COLD STORAGE TO EXTEND SHELF-LIFE AND IMPROVE QUALITY OF CHINESE WATERCHESTNUT
Volker Kleinhenz, David Midmore, Geoff Lodge & Bruce Tomkins
A traditional vegetable in Asia, Chinese waterchestnut [Eleocharis dulcis (Burm. f.) Hensch] is emerging as a new commodity in Australia with potential for export. Postharvest storage protocols are sought by farmers to extend the shelf-life and thereby extend availability at domestic and future overseas markets over time. Since 1997, the Plant Sciences Group of Central Queensland University (Rockhampton, Qld), Eco-lodge-ic (Dhurringile, Vic) and the Institute of Horticultural Development (Knoxville, Vic) have studied the effects of packaging and storage temperatures on shelf-life and quality of waterchestnuts.
While some quality factors are principally affected by the growing environment and cultivation practices (eg corm size, corm colour, and bruises), other quality parameters are more closely connected with postharvest storage. These parameters include weight loss, sprouting, growth of moulds on the external peel, discolouration and development of rots in internal tissues, and changes in sugar content of corms. Once one parameter falls below a critical level the shelf-life of waterchestnuts is essentially terminated. For example, if weight loss causes shrivelling of the peel of corms, they would be unacceptable to consumers even though there might be no signs of moulds and internal rots.
Although recommendations for postharvest storage have been published as early as 1955 (Hodge & Bisset, 1955), only few systematic studies have been conducted to determine the shelf-life of waterchestnuts under various storage conditions. Kays & Sanchez (1984) recommended storing corms in NaCl solution to minimise changes in weight loss, visual quality, and sugar content. However, a 'salty' taste due to increased concentration of sodium ions in corms is not acceptable to Australian customers so such overseas preservation methods cannot be adopted here. The same may apply to storage of waterchestnuts in sodium hypochlorite solution which was promoted by Kanes & Vines (1977).
A few studies (eg Brecht et al., 1992) have tested the effects of storage temperature on weight loss, sprouting and sugar content of waterchestnuts. As in other tuber crops, higher storage temperatures shortened shelf-life by increasing weight loss through transpiration and respiration, and induced sprouting. In tuber crops such as potato, with starch as the primary carbohydrate storage compound, low-temperature storage causes 'sweetening' of tubers that is not acceptable to consumers. This process of formation of sugars from starch has been found to accompany four circumstances: (1) low temperatures (particularly below 10°C), (2) sprouting, (3) senescence, and (4) decay following wounding and fungal infection (Burton et al., 1992). However, 'sweetness' is appreciated in Chinese waterchestnut. Therefore, to minimise weight loss in stored waterchestnuts and to promote sweetness, we tested different packaging materials, storage temperatures, and storage duration. We collected data on the effects of storage temperature and storage period on 'total soluble solids' (TSS) content which is an indicator of sugar content in many other agricultural commodities.
We considered corms which had lost more than 10% of their fresh weight as visually unacceptable. When stored in the open, waterchestnuts lost this proportion of their fresh weight within one week. However, when corms were packaged in low-density polyethylene (LDPE) film (ie 'snap-lock' bags) or microperforated LDPE film (ie 'Longlife' vegetable bags), loss of fresh weight was within the acceptable level over a five-month storage period regardless of storage temperature (1° and 9° C). Packaging materials did not differ in their efficacy in reducing fresh weight loss in stored waterchestnuts.
Waterchestnuts stored in a snap-lock bag
Respiration is the breakdown of plant material into carbon dioxide and water to produce energy for maintenance of cellular integrity. Compared with other horticultural products we measured low to very low respiration rates in waterchestnut (0.6-1.2 ml CO2 kg-1 h-1). Based upon these we calculated weight loss caused by respiration to comprise only 3% of fresh weight loss of corms when stored for one month at 1°C. Therefore, weight loss in openly-stored waterchestnuts is primarily confined to transpiration which can be successfully reduced by packaging materials which are minimally permeable to water-vapour transfer.
Although weight loss of waterchestnuts did not differ across the two storage temperatures tested, difference in storage temperature did eventually affect development of moulds and rots. Not surprisingly, superficial fungal growth was accelerated at the higher storage temperature of 9°C but internal rots developed more rapidly at 1°C. This latter might have been due to fluctuations in storage temperatures which could have decreased corm temperature to below 0°C and, hence, caused chilling injury and associated rots. Moulds and rots shortened shelf-life of waterchestnuts to 3-4 months at either storage temperature.
With the exception of the value for the 2-week storage period, sugar content as indicated by TSS invariably increased over the first 12 weeks of storage (Figure 1). This was accelerated by the lower storage temperature showing that the same (as yet incompletely understood) processes that cause 'low-temperature sweetening' in potato are also active in Chinese waterchestnut. At present, we have no explanation for the subsequent drop in TSS after 12 weeks and then the rise after 20 weeks. However, preliminary results from our current research indicate that apart from the process of sugar formation at low temperature, the other processes which can explain sweetening in potato have no or only limited validity in waterchestnuts: at higher storage temperature sprouting was accelerated but sweetening did not occur; corm senescence was hastened by higher temperatures but this inhibited sweetening and decay after wounding and fungal infection did not enhance sugar content.
Figure 1. Effect of storage temperature & storage period on sugar content (as indicated by TSS)
In our current research on postharvest storage of Chinese waterchestnut we have concentrated on testing a wider range of storage temperatures (4, 5, 8, 12, 15 and 20°C) to determine 'critical' temperatures below which sweetening and above which sprouting occurs. Data presented in Figure 2 show this temperature to be 13.6°C. We are also trying to calculate a 'TSS per cold unit' value (ie the inverse of heat units). This would make it possible to estimate the content of total soluble solids of waterchestnuts when the sum (or perhaps product) of time and temperature at which waterchestnuts were exposed to temperatures below the 'critical' temperature is known. Possibly, exposure duration is independent of whether it occurs during the growing period in the field or in postharvest cold storage. To understand this we have harvested and tested for TSS in corms during different growth stages, and now are studying their development of TSS under subsequent cold storage. Chand (1990) found that application of growth hormones hastened sprouting in waterchestnut grown in soil under ambient temperature. That study indicated that growth hormones might also enhance sugar content of stored waterchestnut and, therefore, we are currently testing the effects of gibberellic acid ('GA3') on corm quality at different storage temperatures.
Figure 2 Relationship between storage temperature, change in total soluble solids and sprout length of waterchestnut.
Brecht, Jk, Bergsma, KA, Sanchez, CA, Snyder, GH, 1992: Harvest maturity and storage temperature effects on quality of Chinese waterchestnuts (Eleocharis dulcis). Acta Horticulturae, 318, 313-319.
Burton, WG, van Es, A, Hartmans, KJ, 1992: The physics and physiology of storage. In: Harris, PM (Ed.): The Potato Crop. Second Edition. Chapman & Hall, London, UK, 608-727.
Chand, G, 1990: Resprouting capability of newly developed tubers of Eleocharis dulcis Trin under the influence of growth regulators and herbicides. Acta Botanica Indica, 18, 137-138.
Hodge, WH, Bisset, DA, 1955: The Chinese waterchestnut. Circular 956. US Department of Agriculture, Washington, USA.
Kanes, CA, Vines, HM, 1977: Storage conditions for Chinese waterchestnuts, Eleocharis dulcis (Burm F.) Trin. Acta Horticulturae, 62, 151-161.
Kays, SJ, Sanchez, MGC, 1984: Storage of Chinese water chestnut [Eleocharis dulcis (Burm. F.) Trin.] corms. Acta Horticulturae, 157, 149-159.
Prof. David J. Midmore / Dr Volker Kleinhenz
Central Queensland University
Tel: (07) 4930 9770/9445
Fax: (07) 4930 9255
Mr Geoff Lodge
Tel: (03) 5826 6375
Mr Bruce Tomkins
Agriculture Victoria, Knoxfield
Tel: (03) 9210 9222