high relative humidity in storage is complicated by the fact that refrigeration removes moisture (Wilson et al., 1995). The storage life of fruits and vegetables can be extended by removing the field heat and cooling as soon as possible after harvesting. The optimum storage temperature of most fruits and vegetables is about 0.5oC to 1oC above their freezing point (FAO, 1995).
Geeson (1989) showed that low temperatures and high humidity can be achieved in a cold store by using either forced air cooling with conventional coils or forced air cooling with wet cooling system. The latter system provides much higher cooling rates than the conventional system and is thus used for rapid cooling and short term holding of fruits and vegetables. (Tassou and Xiang, 1998) reported that the relative humidity and temperature of optimum the air are important parameters in cold store design and operation and he defined relative humidity as the ratio of the partial pressure of water vapour in the air to the partial pressure at saturation at the same temperature. Since it is the water vapour pressure difference between the store air and the surface of the produce that drives the transpiration process, to minimize weight loss it is important to maintain the store air at as high a value of relative humidity as possible, typically 90% relative humidity. (Wilson et al, 1995); Janet and Richard (2000); Lisa and Kader (2004) reported that the relative humidity in refrigerated storage must be within the range for the commodity. For most fruits and vegetables, the optimum relative humidity is 85 – 90 percent and they all concluded that when relative humidity inside the refrigerated storage is less than 85 percent, the facility should use a humidifier. This was possible as a result of varying temperature and relative humidity within the facility.
A refrigerator operates on vapour-compression cycle which involves the use of cyclic refrigerating system. This consists of a refrigeration cycle, where heat is removed from a low – temperature space of source and rejected to a high – temperature with the help of external work, and its inverse, the thermodynamic power cycle. In the power cycle, heat is supplied from a high – temperature source to the engine, part of the heat being used to produce work and the rest being rejected to a low-temperature sink. This satisfies the second law of thermodynamics. A refrigeration cycle describes the changes that take place in the refrigerant as it alternately absorbs and rejects heat as it circulates through a refrigerator. Heat naturally flows from hot to cold. Work is applied to cool a living space or storage volume by pumping heat from a lower temperature heat source into a higher temperature heat sink. Insulation is used to reduce the work and energy needed to achieve and maintain a lower temperature in the cooled space (Wikipedia, 2013).
2.1 Harvest, Handling and Transport of Fruits and Vegetables
Adequate and proper harvesting and handling of fresh produce are crucial preconditions for long storage and shelf lives of fruits and vegetable (NHB, 2010). Considering the harvesting and subsequent handling stage of produce, the thumb rule of the les the better should be followed, in order to minimize chances of mechanical injuries of produce. One damaged, shelf life is considerably compromised (USAID, 2009).
2.2.1 Harvesting Principle of Fruits and Vegetables
For harvest procedure the subsequent points are important to consider:
i. Choosing the correct harvest time refers to both the ripeness and maturity of the produce as well as the right time of the day optimal harvest times for most produce is either early morning hours or the evening when temperature are lower (Thompson, 2002).
ii. Harvesting of delicate, high value produce is best done manually, labour costs are low and fuel costs are high USAID, 2009).
iii. Do not place produce on the ground directly, but use harvesting mats or containers/baskets instead (Sustain n. d.)
