2. LITERATURE REVIEW
2.6 Postharvest Quality Attributes of Tomatoes
2.6.4 Storage time effects on the postharvest quality attributes of tomatoes 26
According to Wills et al. (2007), tomatoes are highly perishable commodities and when stored at a minimum storage temperature of 10°C, have a shelf-life of between one and three weeks. The end of the shelf-life often coincides with the onset of decay, due to attack by microorganisms, including fungi and bacteria. For tomatoes harvested at the mature-green
27
stage, the postharvest storage time is also used for ripening as well as transport from the farmers to the markets. In addition to colour changes and fruit softening discussed in the preceding sections, tomato fruit flavour and acidity are also affected by storage time. Studies have shown that, the TSS content of tomatoes increases with increasing storage time and fruit maturity (Salunkhe et al., 1974; Žnidarčič et al., 2010). Cantwell (2000) reported TSS values of 2.37, 2.42 and 5.15°Brix for tomatoes at the mature-green, breaker and the red-ripe stages, respectively. This increase has been attributed to the breakdown of complex sugars into simple sugars, driven by the respiration process taking place during the ripening process. The acidity, accompanied by a simultaneous reduction in the pH of tomatoes, has also been shown to increase with maturity and an increase in storage period (Davies and Hobson, 1981;
Žnidarčič and Požrl, 2006; Caliman et al., 2010). The pH of maturity green tomatoes was found to be 4.20, 4.17 at the breaker stage and 4.12 at the red-ripe stage (Cantwell, 2000).
2.7 Discussion
The microclimate conditions that are important for crop growth are solar radiation, temperature, RH, internal air velocity and CO2 levels. Of these environmental factors, temperature and relative humidity are the most critical parameters, which can be controlled to ensure optimum growth conditions and improved yields. These factors are functions of the latitude, climate and ventilation system of a greenhouse. Natural ventilation is the most common method of internal environmental control, since it is less costly to install and has lower energy requirements for the manipulation of the internal environment. The common side and/or roof ventilation system in naturally-ventilated structures was developed for the milder environmental conditions prevalent in the northern hemisphere (Boulard et al., 1997).
The internal microclimate has been observed to be variable, with higher air velocity at the vent openings and calmer conditions at the centre of the greenhouse. The internal air temperature and RH, which tend to depend on the internal air flow pattern, are also non- uniform, thus creating non-optimum growth conditions. Because of this variation, the crop growth and yield may also be variable.
Although there is limited literature on greenhouse ventilation systems for any of the variable agro-climatic conditions in Sub-Saharan Africa, the performance of the roof and/or side naturally-ventilated structures may be hampered by high solar radiation and external
28
temperatures that are characteristic of Southern Africa. Maboko et al. (2010) and Mashonjowa et al. (2010) both measured high internal air temperatures in naturally-ventilated greenhouse facilities, which were non-optimal for crop performance.
The fan-pad evaporative cooling and ventilation system may be used as an alternative system to natural ventilation, for modifying the micro-environmental conditions in greenhouse structures. However, this system has high installation, operational and maintenance costs (Ould Khaoua et al., 2006; Flores-Velaquez et al., 2010). Furthermore, in humid and sub- humid climatic zones, such as the coastal areas of Southern Africa, its performance may be limited by high relative humidity (Kumar et al., 2009). Comparison between these two systems shows that fan-pad evaporative cooling maintains lower internal temperatures and higher relative humidity levels than natural ventilation using roof and/or side ventilators. The temperatures in naturally-ventilated structures are often above, or fluctuate around, external ambient air conditions. Although the relative humidity in the naturally-ventilated greenhouse is consistently lower than in the fan-pad evaporative cooled system, it falls within the optimum range recommended for greenhouse crops.
Different researchers report conflicting crop growth and yield results from these two systems.
Although extensive research on greenhouse facilities has not been done in Southern Africa, investigations conducted in South Africa indicated that crops grown in a fan-pad evaporatively-cooled and ventilated greenhouse grew faster and produced higher marketable yields than in a naturally-ventilated greenhouse (Maboko et al., 2010). Investigations conducted in countries in the northern hemisphere indicate that although the naturally- ventilated facilities produced smaller sized produce, the produce was of better marketable quality than in the evaporative cooled facilities (Teitel et al., 2007; Max et al., 2009).
Although it is possible that the difference in the quality of produce could have been due to varying cultivar heat tolerance levels, the internal microclimate also has a profound effect on the quality of the crop. The conflicting results clearly illustrate the dependence of the internal environmental microclimate conditions on the external environment and the need to develop structures that are less expensive and more suited for the variable agro-climate conditions in the Southern African region.
29
Pre-harvest climatic conditions have a profound effect on the postharvest quality of tomatoes.
Growth temperatures above 32°C and below 12°C inhibit the normal development of the red colour of tomatoes (Gruda, 2012). The temperature and RH also affect the cuticular membranes of the tomatoes, influencing the texture of tomatoes (Matas et al., 2005). This has been attributed to the temperature-dependence of the enzymatic activity on the cell wall of tomatoes (Meli et al., 2010). High temperatures during the developmental stages of the fruits, aid the translocation of sugars, mainly sucrose, to the fruits, whereas increased enzymatic activity during the ripening stages results in tomato fruits with high sugar contents.
In addition to pre-harvest climatic conditions, the postharvest qualities of horticultural crops are also subject to genetic variations and postharvest storage temperature and RH. However, genetic responses may be modified by pre-harvest conditions. For example, instead of the normal red colour of tomatoes, a yellow colour may be synthesized in response to high growth temperatures (Camelo and Lopez, 2004). Cold storage temperature and high RH delay the ripening process and extend the shelf-life of tomatoes. Cold temperatures reduce the respiration and transpiration rates and the accompanying biochemical activities, which delays the change in colour and texture that occurs in postharvest. Understanding the response of tomatoes to the greenhouse microclimates of greenhouse facilities available in South Africa, can provide information for selection of both greenhouse type and cultivar. It can also help to minimize the losses that occur after harvesting, as well as the type of storage facilities required.