5.7.2. From preharvest to postharvest quality

As described in the rest of this chapter, genotype (cultivar, see §5.6) and preharvest conditions (cultivation conditions, see §5.1-5.5) determine the quality at harvest (Figure 5.14). Quality at harvest also determines the performance of the plant product in the postharvest chain. Besides nutritional quality, quality at harvest is also determined by other physiological characteristics such as colour, texture, firmness (dependent on cell wall, and water content and turgor), and fragrance.  During the postharvest chain, postharvest conditions such as temperature, relative humidity, gas and light conditions determine how quality changes: quality features decrease, until shelf life has ended. Most postharvest chains use a combination of cooling, controlled humidity, low oxygen and high carbon dioxide, in darkness.

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Figure 5.14: Quality at harvest is determined by genotype and preharvest; shelf life is dependent on postharvest conditions. 

Low temperatures are key in a good postharvest chain. Respiration is reduced, which slows down ripening, and deterioration of nutritional content. There is a risk though, certain products are prone to chilling injury, which can lead to an early end of shelf life. Chilling injury is most common in products from tropical and sub-tropical origin, such as banana, mango, and avocado. It is however, also common in tomato; whose origin is also in the sub-tropical region of Latin America. For most products, it is known what the optimal storage temperature is to slowdown respiration without causing chilling injury. This information is available from the University of California Davis, and in Dutch at koudecentraal.nl.

Proper control of the atmospheric conditions and humidity in the postharvest chain are essential for good postharvest quality conservation. Humidity control because it can prevent excessive transpiration, and atmosphere because it can reduce respiration. Controlled atmosphere (CA) and modified atmosphere packaging (MAP) are two different methods to reduce respiration. By lowering the oxygen concentration and increasing the carbon dioxide levels, reparation is slowed down preserving quality. In the case of CA, the concentrations are maintained at a certain level, and for MAP, conditions start at a specific set point, but over time, respiration of the plant product and permeability of the packaging cause a change in conditions over time. The latter is much cheaper and works well for many products for shorter times. Examples are the long storage (6-12 months) of apples (CA) and bagged lettuce (MAP).

Often the chain is dark, mostly due to practical limitations: light cannot penetrate to the middle of a package, and even so, most products are packed in non-transparent boxes. Absence of light can lead to yellowing and bleaching of green products. For most fruits, dark storage has no severe consequences. Addition of light to the postharvest chain can have a positive effect, and this has been described in chapter 5.4.

The end of shelf life is caused by deterioration. Deterioration is dependent on physiological processes, which are influenced by mechanical damage and pathological contamination. The same physiological processes described before, determine the process of deterioration, the last stage of plant development, senescence. The end of shelf life is different for each product: for lettuce it is determined often by browning, for tomato by softening, and for banana by colour change from yellow to brown/black. Besides these visual observations, phytonutrients also change.

In conclusion, the use of proper postharvest conditions is equally important for the nutritional value of crops as the selection of good genotypes, and optimized cultivation conditions to improve this nutritional value.