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Mazza, G. 1993. Storage, processing, and quality aspects of buckwheat seed. p. 251-254. In: J. Janick and J.E. Simon (eds.), New crops. Wiley, New York.

Storage, Processing, and Quality Aspects of Buckwheat Seed

G. Mazza

  1. METHODOLOGY
  2. RESULTS AND DISCUSSION
  3. REFERENCES
  4. Table 1
  5. Table 2
  6. Table 3
  7. Table 4
  8. Fig. 1
Buckwheat is commonly grown for its black or gray triangular seeds. It can also be grown as a green manure crop, a companion crop, a cover crop, a source of buckwheat honey, and as a pharmaceutical plant yielding rutin (Marshall and Pomeranz 1982). There are three known species of buckwheat: common buckwheat, Fagopryum esculentum Moench; tartary buckwheat, F. tartaricum Gaertn; and perennial buckwheat, F. cymosum L. Common buckwheat, also known as F. sagittatum Gilib., is by far the most economically important species, accounting for over 90% of the world buckwheat production (Mazza 1992). During the past 10 years, world production of buckwheat has averaged about 2 million ha, or about 1 million tonnes, with Russia accounting for about 90% of the production. Other major producers are China, Japan, Poland, Canada, Brazil, the United States, South Africa, and Australia (Mazza 1992).

In eastern Europe, buckwheat is a basic food item in porridges and soups. In North America, it is marketed primarily in pancake mixes, which may contain buckwheat mixed with wheat, maize, rice, or oat flours, plus a leavening agent. In Japan, buckwheat is marketed primarily as flour for manufacturing a variety of noodles (soba) and as groats. Groats, that part of the grain left after the hulls are removed from the seeds, and farina, made from groats are used for breakfast food, porridge, and thickening materials in soups, gravies, and dressings. Buckwheat is also used with vegetables and spices in kasha and with wheat, maize or rice in bread and pasta products (Marshall and Pomeranz 1982; Mazza 1992).

Primary processing of buckwheat includes cleaning, dehulling, and milling. The aim of dehulling is to separate the groats from the hulls by impact or abrasion of seed against emery stones or steel followed by air or screen separation of groats and hulls. The most important quality attributes of buckwheat groats are color and flavor. The color is light green in freshly harvested seed, but gradually changes to reddish brown during storage. The color change is accompanied by loss of desirable flavor, nutrients, and formation of brown pigments.

The objectives of this investigation were to determine: (1) influence of moisture content, temperature, seed size, and shape on dehulling of buckwheat seed; (2) influence of storage, relative humidity, and temperature on color and flavor characteristics of buckwheat groats; (3) to develop recommendations for long-term storage of buckwheat seed.

METHODOLOGY

'Mancan', 'Manor', and 'Tokyo' buckwheat grown in Canada were used in this study. Processing and analytical methods were as previously described (Mazza 1985, 1986, 1988).

RESULTS AND DISCUSSION

Moisture content of seeds, dehulling recovery, percentage of whole and broken groats and Hunter color values of the groats of 'Mancan', 'Tokyo', and 'Manor' buckwheat equilibrated and dehulled at 25°C and 0.23 to 0.97 water activity are presented in Table 1. The moisture sorption isotherms of 'Mancan' buckwheat at 1°, 10°, 25°, and 40°C are shown in Fig. 1. The water concentration (X)-water activity (aw) relationships were unaffected by temperature. However, when the data were plotted using vapor pressure instead of the corresponding activity, the higher temperature isotherms were well below those of lower temperatures, as expected, according to the physical adsorption theory (Labuza 1968).

The dehulling recovery varied between cultivar and water activity of the seed (Table 1). 'Mancan' yielded the highest amount of groats and 'Manor' the lowest. 'Mancan' had larger seeds than either 'Tokyo' or 'Manor'. The number of seeds having wings, paper-like extension of the angles of the hulls, is higher in 'Mancan', intermediate in 'Tokyo' and lower in 'Manor' buckwheat. The difference in dehulling yield between cultivars may, in part, be attributed to variations in seed characteristics as well as to differences in groat to hull ratio which is also slightly higher in 'Mancan' than in the other two cultivars.

Table 2 presents the absorbance of the extracted color and tristimulus color values of buckwheat samples stored at 25°C and 0.11 to 0.67 water activity for 19 months. The maximum concentration of browning pigments occurred at 0.45 to 0.55 water activity.

The calculated BET monolayer for the material stored for 19 months was 7.4 g H2O/100 g of solids. This corresponded to a water activity of 0.275. Dry foods are usually considered to be most stable to chemical reactions if their moisture content is at or near the BET monolayer (Labuza et al. 1970).

Dehulled seeds of 'Mancan', 'Tokyo', and 'Manor' were analyzed for proximate composition, selected mineral profiles and content of total, free, neutral, glyco-, and phospholipids, and each class of lipid was analyzed for fatty acid composition (Table 3, 4). The samples contained from 2.6±0.2 to 3.2±0.1% total lipids of which 81 to 85% were neutral lipids, 8 to 11% phospholipids, and 3 to 5% glycolipids. Free lipids, extracted in petroleum ether, ranged from 2.1±0.1 to 2.6±0.1%. The major fatty acids of all cultivars and of all classes of lipids were palmitic (16:0), oleic (18:1), and linoleic (18:2) acid. Average values of these three fatty acids in the total lipids of all buckwheat samples examined were 14.0±0.8, 36.3±1.9, and 37.0±1.9%, respectively. The corresponding values for the free lipids were 14.8±1.5, 36.5±2.0, and 35.5±1.9% and those for phospholipids were 9.1±0.8, 44.3±4.4, and 41.7±2.8%, respectively. Total lipid content showed significant positive correlation with free, neutral, and glycolipid contents, and there was a highly significant negative correlation between oleic and linoleic acid contents of all lipid classes. There was, however, no significant difference between new and old buckwheat in the content of free, neutral, glyco-, and phospholipids and in the fatty acid composition of total and free lipids.

The influence of water activity, temperature and cultivar on buckwheat dehulling, groat color, and lipids were examined. Of several cultivars examined, 'Mancan' yielded the most groats. Dehulled seeds contained 2.6 to 3.2% total lipids comprised of 81 to 85% neutral lipids, to 11% phospholipids and 3 to 5% glycolipids. Color and flavor characteristics of buckwheat were maximized when seed were stored at a low temperature at a relative humidity below 45%.

REFERENCES

Table 1. Influence of cultivar and moisture content on dehulling characteristics and color of buckwheat seeds stored at 25°C and water activities of 0.23 to 0.97 for 45 days.

Hunter color valuesz
Dehulled groat Whole groat Broken groat Mixed groat
Cultivar Water activity Moisture content (%) Dehulling recovery (%) Whole (%) Broken (%) L a L a L a
Mancan 0.23 5.98 69.2 30.1 69.9 52.0 +0.1 68.7 +0.3 63.5 +0.1
0.52 9.79 67.0 33.3 66.7 53.0 -0.1 60.0 +0.4 58.7 +0.5
0.75 13.47 65.4 44.5 55.5 52.3 +0.3 56.5 -0.4 57.4 +0.4
0.97 19.80 65.0 68.5 31.5 51.0 +0.4 57.87 +1.3 53.2 +0.7
Manor 0.23 5.93 54.9 37.1 62.9 52.6 +1.4 60.6 +0.8 59.0 +1.0
0.52 9.90 50.4 35.7 64.3 52.9 +1.9 58.0 +0.7 58.9 -0.7
0.75 13.50 41.6 48.4 51.6 52.9 +1.5 56.6 +1.4 54.5 +1.2
0.97 19.13 32.5 61.5 38.5 51.7 +2.1 61.3 +1.7 53.2 +0.4
Tokyo 0.23 5.84 66.3 28.7 71.3 51.7 +0.7 62.7 +0.5 60.6 +0.3
0.52 9.77 60.5 33.5 66.5 51.1 +0.7 58.4 +0.1 57.6 +1.3
0.75 13.30 58.2 45.5 54.5 51.3 +0.7 54.8 +0.3 54.6 +0.8
0.97 18.74 51.6 75.1 24.9 50.3 +1.8 58.3 +1.1 52.1 +1.0
zL = lightness; a = redness when + and greenness when -.

Table 2. Absorbance of extracted color and Hunter lab tristimulus color values of 'Mancan' buckwheat samples stored at 25°C and five water activities for 19 months.

Tristimulus values
Water
activity		 Moisture
content
(% dry wt)		 Absorbance
index
(A420)		 X Y Z
0.11 4.1 0.257bz 26.3c 26.2c 15.1b
0.23 6.7 0.233c 26.7b 26.5b 15.1b
0.31 8.7 0.241c 27.1a 27.0a 15.3a
0.51 13.0 0.290a 25.3d 24.7d 13.4d
0.67 13.8 0.238c 26.4bc 25.9c 14.2c
z Means separated by Duncan's multiple range test, p = 0.01.

Table 3. Proximate composition and selected mineral profile of three dehulled buckwheat cultivars (dry weight basis).

Cultivar mean±SD
Assay Mancan Manor Tokyo
Moisture (%) 16.2±0.9 10.1±0.2 10.9±0.1
Proteinz (%) 14.2±0.6 14.6±0.3 11.9±0.4
Crude Fiber (%) 1.57±0.30 1.21±0.03 1.57±0.10
Ash (%) 1.85±0.01 1.66±0.01 1.39±0.01
Lipidsy (%) 2.6±0.3 2.2 ±0.3 2.1±0.2
Carbohydratesx (%) 79.8±1.6 80.3±0.8 83.0±1.1
K (%) 0.44±0.01 0.42±0.01 0.41±0.01
P (%) 0.36±0.02 0.35±0.01 0.26±0.02
Mg (%) 0.21±0.01 0.20±0.01 0.20±0.01
Ca (ppm) 180.5±7.4 180.5±10.6 220.5±6.5
Fe (ppm) 24.8±1.8 21.4±0.3 21.2±0.9
Zn (ppm) 23.4±0.4 22.0±1.6 22.8±1.1
Mn (ppm) 10.2±0.2 10.0±0.5 10.2±0.4
Cu (ppm) 4.6±0.3 3.7±0.1 4.3±0.2
zN x 6.25.
ySoxhlet, petroleum ether for 8 h.
xBy difference.

Table 4. Lipids of dehulled buckwheat seed.

Concentration (% dry weight)
Cultivar Total lipidsz Free lipidsy Neutral lipidsx Glycolipidsw Phospholipidsv
Mancanu 3.2at 2.6a 2.6a 0.15a 0.33a
Mancans 2.9b 2.4a 2.4a 0.13a 0.22a
Manor 2.6c 2.2ab 2.2b 0.09b 0.27a
Tokyo 2.9ab 2.1b 2.5a 0.13a 0.32a
zChloroform-methanol determined by the Folch et al (1975) method.
ySoxhlet, petroleum ether for 8 h.
xSilicic acid column, chloroform extract.
wFlorisil column, acetone extract.
vFlorisil column, methanol extract.
uFresh seed.
tMeans within each column followed by the same letter are not significantly different by using Duncan's multiple range test p = 0.05.
sSeed stored for 25 months at room temperature.


Fig. 1. Adsorption isotherms of 'Mancan' buckwheat at 1°, 10°, 25°, and 40°C.
Last update September 10, 1997 aw