Capsicum fruits, whether they are used as green or red ripe fruit, fleshy or dried, as medicine or dye, or for seed purposes, are to be stored for varying periods before being used. Therefore, an attempt has been made in this chapter to review the research work done on storage aspects of green and red ripe fruits in processed or whole fruit form. Moreover, special emphasis has been given to the storage of seeds, as Capsicum is a seed propagated crop and its productivity and quality are very much influenced by the quality of seeds used for production.
Capsicum fruits, both red ripe and green, are used for imparting pungency and flavour to food. Sweet peppers are mainly used at the green stage as salad and for cooking. In addition to providing pungency and flavour to the food, Capsicum is also a good source of vitamins A and B, and has several medicinal and insecticidal properties. Recently, the paprika types have been used extensively as a source of natural dye because of their deep red fruits. Capsicum fruits, whether they are used as green or red ripe fruit, fleshy or dried, as medicine or dye or for seed purposes, are to be stored for varying periods of time before they are used. Therefore, it is very important that the fruits retain their original characteristics and suffer very little deterioration during storage. Thus, the storage of Capsicum needs special attention.
Storage of green fruits
There is much demand for fresh, green, pungent and sweet peppers as they are a major ingredient of spicy foods, especially of Indian food items. Demand for fresh pungent peppers has greatly increased in the United States as south-western foods have become the most popular ethnic food. Freshly harvested bell peppers have a shelf-life of only a few days since they lose water rapidly after being harvested. Extending post-harvest longevity to meet the demand for fresh peppers, therefore, requires special attention.
Research on extending the shelf-life of fresh peppers is rather sparse. Loss of water content, flaccidity, changes in colour and development of diseases are some of the physical changes that occur during storage and which have a direct impact on the loss of fruit quality. The rate of water loss, flaccidity and colour development increase with an increased temperature. Placing pepper fruits in perforated polythene pouches at 14-20°C reduced water loss rates 20 times or even more, eliminated flaccidity and reduced colour development. The development of diseases in packed fruits, however, results in a reduction of post-harvest quality. In contrast, Ben Yehoshua et al. () reported that bell peppers packaged in perforated film lost less weight during storage and maintained a higher quality than fruits stored in open boxes. They also demonstrated that lower decay levels occurred in the fruits stored in perforated films compared to the non-perforated packing.
Post-harvest water loss of fruits is the major factor influencing the quality of the fresh fruits of peppers. Water loss was found to be positively correlated with the initial water content, ratio of surface area and volume and the cuticular thickness of the fruits and negatively correlated with surface area and epicuticular wax content and hence, is cultivar dependent.
Shelf-life of fruits with thick exocarp is more compared to fruits with thin exocarp. However, the fruits with thick exocarp are tough, difficult to digest and, if processed, the exocarp peels off from flesh, and not preferred. On the other hand, fruits with very thin exocarp which are preferred by consumers and processing industry, are tender and readily bruised, cracked and crushed during post-harvest operations. These mechanical injuries affect pepper quality and subsequent shelf-life which ultimately reduce the market grade. To reduce the damage during post-harvest operations all locations should be cushioned where peppers impact a hard surface and drop height should be limited to 8 cm on hard surface and 20 cm on a cushioned surface.
Storage of fruits under controlled conditions was found to enhance their longevity. Fruits can be stored for longer periods (14-28 days) at 8°C and more than 75% RH (relative humidity) with minimum change in chemical composition and market acceptability. Further, storage can be extended for another six to eight months at freezing temperature with or without blanching. However, Sundstrom reports that bell peppers are subject to chilling injury, characterized by pitting of the fruits, if stored in temperatures less than 7°C. At storage temperatures above 10°C, however, further fruit ripening and the development of anthocyanin and red carotenoid pigments will occur. At these warmer temperatures, bacterial soft rot also becomes a major problem. Hence, the temperature range of 7—10°C at 90—95% RH is ideal for the storage of bell peppers for up to three weeks.
From these studies it is observed that water loss is no longer a limiting factor of fresh fruit longevity if the fruits are stored in a modified atmosphere or in perforated polythene film. However, the limiting factor under these conditions is the incidence of diseases. Bell peppers, after harvest, are graded and frequently run through a hot (53°C) water bath containing 500 ppm chlorine to control bacterial rots. Following this process, most bell pepper fruits are sprayed with a wax emulsion to reduce moisture loss prior to being packed in cardboard cartons. In another study, Fallik et al. () found that dipping naturally infected fruits in water at 50°C for three minutes completely inhibited or significantly reduced the fruit decay caused by Botrytis rinera and Alternaria alternata without any adverse effect on fruit quality.
Since these studies are independent of storage conditions and specific to only a few pathogens, further studies are required to develop suitable storage practices in order to protect fruits from storage decay.
Storage of red ripe fruits
The quality of red chilli is based on colour, pungency and their retention during storage. The development of red colour is attributed to the presence of about 20 carotenoids, of which capsanthin is the major one. Pungency is due to the mixture of seven homologous branched chains of alkyl vanillyl amides, namely, capsaicinoids and these are produced in the glands of the fruit’s placenta.
Factors affecting the quality of red chilli during storage
Colour loss during storage makes red chilli unacceptable to consumers, even though no change occurs in flavour and aroma. It has been found that colour impairment in red chilli starts when the fruits have lost about one-third of their water content and become over-ripe. This colour impairment is accelerated several-fold as a result of the grinding and storage of paprika and is probably due to the development of heat at the time grinding. Positive correlations with high regression coefficient values have been obtained between colour retention by paprika powders and the initial concentrations of tocopherol and ascorbic acid. Further, cultivars have showed substantial variability with respect to carotenoid composition and endogenous antioxidant content and those with higher levels of ascorbic acid and tocopherol have shown greater colour stability during storage in powder form. Although there is an inverse relationship between the degree of pungency and the amount of deterioration during storage, no such correlation between capsanthin content and reduction in quality could be established.
Fruit drying is the most important step that determines the final quality of the product. In India, chilli growers of small holdings dry the fruits using traditional methods. The methods involve sun drying by spreading fruits on either a cement floor, a mud floor, a floor smeared with cow dung, a zinc sheet, a polythene sheet, a granite floor or on the roof of red tiles. In a study involving these traditional methods, including oven drying at 65°C, it has been observed that drying the fruits on the plant itself and in a hot air oven was found to be better as these methods have resulted in a lower percentage of whitened fruits compared to the other methods. However, drying fruits on the plants may not always be possible because of bad weather conditions. Among the traditional methods, drying under house shade or on open ground and even on cement floors were found to be better and practically feasible, especially for large scale drying of fruits, particularly in the tropical countries where sunlight is not a limiting factor. Moreover, this type of drying would reduce the percentage of whitened fruits.
The industrial drying of pepper fruits for paprika production involves mainly two methods namely, oven drying and smoke drying. Each of the drying processes has its own merits and demerits that effect the stability of the final product. One such disadvantage of rapid oven drying at high temperature is the loss of colour stability during storage. On the other hand, slow drying using wood smoke could result in degradative processes which are associated with the post-harvest physiology, as well as rotting arising from breakage, infection, etc. The possible colour stability produced by drying at low temperatures can be diminished by the presence of highly reactive chemicals formed during the smoking process, but these can provide an efficient protection against any infection. However, the same authors in another study comparing the two industrial drying processes noticed a greater loss of carotenoid content in oven drying than in smoke drying. In fact, they have also observed an increase in concentration of some of the pigments during smoke drying, although the final carotenoid content of the dried fruits is less than that of fresh fruits. The loss of carotenoid content during drying and storage is attributed to a drastic reduction in the concentration of antioxidants, such as tocopherol and ascorbic acid, as a result of the antioxidation process. Further, the rate of pigment degradation is greater at a higher storage temperature and lower relative humidity.
These studies suggest that the loss of fruit quality in terms of vitamins and their precursors during drying is very much dependent on the temperature and the rate of drying. Further, drying processes also determine the subsequent storability of chillies.
Dried chillies, when stored, are often attacked by the drugstore beetle Stegobium paniceum L. and the cigarrette beetle Lasioderma serricorne (Fabricius). The Arthrodeis species feed on dried chillies, though the loss caused by them is negligible. If the quantity of chillies infested by these storage pests is small, spreading the fruits in thin layers and exposing them to sunlight will eliminate the infection. If large quantities are infested with these pests, fumigation is the only remedy. Any good fumigant like ethyl dibromide or methyl bromide at one gram per 30 cubic metre space may be used. Fumigation should be done in airtight containers for the effective control of these stored pests.
The storage life of green chillies harvested for vegetable purposes can be extended up to four to six weeks under low temperature and high RH conditions. However, there are contradictory reports on the ideal low temperature range for storage as chilling injury to fruits has been reported at temperatures below 6°C. This aspect of storage requires further study, with a greater number of genotypes, to know if there is any genetic variability for temperature sensitivity. Under ambient conditions, the storage life of fruits can be extended by packing them in perforated polythene film.
Loss of colour during storage is the major concern in dried red chilli. It has been found that the retention of colour during storage is directly correlated with the amount of tocopherol and ascorbic acid present in the fruits and thus varies with the cultivars. Drying temperature is another factor that affects the colour retention during storage; high temperature is harmful as it results in the degradation of antioxidants. Higher storage temperature increases the rate of pigment degradation but an increase in RH level reduces the degradation.
The storage of seeds in fruits is found to maintain viability for longer periods but it is not practically feasible for large-scale seed production. The fruits harvested at the red ripe stage yield good quality seeds, although some studies showed the need for a post-ripening period before and after seed extraction. There is a lot of variation among cultivars for seed longevity during storage. The work on seed drying is negligible. Seeds with a moisture content of 6—8% and stored in airtight containers could maintain seed viability for two years. For long-term storage, seeds can be stored for many years if stored at lower temperatures in airtight containers. There are conflicting reports on ultra drying effects on seed longevity and this aspect of drying requires further study.
Selections from the book: “Capsicum. The genus Capsicum”. Edited by Amit Krishna De. Series: “Medicinal and Aromatic Plants — Industrial Profiles”. 2003.