Chapter 1
Preharvest factors on postharvest life

The quality of a crop at harvest can have a major effect on its postharvest life. There are numerous factors involved and these factors frequently interact giving complex interrelationships. In tree crops, fruit produced on the same tree and harvested at the same time may behave differently during marketing or when stored. The issues that influence produce quality include obvious things, such as harvest maturity and cultivar or variety, but also the climate and soil in which it was grown, chemicals which have been applied to the crop, and its water status. Many of these factors can also interact with time such as when fertilizers or irrigation is applied or the weather conditions near to the time of harvest.

An equation was proposed (David Johnson 1994, personal communication) to predict the probability of low temperature breakdown in apples in storage where variance accounted for 56%. This equation was based on preharvest factors such as temperature, rainfall and nutrient level in the leaves and fruit of the trees as follows:

where:

1. = mean daily maximum temperature in June
2. = difference in mean daily maximum temperature in August and September
3. = total rainfall in August and September
4. = level of nitrogen in the leaves
5. = level of phosphorous in the fruit.

Nutrients

The soil type and its fertility affect the chemical composition of a crop. Excess or deficiency of certain elements from the crop can affect its quality and its postharvest life. Many storage disorders of apples are associated with an imbalance of chemicals within the fruit at harvest (Table 1).

Table 1 Storage disorders and other storage characteristics of Cox's Orange Pippin apples in relation to their mineral content (source: adapted from Rowe 1980)

The relation between the mineral composition of fruits and their quality and behaviour during storage is not always predictable (Table 2) but in some cases the mineral content of fruits can be used to predict storage quality. For good storage quality of Cox's Orange Pippin apples it was found that they required the following composition (on a dry matter basis): 50-70% N, 11% minimum P, 130-160% K, 5% Mg and 5% Ca for storage until December at 3.5 °C or 4.5% Ca with minimum storage in 2% O2 and <1% CO2 at 4 °C until March (Sharples 1980).

Table 2 Summary of the most consistent significant correlations between mineral composition (fruits and leaves) and storage attributes in a 3-year survey (1967, 1968 and 1969) of Cox's Orange Pippin commercial orchards (source: adapted from Sharples 1980)

The physiological disorder that results in the production of colourless fruit in strawberries is called albinism. The fruit, which were suffering from this physiological disorder, were also found to be softer. The ratio of potassium : calcium and nitrogen : calcium was found to be greater in fruit suffering from albinism than in red fruit (Lieten and Marcelle 1993). Albinism was associated with the cultivar Elsanta and some American cultivars, and the recommendation for control was either to grow only resistant cultivars or decrease the application of nitrogen and potassium fertilizers (Lieten and Marcelle 1993).

The application of fertilizers to crops has been shown to influence their postharvest respiration rate. This has been reported for a variety of fertilizers on several crops including potassium on tomatoes, nitrogen on oranges and organic fertilizers on mangoes. An example of this is that an imbalance of fertilizers can result in the physiological disorder of watermelon called blossom-end rot (Cirulli and Ciccarese 1981). However, care must be taken in interpreting experimental results since the application of fertilizers may simply be correcting nutrient deficiencies in the soil that may be having detrimental effects of the physiology of the crop.

Nitrogen

Generally crops that contain high levels of nitrogen typically have poorer keeping qualities than those with lower levels. Results on the effects of nitrogen fertilizer on the storage life have been variable. In shallots the highest incidence of sprouting was found in the treatment combination of 150 kg N ha-1, 50% top fall harvest and non-cured bulbs which accounted for 16.73% sprouting, while the least was observed from zero N, 75% top fall harvest and cured bulbs which was 2.37% at the end of 3 months of storage (Woldetsadik and Workneh 2010). Bhalekar et al. (1987) also observed that sprouting of onions was increased with increasing nitrogen levels from 0 to 150 kg N ha-1. Dankhar and Singh (1991) also reported that high dose of nitrogen produced thick-necked onion bulbs that increased sprouting in storage while Ystaas (1980) showed that the application of nitrogen fertilizer to pear trees did not affect the soluble solids content, firmness, ground colour or quality of the fruit. Kodithuwakku and Kirthisinghe (2009) applied different levels of urea fertilizer to growing cauliflowers and found no significant difference in the postharvest quality of cauliflower curds. However, they found that applying 50% of the recommended N fertilizer resulted in an extension in their postharvest life of 6 days longer than other treatments in storage in ambient conditions. Anonymous (2010) reported that limiting nitrogen fertilizer resulted in improved shape, size and storability of swedes (Brassica napus var. napobrassica).

Application of N fertilizer can affect postharvest quality. Link (1980) showed that high rates of N fertilizer to apple trees could adversely affect the flavour of the fruit. Comis (1989) reported that too much soil nitrogen could reduce the vitamin C content of Swiss chard. Rogozinska and Pinska (1991) found that loss of tuber weight during storage increased with increasing fertilizer rate but loss of starch was high only at high N rates (200 kg N ha-1). They also found a negative effect on the organoleptic value of tubers after 200 kg N ha-1, especially after 6 months of storage. Potatoes grown with high levels of N had lower amounts of free sugars at all times (Roe et al. 1990). High N levels delayed tuber formation resulting in more immature tubers when harvested at the same time compared with tubers grown with lower N levels (Bodin 1988). Admiraal (1988) found that tuber density was less within those that had been grown in 150 kg N ha-1 applied 4 weeks after harvest and after 3 months of storage at 10 °C compared with those grown without N fertilizer. Kolbe et al. (1995) found that at harvest, the glucose and fructose contents in tubers were lower for those that had been grown with high rates of N fertilizer compared to low rates or absence of fertilizers, but throughout storage, reducing sugar accumulation increased, sucrose reduction decreased and ascorbic acid content increased. N decreased reducing and non-reducing sugar content after storage for 3 months at 10 or 15.5 °C (Badshah et al. 1990). During storage of potatoes at 4 °C and 90% r.h. there was an increase in water loss of 54% as a result of N fertilization (Kolbe et al. 1995). Woldetsadik and Workneh (2010) found that with a basic dressing of 92 kg ha-1 P2O5 increasing N levels (0, 50, 75 or 100 kg ha-1) showed proportional increase in the shallot bulb pungency levels, but the dry matter, TSS, total sugars and reducing sugars were not significantly affected either at harvest or during storage. However, there were increments in the percentage bulb rotting, sprouting and weight loss with increased N levels. Since nitrogen fertilizer can affect quality it may be summarized that they could affect their susceptibility to handling damage. However, increasing levels of N fertilizer application did not affect the susceptibility of potato tubers to mechanical damage...