Influencing the storage environment during storage can help to prolong the shelf life of roots and tubers, the important aspects being temperature and relative humidity. In general, a longer shelf life is possible at lower temperatures. Yam dormancy can be prolonged by storage at cool temperatures (15–17 °C) or shortened by exposure to higher temperatures, although temperatures below 10 °C may cause chilling injury (Janssens, 2001). The comparison of various recommended conditions for tropical roots and tubers are presented in Table 6.7.
Table 6.7 Recommended storage conditions for tropical roots and tubers
S. No. | Root/tuber | Temperature (°C) | Relative humidity (%) | Numberof days | References
1 | Taro | 7-10 | 85–90 | 120–150 | Thompson (1996)
2 | Yam | 13–15 | 85–90 | 50-115 | Thompson (1996)
3 | Cassava | 0–2 | 85–90 | ― | Snowdon (1990)
4 | Sweet potato | 12.8-15.6 | 85–90 | 120–150 | Thompson (1996)
In tropical regions, storage losses can assume considerable economic importance (Wills et al., 1998) and deterioration of produce during storage may also have an adverse effect on the processed product. It is for this reason that many traditional and modern methods of storage have been devised to ensure proper storage and combat losses (IITA, 1990).
Most plant tissues have natural mechanisms for healing wounds, a physiological process whose purpose is to prevent entry of microorganisms and loss of moisture, thereby helping to inhibit post-harvest deterioration (UNIFEM, 1993; Van Oirschot et al., 2003). The different techniques can play an important role in the extension of shelf life of roots and tubers. The technique or the combination of techniques such as irradiation, refrigeration, modern atmosphere packaging can be useful for tropical roots and tubers and have shown extension of shelf life for shredded and cubed from sweet potatoes (Table 6.8). The various types of techniques employed for storage are given in Table 6.8.
Table 6.8 Various techniques employed for tropical roots and tubers to enhance quality and shelf life
S. no. | Root/tuber | Storage facilities/Treatment | Conditions | Conclusions | References
Sweet potato | X-ray irradiation and refrigerated storage | Packaged sweet potato cubes were treated with 0, 250, 500, 750 or 1,000 Gy X-ray irradiation and stored at 4 ± 1 °C for 14 days. | Mean day temperature during harvesting season (32–35 °C) and relative humidity (80–95 %) | X-ray irradiation treatment at doses up to 1,000 Gy can reduce microbial populations while maintaining the physical quality and anthocyanin content to 14 days of storage. | Oner and Wall, (2013)
Sweet potato | Use of semi permeable packaging materials and modified atmosphere packaging | Curing efficiency of varieties differed in response to curing periods. Other treatments like fungicide, bio-control, gamma irradiation, hydro warming, and storage in sand and saw dust were found to have intermediate impacts in controlling spoilage | Ray and Ravi (2005)
Sweet potato | Curing | Two major commercial cultivars (Beauregard and Hernandez) were packed in low and medium O2 permeability bags and flushed with gas composed of 5 % O2, 4 % CO2 and 91 % N2. The quality of shredded sweet potatoes could be maintained for 7 days at 4 °C in air, but extended up to 14 days in MAP | Best results were obtained by MAP using moderately O2-permeable film (7,000 cm3/atm/m2/24 h). Shredded sweet potatoes stored in MAP showed lesser changes in tissue firmness, dry matter, ascorbic acid and starch than shredded sweet potatoes stored in air | McConnell et al. (2005)
4 | Yam | Combined treatment of aqueous chlorine dioxide (ClO2) and ultraviolet-C (UV–C) | Samples were treated with distilled water, 50 ppm aqueous ClO2, 5 kJ m2 UV–C or a combination of 50 ppm aqueous ClO2 and 5 kJ m2 UV–C | Reduction of populations of total aerobic bacteria, yeast and mould, and coliform bacteria by 3.2, 3.4 and 3.8 log CFU/g, respectively. Moreover, the combination improved microbial safety and shelf life of the yams | Chun et al. (2013)
5 | Yam | Two traditional yam barns, one with fan to aid air circulation and the other without fan. | Intermittent air flow on stored yam tubers reduces sprouting, weight loss and rot development, thereby reducing the overall loss in stored yam tuber
6 | ―"― | Treatment 1: Storage in pit (50 × 50 × 70 cm3) with alternate layers of grasses and finally covered with soil. | Temperature and relative humidity were measured three times a week and four times a day at 0800 h, 1,200 h, 1,600 h and 2,000 h. | Two storage methods moist sawdust in wooden box and pit storage with layer of river sand have good potentials for storage of sweet potatoes up to 5 months without serious change in nutrient content and could therefore be recommended to farmers | Osunde and Orhevba (2009)
―"― | ―"― | Treatment 2: Storage in pit (50 × 50 × 70 cm3) sprinkled with 30 kg of fresh river sand then layers of sweet potato alternated with layers of sand and finally covered with a layer of soil. | Pits of 50 × 50 × 70 cm3 were dug and storage conditions varied. | ―"― | Dandago and Gungula (2011)
―"― | ―"― | Treatment 3: Storage in modified pit (50 × 50 × 70 cm3) constructed inside a hut 1.5 × 1.8 × 1.0 m3 and filled with alternate layers of 5 kg of fresh sand and potato (sprinkled with water) and finally 2 cm thick sand and soil.
―"― | ―"― | Treatment 4: Storage in moist sawdust packed in a wooden box (50 × 50 × 50 cm3) and potatoes alternated with sawdust and kept wet by sprinkling water every week.
―"― | ―"― | Treatment 5: Heap storage on a layer of sand in a cool corner of the laboratory and covered with a layer of grasses.
7 | Taro | The structures consisted of pit 1 with two PVC-vents which served as the improved pit, pit 2 with one vent which was regarded as a semi-improved pit and pit 3 without any vent and served as the control pit | Pit air temperatures were monitored and these varied from 27.5-29.4 °C with a mean of ~28.6 °C in pit 1; and from 28.8-30.5 °C with a mean temperature of 29.6 °C in pit 2 while in pit 3 the temperatures varied from 30.0-31.8 °C with a mean of 30.9 °C. The relative humidities attained in the three pits were 83 %, 82.5 % and 72 % respectively. | Corms stored in pit 1 sustained a weight loss of 1.5-11 % in 8 weeks of storage. Pit 2 sustained a weight loss of 1.5-15 % in 8 weeks while its sprouting index was 37.7 % In pit 3, weight loss was from 2.5 to 16.1 % in 2 months with sprouting index of 47 %. | Obetta et al. (2007)
Whatever type of traditional store is used for roots and tubers, the structure should have adequate shade, good ventilation, security against animals, pests and thieves and protection from the sun to avoid overheating and excessive moisture loss (Etejere and Bhat, 1986). The types of storage structures used are also influenced by the purpose of storage, the type of building materials available and the resources of the farmer such as labour and capital (FAO, 1990; Knoth, 1993). Social conditions and the specific consumer requirements are also important considerations, and they point to the fact that storage requirements are location specific with different systems being more or less appropriate under different technical, social and economic conditions (Booth and Shaw, 1981).