Isotherms for Australian canola varieties

Transcription

1 From: E.J. Wright, M.C. Webb and E. Highley, ed., Stored grain in Australia Proceedings of the Australian Postharvest Technical Conference, Canberra, June CSIRO Stored Grain Research Laboratory, Canberra. Isotherms for Australian canola varieties J.A. Cassells, L.P. Caddick, J.R. Green and R. Reuss Stored Grain Research Laboratory, CSIRO Entomology, GPO Box 1700, Canberra, ACT 2601 Abstract. Canola seed needs to be stored cool and dry to minimise spoilage of the oil. Excessive moisture levels in stored canola lead to rapid heating and loss of oil quality. The equilibrium relative humidity (ERH) of air surrounding bulk stored canola reaches conditions where heating and spoilage occur at lower grain moisture contents than in cereals and coarse grains. A moisture isotherm describes the relationship between grain moisture and oil content, and ERH, when water in the air is in equilibrium with water in the grain at different temperatures. Commercial grain handlers can use this to determine safe conditions for long-term storage of canola. In this paper, moisture isotherms are presented for selected current canola varieties at 20 and 30 C. The data were modelled using the enhanced Halsey equation and compared with previously published data on Australian varieties. Conditions suitable for safe long-term storage of canola are discussed. Introduction Storage conditions play a vital role in determining canola seed and oil quality. High moisture and temperature can be detrimental, causing loss in seed viability (Reuss and Cassells 2003), and increased levels of free fatty acids (FFA) and products of oxidation in the oil. The level of FFA is a major quality parameter used by the oilseed industry to indicate loss of oil quality at harvest receival and after storage (Steele 1991; Banks 1998). Moulds are also a problem when oilseeds are stored at high moisture and temperature (Steele 1991), and are a primary cause of rapid heating in oilseed bulks. Heating and spoilage of canola occur at lower grain moisture contents than they do in cereals. Cereals contain approximately 70% carbohydrates, whereas in canola they are in the range 25 30%, depending on oil content. Starch has a stronger affinity for water than does oil, and the oil fraction in oilseeds is essentially hydrophobic, with minimal amounts of water (2 3% w/w) absorbed. The capacity of canola to absorb water on a whole seed basis is therefore much less than it is in cereals. This is fundamental to understanding the difference in the storage potential of canola compared with cereals under the same conditions. In essence, the greater the oil content of the seed, the lower the total seed moisture level required to produce heating and loss of quality. Small changes in the moisture content of canola seed also result in substantial changes in the equilibrium relative humidity (ERH) of a canola bulk. Cereals are buffered from this effect, within limits, and the response in ERH to changing moisture levels is less. To assess storage risks, the relationship between canola moisture, oil content and ERH needs to be defined at different temperatures. A moisture isotherm describes this relationship, where ERH is the amount of water in the air surrounding the canola seed when in equilibrium with water in the grain. Moisture isotherms have been published for several overseas canola varieties (Anon. 1997), and for Australian rapeseed (Steele 1991). This paper provides a moisture isotherm for a selection of current Australian canola varieties. It will enable grain storage operators to determine safe storage conditions for long-term storage of canola. Materials and methods Different varieties of canola, Brassica napus L., with a range of oil contents, were collected during the and harvest seasons. The varieties were Pinnacle, Oscar, Monty, Scoop, Rainbow, Karoo, Hyola and Grouse. Seed was conditioned to higher moisture contents at 4 C and 85% RH, and stored at 4 C before testing. Moisture content The International Standards Organization (ISO) standard oven-dried method No. 665 (Anon. 1977) was used to determine seed moisture content. Moisture content was expressed on an oil-free, dry matter basis (% of d.b.) using the calculations given in Steele (1991). Equilibrium relative humidity Approximately 400 g of seed was placed into sealed glass vessels with a sintered glass plenum. The seed was 59

2 Stored grain in Australia 2003 maintained for one week at 20 or 30 C in a controlled temperature water bath to establish equilibrium before testing. ERH of the canola was determined in a closed loop, using a MBW Model DP 3-D dewpoint meter (MBW Elektronic AG, Wettingen, Switzerland). All testing was undertaken in a controlled temperature environment. Oil content The method used for determining oil content was based on ISO standard No. 659 (Anon. 1988). Twelve g of canola was ground with a Moulinex Type 843 coffee/spice grinder for 30 seconds. The meal was placed in an extraction thimble and the oil extracted using a Soxhlet extraction apparatus. Oil was extracted at 60 C with 300 ml of petroleum spirit (BP C). After 6 hours of extraction, the solvent was evaporated under a flow of nitrogen, and the oil weighed. Seed oil content was expressed as percent by mass of the dry matter. Results and discussion Canola isotherms at 20 and 30 C are shown in Figure 1. Moisture content (MC) is given on an oil-free dry basis (% of db). The amount of oil in the seed influences the behaviour of water in a bulk, and canola samples used in this study ranged from 32 to 48% oil content. To establish a moisture isotherm based on seed with different oil contents, the oil is assumed to be independent of moisture and removed from the calculation. This is a valid approach since canola oil is essentially hydrophobic, with the amount of water absorbed by the oil being negligible. Conversion of moisture content to an oil-free, dry basis allows the data to be modelled using the modified Halsey equation (equation 1). ERH = 100 exp{ [exp (A + B.T)]/(MC D ) C } (1) where ERH is equilibrium relative humidity, T is temperature ( C), MC D is moisture content dry basis and A, B and C are constants. The constants are A = 3.997, B = and C = The modified Halsey equation provides a useful model for high oil and protein products (Chen and Morey 1989). The isotherm curve or line of best fit is calculated from the least sum difference between data points. Variations observed in the data set are due to factors such as canola variety, and paddock or preharvest history, i.e. weathering and seed ageing. Table 1 shows the conversion of moisture content, dry to wet basis, against oil content, dry basis. This type of conversion table provides a useful guide to establish management strategies for long-term storage of canola over a range of oil contents. The highlighted areas indicate the ERH of a given batch of canola when it is received and stored at 20 or 30 C. The current industry moisture content receival limit of 8% (wet basis) is underlined. Steele (1991) showed that mould infection levels of 100% resulted when rapeseed was stored at 72% ERH for 4 months at 35 C and 6 months at 25 C. When rapeseed was stored at 68% ERH and 35 C, mould infection levels reached 100% after 12 months, while at 25 C mould levels remained low. Moisture content (%, oil-free dry basis) C 30 C Equilibrium relative humidity (%) Figure 1. Moisture isotherm for canola at 20 and 30 C. Data fitted with the modified Halsey equation: ERH = 100 exp{ [exp (A + B.T)]/(MC D ) C } where ERH is equilibrium relative humidity, T is temperature, MC D is moisture content dry basis and A, B, C are constants. The constants are A = 3.997, B = and C =

3 Stored grain in Australia 2003 Table 1. Conversion of seed moisture and oil content dry basis to wet basis, indicating potentially unsafe long-term storage at 20 C and 30 C. Seed moisture content Conversion of seed oil content from % dry basis to % wet basis % w.b. % d.b ERH % Storage temperature 20 C Storage temperature 30 C Recommended management of bulk canola Cooling Cooling Cooling and in-store drying Rapid cooling and in-store drying Rapid drying followed by cooling 61

4 Stored grain in Australia 2003 Xerophilic (storage) moulds were not able to grow on stored wheat at 30 C when the ERH was at or below 68% (Caddick 1998; Caddick and Shelton 1998). The interaction between ERH and temperature largely determines the range of individual fungi that can germinate, grow and sporulate (Lacey and Magan 1991). The growth of xerophilic moulds on grain where moisture levels are marginal for their requirement will be slow, even at favourable temperatures. However, as the ERH increases, the rate at which xerophilic moulds can develop also increases. Steele (1991) and Caddick (1998) show that cooling stored grain curbs the growth of xerophilic moulds when ERH favours their development. Water is more tightly bound to the grain substrate at lower temperatures, which reduces the amount available in the air surrounding the grain. The different moisture relationship of stored canola at 20 C compared to 30 C shown in Figure 1 reflects this behaviour. The data from this study show that canola with high oil contents ( 42%) received at the maximum 8% moisture receival limit, and stored at 30 C, presents conditions favourable for mould growth, and supports the current practice of bulk handling companies to cool canola immediately after harvest. The level of free fatty acids (FFA) present in oil derived from oilseeds is one of the most useful direct and quantitative indicators of oil (Robertson et al. 1985; Steele 1991). There is a maximum limit of 1.5% FFA in freshly harvested canola, although some marketers and processors may require even lower levels. Canola received with higher FFA levels is downgraded, and is rejected when levels exceed 2.5% (Anon. 2002). Fatty acids in oilseeds are subjected to slow, consistent breakdown during storage. Heating of canola increases the rate of hydrolysis and oxidation of fatty acids in the seed, leading to a higher level of undesirable compounds and free fatty acids. Higher moisture levels also influence the rate that these chemical processes occur. Loss in oil quality is often accompanied by a darkening in oil colour. Reuss and Cassells (2003) showed that, over a 9-month storage period, FFA levels at 30 C and 8% MC (w.b.) increased by %, whereas at 20 C the increase was lower at %. Oil and seed quality is maintained when canola is stored under safe storage conditions, indicated by the unshaded zone in Table 1. Canola stored at moderate to high temperatures and high moisture levels is therefore at risk of rapid loss of oil quality. Where canola is stored for a short time at moisture contents below 8%, the potential loss of oil quality may be minimal without cooling. Freshly harvested canola that has not been weathered is particularly robust, with greater storage potential than weathered seed. Storage handlers nevertheless need to check an unaerated bulk regularly for possible signs of heating, and moisture aggregation and moulding. Safe storage conditions for oilseeds are often emphasised from the point of reducing or preventing heating of bulk seed. Losses in oil quality, however, can occur under moderate storage conditions, especially where seed that has been weathered is stored for prolonged periods. An ERH of 60% is recommended for long-term storage of canola due to the susceptibility of the seed to rapid heating. The presence of temperature gradients, or differences in oil content, will result in different levels of Figure 2. Comparison of the storability of current canola varieties and rapeseed (Steele 1991) at 60% RH. 62

5 Isotherms for Australian canola varieties water activity for oilseeds at the same moisture content. Therefore, parts of an oilseed bulk can be at higher risk of heating and quality loss. Storage operators need to be aware that oilseeds are unique in this regard. Where the ERH exceeds 65%, the storage potential of canola is reduced, and rapid cooling and in-store drying would be required to maintain canola quality during long-term storage. In-store drying using aeration has limitations, and excessively wet canola is likely to deteriorate before the seed can be cooled and dried. Harvesting of canola at high moisture contents should be avoided, unless the storage handler has the facilities and experience to rapidly dry the seed. Information available to the oilseed industry on storage of Australian rapeseed and canola varieties has been limited to that gathered in a study of rapeseed by Steele (1991). The data are summarised by Banks (1998). The comparative data of Steele (1991) for Australian rapeseed are shown in Figure 2. Data for canola indicate that canola at 60% RH and 30 C has a lower storage potential than rapeseed, but greater potential at lower temperatures. Seed type and variety, and paddock history of the seed used in the two studies, would account for some of the variation. Experimental methods used in the two studies would also account in part for the difference. Steele (1991) used a calibrated relative humidity sensor to determine relative humidity. Rapeseed was also initially dried to 6% moisture content (w.b.) at 35 C, and then conditioned to higher moisture levels through the addition of water using a fine atomised spray. Conclusion Canola is less storable at higher temperatures, moisture and oil contents. Aeration can be used to even-out moisture and temperature within a canola bulk. The prevention of temperature gradients and localised heating within a canola bulk will minimise quality loss. Active storage management practices, such as cooling and instore drying, are required to maintain the quality of canola with high moisture, especially when seed also has high oil content. The data on moisture isotherms at 20 and 30 C for Australian canola varieties provide a basis for commercial grain storage operators to determine appropriate conditions of safe storage for canola. References Anon ISO International Standard No Oilseeds Determination of moisture and volatile matter content. Switzerland, International Standards Organization, 2 p. Anon ISO International Standard No Oilseeds Determination of hexane extract (or light petroleum extract), called oil content. International Organisation of Standardization, Switzerland, 5 p. Anon ASAE Standard D Moisture relationships of plant-based agricultural products. St Joseph, USA, American Society of Agricultural Engineers, Anon Agricultural commodity standards manual. Oilseeds. National Agricultural Commodities Marketing Association. Wilberforce, NSW. Banks, H.J Effect of storage conditions on quality change in canola. In: Banks, H.J., Wright, E.J. and Damcevski, K.A., ed., Stored grain in Australia. Canberra, CSIRO Stored Grain Research Laboratory, Caddick, L.P Effect of grain moisture content and temperature on storability and end-product quality of white-grained wheat. In: O Brien, L., Blakeney, A.B., Ross, A.S. and Wrigley, C.W., ed., Proceedings of the 48 th Australian Cereal Chemistry Conference, August, Cairns, Queensland. Royal Australian Chemical Institute, Melbourne, Caddick, L.P. and Shelton, S.P Storability of wheat at 13% moisture content: field study and review. In: Banks, H.J., Wright, E.J. and Damcevski, K.A., ed., Stored grain in Australia. Canberra, CSIRO Stored Grain Research Laboratory, Chen, C. and Morey, R.V Comparison of four EMC/ERH equations. American Society of Agricultural Engineers, 32, Lacey, J. and Magan, N Fungi in cereal grains: their occurrence and water and temperature relationships. In: Chelkowski, J., ed., Cereal grains: mycotoxins, fungi and quality in drying and storage. Elsevier, Amsterdam, Reuss, R. and Cassells, J The effect of storage conditions on the quality of Australian canola (rapeseed), Brassica napus L. In: Credland, P.F., Armitage, D.M., Bell, C.H., Cogan, P.M. and Highley, E., ed., Advances in stored product protection. Wallingford, Oxon, CAB International, Robertson, J.A., Roberts, R.G. and Chapman, G.W An evaluation of heat damage and fungi in relation to sunflower seed quality. Phytopathology, 75, Steele, R.J Sub-project 1. Acceptance standards. In: Safe storage of rapeseed and other oilseeds. Oilseeds Research Council, Canberra. Reprinted by Stored Grain Research Laboratory, Canberra, Acknowledgments The partners of the Stored Grain Research Laboratory funded this work. Thanks are extended to Graincorp, Vicgrain and Co-operative Bulk Handling Limited for supplying canola samples, and Fiona Spier and Ben Boyd, CSIRO Entomology, for their assistance. 63

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