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Effect of Storage Time and Conditions on the Hardness and Cooking Quality of Adzuki (Vigna angularis)
Lebensm.-Wiss. u.-Technol., 35, 338–343 (2002)
Effect of Storage Time and Conditions on the Hardness and Cooking Quality of Adzuki (Vigna angularis) A. M. Yousif*, H. C. Deeth, N. A. Caffin and A. T. Lisle
A. M. Yousif, H. C. Deeth, N. A. Caffin: University of Queensland, School of Land and Food Sciences, Gatton, Qld 4343 (Australia) A. T. Lisle: University of Queensland, School of Agriculture and Horticulture, Gatton, Qld 4343 (Australia) (Received August 30, 2001; accepted January 2, 2002) A storage trial of two varieties of adzuki (Vigna angularis), Bloodwood and Erimo, produced in Australia, was conducted to determine the effect of various combinations of temperature, humidity and length of storage on bean quality. The beans were stored for up to 6 mo under the following conditions: temperature (10, 20 and 30 1C), relative humidity (RH) (40 and 65%). Storage of adzuki at elevated temperature (30 1C) and low relative humidity (40%) resulted in the greatest loss of bean moisture, increase in hydration times and decrease in bean cooking quality, i.e. increased hardness of cooked beans. The best storage conditions for the preservation of adzuki quality were 10 1C and 65% RH.
r 2002 Elsevier Science Ltd. All rights reserved. Keywords: adzuki; beans; storage; hydration; cooking quality
Introduction In East Asian countries, adzuki (Vigna angularis) are cooked and consumed as a dessert or snack food, especially during celebrations and traditional festivals such as the Chinese New Year. In Japan, adzuki are consumed as ‘ann’ (bean paste) used alone or as a filler for various sweet pastries, and as whole beans boiled and sweetened for snacks and confectionery items. At least 50 other beans and legumes are used to make ann, but, due to the desirable colour, delicate flavour and characteristic texture of its paste, the adzuki is preferred (Breene and Hardman, 1987). Preservation of legume seeds in dry storage is the main way of maintaining a year-round supply of this food source. Unfavourable storage conditions can reduce bean quality and result in inadequate cooking, i.e. the beans remain hard due to insufficient water uptake and softening of the cotyledon tissue. Processing poor quality beans results in inferior texture and mouthfeel of ann which is believed to be caused by poor cell separation, improper starch gelatinisation and changes to the protein denaturation temperature (Ozawa, 1978; Hatai, 1982). The nutritional quality of the beans is also reduced. Overall, the changes in the beans due to *To whom correspondence should be addressed. E-mail: [email protected]
0023-6438/02/$35.00 r 2002 Elsevier Science Ltd. All rights reserved.
unsatisfactory storage conditions lead to a reduction in their commercial value. The moisture content of the seeds, storage temperature and relative humidity are important parameters for good bean preservation (Burr et al., 1968; Antunes and Sgarbieri, 1979; Kon and Sanshuck, 1981; Stanley, 1992; Garcia and Lajolo, 1994). This paper presents the results of a storage trial conducted to investigate the effects of various combinations of temperature, humidity and length of storage on the moisture content and cooking quality of two Australian adzuki varieties, Bloodwood and Erimo.
Materials and Methods Beans Bloodwood and Erimo varieties of adzuki (Vigna angularis) (produced near Grafton, New South Wales, Australia, and provided by Bean Growers Australia) were used in the storage trials. Freshly harvested beans were commercially cleaned and size-graded to exclude beans smaller than 4.76 mm and larger than 5.56 mm. Immediately after grading, the beans (250 kg of each variety in 25 kg synthetic bags) were transported to the University of Queensland, Gatton Campus, where they were placed in large plastic containers at ambient temperature (B19 1C) and hand-sorted to further
doi:10.1006/fstl.2001.0878 All articles available online at http://www.idealibrary.com on
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remove imperfect beans and foreign matter. The cleaned, size-graded beans were then weighed into 20 kg batches and placed into 40-L plastic drums for the storage trial.
Storage system The drums containing the beans were placed into three storage rooms set at 10, 20 and 30 1C. At each temperature, the beans were stored at two relative humidities (RH), 40 and 65%. All treatments were performed in duplicate. Thus each storage room contained eight 40-L drums (two varieties at 2 RHs, in duplicate). For each temperature, storage drums were maintained at two relative humidities by a constant supply of compressed air at approximately 3 L/min which was adjusted to attain the desired RH. The compressed air was conveyed via two clear vinyl food-grade Nylex tubes, 6 mm in diameter. The first one conveyed air to a silica gel column in order to produce dry air while the second tube conveyed air to a sandstone block placed at the bottom of a water column to produce air with a high relative humidity (close to 100%). The air was remixed via control valves to attain the desired RH. Relative humidity stability was verified with the use of a Hygro-M2 optical dew point/humidity monitor (General Easton instrument, Woburn, MA, U.S.A.). The 40 and 65% RH air was then conveyed via a distribution system with a constant head pressure of air to the 40-L storage drums. The drums contained an air inlet at the bottom below a metal screen covered with rust-proof fly screen which supported the beans. The drums had an air outlet in the lids. Bean sampling times were at zero time, 1.5, 3 and 6 mo.
Storage trial design The storage trial was designed to simulate different storage conditions: 10 1C to simulate cold or refrigerated storage; 20 1C to simulate storage at moderate ambient temperatures; and 30 1C to simulate elevated temperature storage that can be encountered in silo storage conditions. Similarly, the relative humidities used, 40 and 65%, were chosen to simulate dry and humid conditions, respectively. Storage at a higher relative humidity was not attempted because of the risk of mould and fungal growth (Hayakawa and Breene, 1982) which would have jeopardised the success of the storage trial.
weighing. Bean hydration was expressed as percentage weight increase. Bean cotyledon to testa ratio and hard bean percentage. After the bean hydration results were recorded, the same duplicate samples were used to assess the bean cotyledon to testa ratio and hard bean percentage. The testa of each sample was separated from the cotyledons. Testa, cotyledons and hard beans (beans which had not increased in size) were dried in a vacuum oven at 98–100 1C, with partial vacuum of 25 mmHg for approximately 48 h. Testa, cotyledons and hard beans were calculated as a percentage of total dry weight.
Chemical analyses of beans Sample preparation. A Glen Creston hammer mill with screen size of 1.4 mm was used to grind 40 g samples of adzuki from each treatment. The sample was mixed thoroughly and placed in an airtight container to full capacity and stored at 4 1C (for o2 d) until analysed. Moisture content was determined by the modified vacuum oven method (AACC, 1995, 44-40). Ash content was determined by the ash basic method (AACC 1995, 08-01). Fat content was determined by the AACC method (AACC, 1995, 30-20), (Crude fat in grain and stock feed). Total dietary fibre content was determined by the official method (AOAC, 1995, 985.29), (Total dietary fibre in foods, enzymatic gravimetric method). Total crude protein was determined using the Leco Corporation protein/nitrogen Auto Analyser, model EP-2000; the resulting % N was converted into % total crude protein by multiplying by 6.25. Total starch was estimated by difference.
Cooking of beans Beans (6070.5 g) were washed and placed in 800 mL cold deionized water and cooked for 40 min at 95–100 1C in a 1-L glass beaker. After cooking, the beans were left to cool to room temperature. Beans were not soaked before cooking as that it is not common practice in the preparation of ann in Japan (Breene and Hardman, 1987). The consistency of the cooking results was tested by cooking five 60 g bean batches using the same procedure and testing for cooked bean hardness with a texture analyser. The appropriate cooking time was determined by cooking beans for 30, 40, 50, 60, 70, 80, 90 and 100 min and assessing the bean hardness with a texture analyser.
Physical analyses of beans Bean size. Triplicate one-hundred-bean batches of each variety were chosen at random, counted and weighed. Bean size was expressed as grams/100 beans. Bean hydration. Duplicate random 10 g samples of adzuki were placed in 50-mL sealed plastic containers with 50-mL water. The beans were incubated at 25 1C for 3, 6, 12 and 24 h before draining, blotting dry and
Hardness measurement A TA-XT2 texture analyser (Stable Micro Systems Ltd, Surrey, UK) was used for the textural analyses of cooked beans. Thirty grams of beans were weighed into a cylinder 51 mm in length and 38 mm internal diameter. The cooked beans were levelled with a plunger, 36 mm in diameter and weighing 500 g. The bean hardness was determined as the vertical force required to move a
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Table 1
Average (n=3) chemical composition of adzuki varieties Erimo and Bloodwood at zero time
Variety
Moisture (%)
Ash (%)
Protein (%)
Fat (%)
Fibre (%)
Starch (%)
Bloodwood Erimo
12.44 12.19
3.03 3.10
24.43 23.09
0.45 0.45
16.41 16.71
55.68 56.65
cylindrical probe 20 mm in diameter and 40 mm in length a distance of 25.0 mm. This measurement was taken three times (on separate samples) and the average reading recorded.
Statistical analyses Data were subjected to a split unit analysis of variance using the SAS (version 6.12) statistical package. The 24 ‘main units’ consisted of 2 replicates of 3 temperatures 2 varieties 2 humidities, with the ‘subunits’ being storage times. Means were compared using a protected LSD procedure.
Results and Discussion Chemical composition Moisture, ash, fat and fibre contents of beans at zero time were similar for Bloodwood and Erimo (Table 1). The compositional data are comparable to those previously reported by some authors (e.g. Hayakawa and Breene, 1982; Hsieh et al., 1992) but slightly different from those reported by others (e.g. Ogawa et al., 1983; Tjahjadi et al., 1988; Fujimura and Kugimiya, 1993) (Table 1). The differences may be attributed to adzuki varietal variation and/or the methods of analyses.
Bean physical characteristics Bean size and bean coat thickness are important factors in bean hydration rate and cooking quality. The Bloodwood sample exhibited 7% more bean coat (testa) than did Erimo (Table 2). A greater bean coat thickness results in a slower hydration rate (Sefa-Dedeh and Stanley, 1979) which may explain the lower hydration rate observed for fresh Bloodwood than for the fresh Erimo (see data below). Fresh Erimo exhibited 1.76% of hard beans while fresh Bloodwood exhibited no hard beans (Table 2). Adzuki have been reported to have a hard bean range of 0–13% which may be related to variety, growing environment, maturity, and storage time and conditions (Sefa-Dedeh and Stanley, 1979; Yoshida et al., 1995). It has been shown that the hydration rate is related to the adzuki size. A 10% reduction in size within a population of regular-sized adzuki would cause a significant reduction in the water imbibition rate, resulting in a higher incidence of hard beans, i.e. beans with the ‘hard shell’ defect or impermeable bean coat (Bourne, 1967; Sefa-Dedeh and Stanley, 1979; Yoshida et al., 1995). The average bean size of Bloodwood (11.2 g/100 beans, i.e. 112 mg/seed) was only slightly
Table 2 Average (n=3) percentage of testa, cotyledon and hard bean of adzuki varieties, Erimo and Bloodwood at zero time Variety
Testa (%)
Cotyledon (%)
Hard bean (%)
Bloodwood Erimo
9.83 9.11
90.12 90.89
0.0 1.76
smaller (by 4.5%) than that of Erimo (11.7 g/100 beans, i.e., 117 mg/seed). In comparison, Yoshida et al. (1995) found the overall seed size of adzuki (Erimo variety) produced in four districts of Hokkaido to be 129 mg/ seed, whereas the overall size of hard bean from the same population was 116 mg/seed. In the current trial, the average bean size did not appear to have a major influence on the incidence of hard beans.
Bean moisture content Both Bloodwood and Erimo samples behaved in a similar manner with regard to bean moisture content during storage (Fig. 1), with Bloodwood always having a slightly higher moisture content than Erimo. This behaviour was significantly (Po0.01) related to the storage RH, which interacted strongly with the other storage factors. Fresh bean moisture content for both varieties was 12%. Beans stored at 65% RH maintained a reasonably steady moisture content regardless of temperature, with the lowest recorded mean being 11.6%. When stored at 40% RH, however, both varieties showed a significant (Po0.01) response to temperature and storage time. Moisture content declined as storage time increased, with the moisture loss increasing as temperature increased. After 6 mo at 30 1C and 40% RH, the moisture content of both varieties had declined to less than B10%. The loss of moisture was also significantly (Po0.01) temperature-related with the least loss of moisture occurring at 10 1C storage and the greatest at 30 1C. Therefore, beans of both varieties (Bloodwood and Erimo) stored at 10 1C and 65% RH experienced the least loss of moisture, and beans of both varieties stored at 30 1C and 40% RH experienced the most moisture loss. The recommended moisture level for stored adzuki is 13% (Duan, 1989). Storage of adzuki (Var. Bloodwood and Erimo) at low relative humidity (40%) leads to significant moisture loss from the bean coat, particularly at higher temperatures. These results are consistent with those of Hayakawa and Breene (1982), who reported that storage of adzuki (Var. Takara) at low relative humidity (12.4–33.6%) caused a decrease in moisture content to below 8%.
340
13
120
12
90
11
% wgt increase
Moisture %.
lwt/vol. 35 (2002) No. 4
Erimo.
10
60 30
9 13
0
Moisture %.
0
3
6
9
12
15
18
21
24
12
Soaking time in hours. 11
Bloodwood.
Fig. 2 Comparison of hydration rates of fresh adzuki (&) Erimo and (^) Bloodwood
10 9 3 4.5 Storage time in months.
6
40% 65%
Fig. 1 Effect of storage time and conditions on Erimo and Bloodwood moisture content (averaged over rep.), (*) 10 1C 40%, (*) 10 1C 65%, (&) 20 1C 40%, (&) 20 1C 65%, (~) 30 1C 40%, (~) 30 1C 65%. The bar indicates the LSD at 5%
(A)
(B) % Wt increase
1.5
% Wt increase
0
100 80 60 40 20 0 Fresh
Bean hydration (water imbibition) Comparison of the bean hydration rates indicated that fresh Erimo imbibed 10% more water after 12 h of soaking than did fresh Bloodwood (Fig. 2). The difference may be attributed to the Bloodwood seed coat thickness. The 6- and 12-h hydration readings exhibited the same trends. The 12-h soaking time was chosen for discussion because after 12 h any differences in hydration were maximised. After 6-mo storage, beans of both varieties stored at 40% RH absorbed significantly (Po0.01) less water after 12 h of soaking than beans stored at 65% RH (Fig. 3). Bloodwood absorbed significantly (Po0.01) less water than Erimo, regardless of storage conditions. As with moisture content, there was a strong interaction between storage temperature and humidity. At 65%, there were no significant differences between the amounts of water absorbed by beans stored at different temperatures (within each variety). In contrast, under low humidity conditions, beans stored at 30 1C absorbed significantly less water than those stored at the two lower temperatures. Storage time interacted significantly with the storage relative humidity and temperature (Po0.05 and Po0.01) respectively, thereby having a cumulative negative effect on the hydration rate of the stored beans. The above is in agreement with Moscoso et al. (1984) who reported that red kidney beans stored at 32 1C for 9 mo exhibited a 10% decrease in bean hydration. The effect of storage conditions on bean hydration followed the same trend as the bean moisture content. This indicates that bean moisture content plays an important role in bean hydration rate. A possible explanation is that, due to the loss of bean moisture, the germ portion (hilum), which is the main water imbibition area in the seed coat, becomes inactive. Therefore, the rate at which water can be imbibed into the bean is restricted (Hayakawa and Breene, 1982). It is
10°C 20°C 30°C Storage temperature
100 80 60 40 20 0 Fresh 10°C 20°C 30°C Storage temperature.
Fig. 3 Effect of 6 mo storage on (A) Erimo, (B) Bloodwood hydration after 12 hours soaking. The bars indicate 7 standard error
evident that the bean coat contributes to a large extent to the loss of cooking quality via the failure of beans to absorb water within a reasonable soaking time (Bourne, 1967). Beans stored for six months at 65% RH absorbed, after 12 hours of soaking, significantly (Po0.01) more water than (zero time) fresh beans. This phenomenon was also reported by Plhak et al. (1989) who observed that aged black beans absorbed more water than fresh black beans in the first 11–15 h of soaking. They explained that free water collects between the seed coat and the cotyledon and in the fissure between the two cotyledons. Fresh and stored beans absorb the same amount of water after soaking for 24 h (Shiota et al., 1983), suggesting that water penetration into the cotyledon tissue is retarded. This indicates that the hard shell phenomenon is not the only factor that influences adzuki water hydration, but that the hard-to-cook phenomenon may also be involved. This phenomenon is defined as the failure of seeds to soften enough after cooking for a reasonable time (Aguilera and Rivera, 1992; Stanley, 1992) and relates to reduced cotyledon water absorption and impaired starch gelatinisation.
Hardness of cooked beans Forty minutes was found to be the optimal cooking time for this investigation for determination of the hardness of cooked beans. Beans cooked for longer than 40 min were so well cooked that any differences in hardness between batches of beans could not be detected.
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Force in kilograms
21.0
Erimo.
18.0 15.0 12.0 9.0 6.0
Force in kilograms.
21.0
Bloodwood.
18.0 15.0 12.0 9.0 6.0 0
1.5
3
4.5
6
Storage time in months.
Fig. 4 Effect of storage time and conditions on the cooked bean hardness of Erimo and Bloodwood (averaged over RH & rep.), (*) 10 1C, (&) 20 1C, (~) 30 1C. The bar indicates LSD at 5%
consistent with the findings of Hayakawa and Breene (1982), that adzuki stored at low relative humidity (12.4–33.6%) decreased in moisture content and also exhibited a slower water uptake during cooking. The presence of the thicker bean coat of Bloodwood plus the loss of moisture during storage at 301C and 40% RH resulted in increased impermeability of the bean coat to water, causing loss of cooking quality. This agrees with the findings of Ozawa (1978) who also observed that adzuki stored at 30 1C experienced a large increase in cooked bean hardness. However, Ozawa (1978) also observed that high RH rather than low RH caused loss of bean cooking quality. On the other hand, El-Tabey Shehata (1992) commented that beans stored at high temperature developed hard shell whether they were stored under humid or very dry conditions. This suggests that other factors can come into play that might cause storage-related adzuki hardening, such as the hard-to-cook phenomenon.
Conclusion Storage of beans for 6 mo at 30 1C resulted in a reduction of cooking quality as measured by a significant increase in cooked bean textural strength (Fig. 4). This indicates that adzuki stored for extended periods (6 mo) under unfavourable conditions of high temperature (30 1C) maintained a harder texture after cooking in relation to beans stored for the same period under favourable conditions (10 1C). It follows that adzuki stored for extended periods under high temperatures need a longer cooking time to attain the same tenderness as beans stored under favourable conditions (Kato et al., 2000). Unlike its effect on moisture content and hydration rate, the effect of RH on the hardness of cooked beans was independent of all other factors. Storage at 40% RH was found to result in cooked beans being significantly (Po0.01) harder than those stored at 65% RH. Figure 4 shows that the Erimo and Bloodwood samples responded to storage time and temperature somewhat differently, reflecting the significant (Po0.01) interaction between storage time, temperature and variety. Erimo experienced little change in hardness (of the cooked beans) over time at 10 and 20 1C, and, when stored at 30 1C, hardness showed a relatively modest increase between 3 and 6 mo. Bloodwood, on the other hand, showed no notable change when stored at 10 1C, a small though significant (Po0.01) increase when stored at 20 1C, and a quite dramatic increase when stored at 30 1C, especially between 3 and 6 mo of storage (Fig. 4). Furthermore, Bloodwood, on average, required a further 20 min cooking time in relation to Erimo in order to attain the same tenderness (Kato et al., 2000). The hardness pattern of the cooked, stored beans reflects those of the moisture content and hydration rate for both varieties. It may be deduced that the bean moisture content affects bean hydration rate and also has an effect on the cooked bean hardness. This is
Storage of adzuki (Bloodwood and Erimo) at elevated temperatures (30 1C) caused an increase in cooked bean hardness. This effect was more pronounced when the beans were stored at low RH (40%). Adzuki maintained relatively good cooking quality when stored at 10 or 20 1C with 65% RH. The Erimo variety exhibited a higher tolerance than Bloodwood to storage and maintained a relatively stable cooking quality up to 6 mo, even when stored at 30 1C.
Acknowledgements The authors wish to acknowledge the support of Bean Growers Australia and the Grains Research and Development Corporation for this project.
References AACC. Approved Methods of the American Association of Cereal Chemists, 9th edn. St Paul, MN. American Association of Cereal Chemists (1995) AGUILERA, J. M. AND RIVERA, R. Hard-to-cook defect in black beans: Hardening rates, water imbibition and multiple mechanism hypothesis. Food Research International, 25, 101–108 (1992) AOAC. Official Methods of Analysis of the Association of Official Analytical Chemists, 16th edn. Washington DC: Association of Official Analytical Chemists (1995) ANTUNES, P. L. AND SGARBRIERI, V. C. Influence of time and conditions of storage on technological and nutritional properties of a dry bean (Phaseolus vulgaris, L.) variety Rosinha G2. Journal of Food Science, 44, 1703–1706 (1979) BOURNE, M. C. Size, density, and hard shell in dry beans. Food Technology, 21, 335–338 (1967) BREENE, W. M. AND HARDMAN, L. L. Anatomy of a specialty crop F the adzuki bean experience. A Symposium on Grain Legumes as Alternative Crops. Centre for Alternative Crops and Products, University of Minnesota, pp. 67–77 (1987)
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BURR, H. K., KON, S. AND MORRIS, H. J. Cooking rates of dry beans as influenced by moisture content and temperature and time of storage. Food Technology, 22, 88–90 (1968) DUAN, H. Small bean (in Chinese). In: LONG, J., LIN, L., XUSHEN, H. AND DUAN, H. (Eds), Edible Bean Crops. Beijing: Science Publishing House, pp. 160–171 (1989) EL-TABEY SHEHATA, A. M. Hard-to-cook phenomenon in legumes. Food Reviews International, 8, 191–221 (1992) FUJIMURA, T. AND KUGIMIYA, M. Gelatinisation of starches inside cotyledon cells of adzuki bean. (in Japanese, English summary). Journal of the Japanese Society for Food Science and Technology [Nippon Shokuhin Kogyo Gakkaishi], 40, 490–495 (1993) GARCIA, E. AND LAJOLO, F. M. Starch alterations in hard-tocook beans (Phaseolus vulgaris). Journal of Agricultural and Food Chemistry, 42, 612–615 (1994) HATAI, A. Influence of storage temperatures on cooking characteristics of adzuki beans Vigna angularis. Ohwi and Ohashi. Kaseigaku Zasshi, 33, 621–627 (1982) HAYAKAWA, I. AND BREENE, W. M. A study on the relationship between cooking properties of adzuki bean and storage conditions. Journal of the Faculty of Agriculture. Kyushu University, 27, 83–88 (1982) HSIEH, Y., POMERANZ, Y. AND SWANSON, B. G. Composition, cooking time, and maturation of adzuki (Vigna angularis) and common bean (Phaseolus vulgaris). Cereal Chemistry, 69, 244–248 (1992) KATO, J., YOUSIF, A. M., DEETH, H. C., SUZUKI, M. M., CAFFIN, N. A. AND MEGURO, T. Differences in the cooking quality between two adzuki varieties harvested in Australia and stored at different temperatures. Journal of the Society of Japanese Cookery Science, 33, 127–136 (2000) KON, S. AND SANSHUCK, D. W. Phytate content and its effect on cooking quality of beans. Journal of Food Processing and Preservation, 5, 169–178 (1981)
MOSCOSO, W., BOURNE, M. C. AND HOOD, L. F. Relationships between the hard-to-cook phenomenon in red kidney beans and water absorption, puncture force, pectin, phytic acid and minerals. Journal of Food Science, 49, 1577–1583 (1984) OGAWA, T., ABE, S. AND KUGIMIYA, M. Gelatinisation of starch in dried adzuki Ann granules. (in Japanese, English summary). Journal of the Japanese Society for Food Science and Technology [Nippon Shokuhin Kogyo Gakkaishi], 30, 323–330 (1983) OZAWA, Y. On the viability of stored adzuki beans (Phaseolus angularis) and its hardness after cooking (in Japanese, English summary). Report of the National Food Research Institute, 33, 37–40 (1978) PLHAK, L. C., CALDWELL, K. B. AND STANLEY, D. W. Comparison of methods used to characterise water imbibition in hard-to-cook beans. Journal of Food Science, 54, 326–336 (1989) SEFA-DEDEH, S. AND STANLEY, D. W. Textural implications of the microstructure of legumes. Food Technology, 33, 77–83 (1979) SHIOTA, Y., KURATA, M. AND TSUCHIYA, F. Water absorption by adzuki beans (Vigna angularis) (in Japanese, English Summary). Japan Society of Home Economics, 34, 775–781 (1983) STANLEY, D. W. A possible role for condensed tannins in bean hardening. Food Research International, 25, 187–192 (1992) TJAHJADI, C., LIN, S. AND BREENE, W. M. Isolation and characterisation of adzuki bean (Vigna angularis cultivar Takara) proteins. Journal of Food Science, 53, 1438–1443 (1988) YOSHIDA, K., SATOH, H. AND SATO, M. The extent and its source of variation for characteristics related to seed quality of azuki beans. III The water uptake of seeds and hardseediness (in Japanese, English summary). Japanese Journal of Crop Science, 64, 7–13 (1995)
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Effect of Storage Time and Conditions on the Hardness and Cooking Quality of Adzuki (Vigna angularis) A. M. Yousif*, H. C. Deeth, N. A. Caffin and A. T. Lisle
A. M. Yousif, H. C. Deeth, N. A. Caffin: University of Queensland, School of Land and Food Sciences, Gatton, Qld 4343 (Australia) A. T. Lisle: University of Queensland, School of Agriculture and Horticulture, Gatton, Qld 4343 (Australia) (Received August 30, 2001; accepted January 2, 2002) A storage trial of two varieties of adzuki (Vigna angularis), Bloodwood and Erimo, produced in Australia, was conducted to determine the effect of various combinations of temperature, humidity and length of storage on bean quality. The beans were stored for up to 6 mo under the following conditions: temperature (10, 20 and 30 1C), relative humidity (RH) (40 and 65%). Storage of adzuki at elevated temperature (30 1C) and low relative humidity (40%) resulted in the greatest loss of bean moisture, increase in hydration times and decrease in bean cooking quality, i.e. increased hardness of cooked beans. The best storage conditions for the preservation of adzuki quality were 10 1C and 65% RH.
r 2002 Elsevier Science Ltd. All rights reserved. Keywords: adzuki; beans; storage; hydration; cooking quality
Introduction In East Asian countries, adzuki (Vigna angularis) are cooked and consumed as a dessert or snack food, especially during celebrations and traditional festivals such as the Chinese New Year. In Japan, adzuki are consumed as ‘ann’ (bean paste) used alone or as a filler for various sweet pastries, and as whole beans boiled and sweetened for snacks and confectionery items. At least 50 other beans and legumes are used to make ann, but, due to the desirable colour, delicate flavour and characteristic texture of its paste, the adzuki is preferred (Breene and Hardman, 1987). Preservation of legume seeds in dry storage is the main way of maintaining a year-round supply of this food source. Unfavourable storage conditions can reduce bean quality and result in inadequate cooking, i.e. the beans remain hard due to insufficient water uptake and softening of the cotyledon tissue. Processing poor quality beans results in inferior texture and mouthfeel of ann which is believed to be caused by poor cell separation, improper starch gelatinisation and changes to the protein denaturation temperature (Ozawa, 1978; Hatai, 1982). The nutritional quality of the beans is also reduced. Overall, the changes in the beans due to *To whom correspondence should be addressed. E-mail: [email protected]
0023-6438/02/$35.00 r 2002 Elsevier Science Ltd. All rights reserved.
unsatisfactory storage conditions lead to a reduction in their commercial value. The moisture content of the seeds, storage temperature and relative humidity are important parameters for good bean preservation (Burr et al., 1968; Antunes and Sgarbieri, 1979; Kon and Sanshuck, 1981; Stanley, 1992; Garcia and Lajolo, 1994). This paper presents the results of a storage trial conducted to investigate the effects of various combinations of temperature, humidity and length of storage on the moisture content and cooking quality of two Australian adzuki varieties, Bloodwood and Erimo.
Materials and Methods Beans Bloodwood and Erimo varieties of adzuki (Vigna angularis) (produced near Grafton, New South Wales, Australia, and provided by Bean Growers Australia) were used in the storage trials. Freshly harvested beans were commercially cleaned and size-graded to exclude beans smaller than 4.76 mm and larger than 5.56 mm. Immediately after grading, the beans (250 kg of each variety in 25 kg synthetic bags) were transported to the University of Queensland, Gatton Campus, where they were placed in large plastic containers at ambient temperature (B19 1C) and hand-sorted to further
doi:10.1006/fstl.2001.0878 All articles available online at http://www.idealibrary.com on
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lwt/vol. 35 (2002) No. 4
remove imperfect beans and foreign matter. The cleaned, size-graded beans were then weighed into 20 kg batches and placed into 40-L plastic drums for the storage trial.
Storage system The drums containing the beans were placed into three storage rooms set at 10, 20 and 30 1C. At each temperature, the beans were stored at two relative humidities (RH), 40 and 65%. All treatments were performed in duplicate. Thus each storage room contained eight 40-L drums (two varieties at 2 RHs, in duplicate). For each temperature, storage drums were maintained at two relative humidities by a constant supply of compressed air at approximately 3 L/min which was adjusted to attain the desired RH. The compressed air was conveyed via two clear vinyl food-grade Nylex tubes, 6 mm in diameter. The first one conveyed air to a silica gel column in order to produce dry air while the second tube conveyed air to a sandstone block placed at the bottom of a water column to produce air with a high relative humidity (close to 100%). The air was remixed via control valves to attain the desired RH. Relative humidity stability was verified with the use of a Hygro-M2 optical dew point/humidity monitor (General Easton instrument, Woburn, MA, U.S.A.). The 40 and 65% RH air was then conveyed via a distribution system with a constant head pressure of air to the 40-L storage drums. The drums contained an air inlet at the bottom below a metal screen covered with rust-proof fly screen which supported the beans. The drums had an air outlet in the lids. Bean sampling times were at zero time, 1.5, 3 and 6 mo.
Storage trial design The storage trial was designed to simulate different storage conditions: 10 1C to simulate cold or refrigerated storage; 20 1C to simulate storage at moderate ambient temperatures; and 30 1C to simulate elevated temperature storage that can be encountered in silo storage conditions. Similarly, the relative humidities used, 40 and 65%, were chosen to simulate dry and humid conditions, respectively. Storage at a higher relative humidity was not attempted because of the risk of mould and fungal growth (Hayakawa and Breene, 1982) which would have jeopardised the success of the storage trial.
weighing. Bean hydration was expressed as percentage weight increase. Bean cotyledon to testa ratio and hard bean percentage. After the bean hydration results were recorded, the same duplicate samples were used to assess the bean cotyledon to testa ratio and hard bean percentage. The testa of each sample was separated from the cotyledons. Testa, cotyledons and hard beans (beans which had not increased in size) were dried in a vacuum oven at 98–100 1C, with partial vacuum of 25 mmHg for approximately 48 h. Testa, cotyledons and hard beans were calculated as a percentage of total dry weight.
Chemical analyses of beans Sample preparation. A Glen Creston hammer mill with screen size of 1.4 mm was used to grind 40 g samples of adzuki from each treatment. The sample was mixed thoroughly and placed in an airtight container to full capacity and stored at 4 1C (for o2 d) until analysed. Moisture content was determined by the modified vacuum oven method (AACC, 1995, 44-40). Ash content was determined by the ash basic method (AACC 1995, 08-01). Fat content was determined by the AACC method (AACC, 1995, 30-20), (Crude fat in grain and stock feed). Total dietary fibre content was determined by the official method (AOAC, 1995, 985.29), (Total dietary fibre in foods, enzymatic gravimetric method). Total crude protein was determined using the Leco Corporation protein/nitrogen Auto Analyser, model EP-2000; the resulting % N was converted into % total crude protein by multiplying by 6.25. Total starch was estimated by difference.
Cooking of beans Beans (6070.5 g) were washed and placed in 800 mL cold deionized water and cooked for 40 min at 95–100 1C in a 1-L glass beaker. After cooking, the beans were left to cool to room temperature. Beans were not soaked before cooking as that it is not common practice in the preparation of ann in Japan (Breene and Hardman, 1987). The consistency of the cooking results was tested by cooking five 60 g bean batches using the same procedure and testing for cooked bean hardness with a texture analyser. The appropriate cooking time was determined by cooking beans for 30, 40, 50, 60, 70, 80, 90 and 100 min and assessing the bean hardness with a texture analyser.
Physical analyses of beans Bean size. Triplicate one-hundred-bean batches of each variety were chosen at random, counted and weighed. Bean size was expressed as grams/100 beans. Bean hydration. Duplicate random 10 g samples of adzuki were placed in 50-mL sealed plastic containers with 50-mL water. The beans were incubated at 25 1C for 3, 6, 12 and 24 h before draining, blotting dry and
Hardness measurement A TA-XT2 texture analyser (Stable Micro Systems Ltd, Surrey, UK) was used for the textural analyses of cooked beans. Thirty grams of beans were weighed into a cylinder 51 mm in length and 38 mm internal diameter. The cooked beans were levelled with a plunger, 36 mm in diameter and weighing 500 g. The bean hardness was determined as the vertical force required to move a
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Table 1
Average (n=3) chemical composition of adzuki varieties Erimo and Bloodwood at zero time
Variety
Moisture (%)
Ash (%)
Protein (%)
Fat (%)
Fibre (%)
Starch (%)
Bloodwood Erimo
12.44 12.19
3.03 3.10
24.43 23.09
0.45 0.45
16.41 16.71
55.68 56.65
cylindrical probe 20 mm in diameter and 40 mm in length a distance of 25.0 mm. This measurement was taken three times (on separate samples) and the average reading recorded.
Statistical analyses Data were subjected to a split unit analysis of variance using the SAS (version 6.12) statistical package. The 24 ‘main units’ consisted of 2 replicates of 3 temperatures 2 varieties 2 humidities, with the ‘subunits’ being storage times. Means were compared using a protected LSD procedure.
Results and Discussion Chemical composition Moisture, ash, fat and fibre contents of beans at zero time were similar for Bloodwood and Erimo (Table 1). The compositional data are comparable to those previously reported by some authors (e.g. Hayakawa and Breene, 1982; Hsieh et al., 1992) but slightly different from those reported by others (e.g. Ogawa et al., 1983; Tjahjadi et al., 1988; Fujimura and Kugimiya, 1993) (Table 1). The differences may be attributed to adzuki varietal variation and/or the methods of analyses.
Bean physical characteristics Bean size and bean coat thickness are important factors in bean hydration rate and cooking quality. The Bloodwood sample exhibited 7% more bean coat (testa) than did Erimo (Table 2). A greater bean coat thickness results in a slower hydration rate (Sefa-Dedeh and Stanley, 1979) which may explain the lower hydration rate observed for fresh Bloodwood than for the fresh Erimo (see data below). Fresh Erimo exhibited 1.76% of hard beans while fresh Bloodwood exhibited no hard beans (Table 2). Adzuki have been reported to have a hard bean range of 0–13% which may be related to variety, growing environment, maturity, and storage time and conditions (Sefa-Dedeh and Stanley, 1979; Yoshida et al., 1995). It has been shown that the hydration rate is related to the adzuki size. A 10% reduction in size within a population of regular-sized adzuki would cause a significant reduction in the water imbibition rate, resulting in a higher incidence of hard beans, i.e. beans with the ‘hard shell’ defect or impermeable bean coat (Bourne, 1967; Sefa-Dedeh and Stanley, 1979; Yoshida et al., 1995). The average bean size of Bloodwood (11.2 g/100 beans, i.e. 112 mg/seed) was only slightly
Table 2 Average (n=3) percentage of testa, cotyledon and hard bean of adzuki varieties, Erimo and Bloodwood at zero time Variety
Testa (%)
Cotyledon (%)
Hard bean (%)
Bloodwood Erimo
9.83 9.11
90.12 90.89
0.0 1.76
smaller (by 4.5%) than that of Erimo (11.7 g/100 beans, i.e., 117 mg/seed). In comparison, Yoshida et al. (1995) found the overall seed size of adzuki (Erimo variety) produced in four districts of Hokkaido to be 129 mg/ seed, whereas the overall size of hard bean from the same population was 116 mg/seed. In the current trial, the average bean size did not appear to have a major influence on the incidence of hard beans.
Bean moisture content Both Bloodwood and Erimo samples behaved in a similar manner with regard to bean moisture content during storage (Fig. 1), with Bloodwood always having a slightly higher moisture content than Erimo. This behaviour was significantly (Po0.01) related to the storage RH, which interacted strongly with the other storage factors. Fresh bean moisture content for both varieties was 12%. Beans stored at 65% RH maintained a reasonably steady moisture content regardless of temperature, with the lowest recorded mean being 11.6%. When stored at 40% RH, however, both varieties showed a significant (Po0.01) response to temperature and storage time. Moisture content declined as storage time increased, with the moisture loss increasing as temperature increased. After 6 mo at 30 1C and 40% RH, the moisture content of both varieties had declined to less than B10%. The loss of moisture was also significantly (Po0.01) temperature-related with the least loss of moisture occurring at 10 1C storage and the greatest at 30 1C. Therefore, beans of both varieties (Bloodwood and Erimo) stored at 10 1C and 65% RH experienced the least loss of moisture, and beans of both varieties stored at 30 1C and 40% RH experienced the most moisture loss. The recommended moisture level for stored adzuki is 13% (Duan, 1989). Storage of adzuki (Var. Bloodwood and Erimo) at low relative humidity (40%) leads to significant moisture loss from the bean coat, particularly at higher temperatures. These results are consistent with those of Hayakawa and Breene (1982), who reported that storage of adzuki (Var. Takara) at low relative humidity (12.4–33.6%) caused a decrease in moisture content to below 8%.
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13
120
12
90
11
% wgt increase
Moisture %.
lwt/vol. 35 (2002) No. 4
Erimo.
10
60 30
9 13
0
Moisture %.
0
3
6
9
12
15
18
21
24
12
Soaking time in hours. 11
Bloodwood.
Fig. 2 Comparison of hydration rates of fresh adzuki (&) Erimo and (^) Bloodwood
10 9 3 4.5 Storage time in months.
6
40% 65%
Fig. 1 Effect of storage time and conditions on Erimo and Bloodwood moisture content (averaged over rep.), (*) 10 1C 40%, (*) 10 1C 65%, (&) 20 1C 40%, (&) 20 1C 65%, (~) 30 1C 40%, (~) 30 1C 65%. The bar indicates the LSD at 5%
(A)
(B) % Wt increase
1.5
% Wt increase
0
100 80 60 40 20 0 Fresh
Bean hydration (water imbibition) Comparison of the bean hydration rates indicated that fresh Erimo imbibed 10% more water after 12 h of soaking than did fresh Bloodwood (Fig. 2). The difference may be attributed to the Bloodwood seed coat thickness. The 6- and 12-h hydration readings exhibited the same trends. The 12-h soaking time was chosen for discussion because after 12 h any differences in hydration were maximised. After 6-mo storage, beans of both varieties stored at 40% RH absorbed significantly (Po0.01) less water after 12 h of soaking than beans stored at 65% RH (Fig. 3). Bloodwood absorbed significantly (Po0.01) less water than Erimo, regardless of storage conditions. As with moisture content, there was a strong interaction between storage temperature and humidity. At 65%, there were no significant differences between the amounts of water absorbed by beans stored at different temperatures (within each variety). In contrast, under low humidity conditions, beans stored at 30 1C absorbed significantly less water than those stored at the two lower temperatures. Storage time interacted significantly with the storage relative humidity and temperature (Po0.05 and Po0.01) respectively, thereby having a cumulative negative effect on the hydration rate of the stored beans. The above is in agreement with Moscoso et al. (1984) who reported that red kidney beans stored at 32 1C for 9 mo exhibited a 10% decrease in bean hydration. The effect of storage conditions on bean hydration followed the same trend as the bean moisture content. This indicates that bean moisture content plays an important role in bean hydration rate. A possible explanation is that, due to the loss of bean moisture, the germ portion (hilum), which is the main water imbibition area in the seed coat, becomes inactive. Therefore, the rate at which water can be imbibed into the bean is restricted (Hayakawa and Breene, 1982). It is
10°C 20°C 30°C Storage temperature
100 80 60 40 20 0 Fresh 10°C 20°C 30°C Storage temperature.
Fig. 3 Effect of 6 mo storage on (A) Erimo, (B) Bloodwood hydration after 12 hours soaking. The bars indicate 7 standard error
evident that the bean coat contributes to a large extent to the loss of cooking quality via the failure of beans to absorb water within a reasonable soaking time (Bourne, 1967). Beans stored for six months at 65% RH absorbed, after 12 hours of soaking, significantly (Po0.01) more water than (zero time) fresh beans. This phenomenon was also reported by Plhak et al. (1989) who observed that aged black beans absorbed more water than fresh black beans in the first 11–15 h of soaking. They explained that free water collects between the seed coat and the cotyledon and in the fissure between the two cotyledons. Fresh and stored beans absorb the same amount of water after soaking for 24 h (Shiota et al., 1983), suggesting that water penetration into the cotyledon tissue is retarded. This indicates that the hard shell phenomenon is not the only factor that influences adzuki water hydration, but that the hard-to-cook phenomenon may also be involved. This phenomenon is defined as the failure of seeds to soften enough after cooking for a reasonable time (Aguilera and Rivera, 1992; Stanley, 1992) and relates to reduced cotyledon water absorption and impaired starch gelatinisation.
Hardness of cooked beans Forty minutes was found to be the optimal cooking time for this investigation for determination of the hardness of cooked beans. Beans cooked for longer than 40 min were so well cooked that any differences in hardness between batches of beans could not be detected.
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Force in kilograms
21.0
Erimo.
18.0 15.0 12.0 9.0 6.0
Force in kilograms.
21.0
Bloodwood.
18.0 15.0 12.0 9.0 6.0 0
1.5
3
4.5
6
Storage time in months.
Fig. 4 Effect of storage time and conditions on the cooked bean hardness of Erimo and Bloodwood (averaged over RH & rep.), (*) 10 1C, (&) 20 1C, (~) 30 1C. The bar indicates LSD at 5%
consistent with the findings of Hayakawa and Breene (1982), that adzuki stored at low relative humidity (12.4–33.6%) decreased in moisture content and also exhibited a slower water uptake during cooking. The presence of the thicker bean coat of Bloodwood plus the loss of moisture during storage at 301C and 40% RH resulted in increased impermeability of the bean coat to water, causing loss of cooking quality. This agrees with the findings of Ozawa (1978) who also observed that adzuki stored at 30 1C experienced a large increase in cooked bean hardness. However, Ozawa (1978) also observed that high RH rather than low RH caused loss of bean cooking quality. On the other hand, El-Tabey Shehata (1992) commented that beans stored at high temperature developed hard shell whether they were stored under humid or very dry conditions. This suggests that other factors can come into play that might cause storage-related adzuki hardening, such as the hard-to-cook phenomenon.
Conclusion Storage of beans for 6 mo at 30 1C resulted in a reduction of cooking quality as measured by a significant increase in cooked bean textural strength (Fig. 4). This indicates that adzuki stored for extended periods (6 mo) under unfavourable conditions of high temperature (30 1C) maintained a harder texture after cooking in relation to beans stored for the same period under favourable conditions (10 1C). It follows that adzuki stored for extended periods under high temperatures need a longer cooking time to attain the same tenderness as beans stored under favourable conditions (Kato et al., 2000). Unlike its effect on moisture content and hydration rate, the effect of RH on the hardness of cooked beans was independent of all other factors. Storage at 40% RH was found to result in cooked beans being significantly (Po0.01) harder than those stored at 65% RH. Figure 4 shows that the Erimo and Bloodwood samples responded to storage time and temperature somewhat differently, reflecting the significant (Po0.01) interaction between storage time, temperature and variety. Erimo experienced little change in hardness (of the cooked beans) over time at 10 and 20 1C, and, when stored at 30 1C, hardness showed a relatively modest increase between 3 and 6 mo. Bloodwood, on the other hand, showed no notable change when stored at 10 1C, a small though significant (Po0.01) increase when stored at 20 1C, and a quite dramatic increase when stored at 30 1C, especially between 3 and 6 mo of storage (Fig. 4). Furthermore, Bloodwood, on average, required a further 20 min cooking time in relation to Erimo in order to attain the same tenderness (Kato et al., 2000). The hardness pattern of the cooked, stored beans reflects those of the moisture content and hydration rate for both varieties. It may be deduced that the bean moisture content affects bean hydration rate and also has an effect on the cooked bean hardness. This is
Storage of adzuki (Bloodwood and Erimo) at elevated temperatures (30 1C) caused an increase in cooked bean hardness. This effect was more pronounced when the beans were stored at low RH (40%). Adzuki maintained relatively good cooking quality when stored at 10 or 20 1C with 65% RH. The Erimo variety exhibited a higher tolerance than Bloodwood to storage and maintained a relatively stable cooking quality up to 6 mo, even when stored at 30 1C.
Acknowledgements The authors wish to acknowledge the support of Bean Growers Australia and the Grains Research and Development Corporation for this project.
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