A cold chain, for the purpose of this research, is defined as per the PPECB as, “the seamless and uninterrupted movement of fresh, chilled or frozen products, from the production area to the market, through various storage and transport mediums, without any change in the optimum storage temperature and relative humidity” (PPECB Cold Chain Management, n.d.).
38 As the definition implies, different products have different protocol temperatures at which they need to be kept to ensure optimal quality upon delivery. To facilitate this, logisticians and transporters alike must ensure the cold chain remains unbroken so that the goods remain at their specified protocol temperatures in order to be delivered to their final destination at their optimum quality. In conjunction to this, a further challenge for transporters is the segmentation found within cold chains as the goods move between locations during processing and within transport modes. An effective cold chain starts at the farms where the produce is harvested and ends once that produce is in the consumer’s refrigerator.
3.6.1 Fruit within the cold chain
Fruit is classified as a temperature sensitive commodity (Aung & Chang, 2014). As a result, it is imperative to ensure that the temperatures experienced by the fruit throughout the cold chain are carefully monitored and adhere to regulated stipulations to ensure the fruit is at its optimal quality for the end consumer. Fruits differ in many respects, but one of the most important aspects is its ripening category. There are two ripening categories into which fruit can be classified, namely, climacteric and non- climacteric fruits. The distinction is made based on the rate of respiration of the fruits and ethylene synthesis (PPECB HP02, n.d.; Fresh Produce Exporters’ Forum, 2016:53).
3.6.1.1 Rate of respiration and ethylene
A fruit is still a living organism even once it has been detached from the plant post- harvest. Thus, it still requires energy for the organism to survive. The energy required is generated through the process known as respiration (Silva, 2010). The respiration rate in fruits is a chemical process through which the fruit converts carbohydrates, usually in the form of either starches or sugars, and oxygen into energy, mostly in the form of heat, as well as water and carbon dioxide (Silva, 2010 ; Goedhals-Gerber, Haasbroek, Freiboth & Van Dyk, 2015). As respiration continues and more carbohydrates are used for energy creation, compounds affecting the fruit’s turgor, flavour, sweetness and nutritional value are diminished (Silva, 2010), which has a detrimental effect on the fruit’s overall quality. Different fruits have different rates of respiration, but temperature has a substantial effect on the rate of respiration. Temperature has an accelerating effect on the rate of respiration in fruits, which
39 diminishes the fruit’s shelf life as well as quality. According to Silva (2010), “for every 10°C rise in temperature, the respiration rate will double or even triple.”
Ethylene is naturally produced by plants during a biological phenomenon known as allelopathy. It is a colourless gas, which is flammable at room temperature, but functions similarly to a hormone as it triggers specific events such as ripening, seed germination and flower initiation among others (Silva, 2010 ; Fresh Produce Exporters’ Forum, 2016:53). During ripening, ethylene induces changes such as colour and texture changes as well as tissue degradation (Silva, 2010). Ethylene can also be produced through other means including internal combustion engines, which are found in the vast majority of vehicles. As the effects of ethylene can have desirable and undesirable results, precautions should be taken to minimize unwanted exposure as even small concentrations of ethylene, from natural or artificial sources, can have a detrimental effect on fruit quality.
3.6.1.2 Climacteric and non-climacteric fruit
Climacteric fruits continue to ripen once harvested and exhibit a distinctive peak in their rate of respiration during their ripening phase. They also experience two ethylene production peaks. Firstly, during the flowering phase of development, whereafter it decreases and remains constant during the growth phase. The second peak is experienced at the onset of ripening, where production increases and continues at peak levels until the fruit is ripe (Fresh Produce Exporters’ Forum, 2016:53). The rapid increase in the production of ethylene during ripening is mirrored by the fruit’s rate of respiration. The respiration rate of climacteric fruits slows during development, but rapidly increases during the ripening process (Fresh Produce Exporters’ Forum, 2016:53). This enables the fruit to be harvested once it is mature, but unripe (PPECB HP02, n.d.) as well as handled and transported with minimal damage. This early harvesting property also prolongs the fruit’s shelf life. Examples of climacteric fruits include pome fruit, namely apples and pears as well as tomatoes, avocados, bananas, persimmons and kiwis among others.
Non-climacteric fruits do not continue to ripen once harvested and do not experience any significant increases in ethylene production or in their rate of respiration during the ripening process. Thus, non-climacteric fruits must only be harvested when mature and at the ripe stage of development (PPECB HP02, n.d.). As a result, non-climacteric
40 fruits have a shorter shelf life than climacteric fruits, as they simply begin to decay after being harvested. Examples of non-climacteric fruits include grapes, citrus, pineapples, blueberries, raspberries, strawberries and pomegranates among others. The primary goal of a cold chain is to maintain the optimal temperature conditions, in line with all specialized and specific requirements and regulations, for the temperature sensitive commodities being transported. It is important to take into account the three most important sources of heat, namely, environmental heat, such as the field heat, respirative heat generated by the product itself and lastly, generated heat from the operation of the cooling system itself such as during the defrost cycle. Understanding the pivotal role of temperature within a cold chain is imperative to its success.