J Food Sci Technol DOI 10.1007/s13197-013-1182-9
ORIGINAL ARTICLE
Microwave assisted dehulling of black gram (Vigna mungo L) J. Jerish Joyner & B. K. Yadav
Revised: 17 September 2013 / Accepted: 26 September 2013 # Association of Food Scientists & Technologists (India) 2013
Abstract This article summarises the results of the investigation of application of microwave exposure on the dehulling characteristics of the black gram and the properties of the dehulled grains. Black gram was exposed to 3 microwave power levels, viz., 450, 630 and 810 W for 7 different exposure, ranging from 60 to 150 s at an interval of 15 s with a view to determine the suitable combination of dosage in order to get the maximum yield with little change in colour. Related changes in properties were also studied. It was observed that the surface temperature of the grain increased with the increase in microwave power level from 450 to 810 W as well as exposure time from 60 to 150 s in the range from 58 to 123 °C while the dehulling time reduced from 445 to 170 s. The dehulling yield increased with increasing microwave dosage in the beginning and reached to the maximum value followed by decreasing trend. The colour of the dehulled grain changed slowly up to a microwave dosage of 972 J/g after that it changed vividly darker than the control. The dehulling and dhal yields and colour change were polynomial functions of microwave dosage. The highest yield of 73.7 % was achieved at about 972 J/g with a little change in colour (CIELAB ΔE* value of 2.58). The corresponding dehulling time, cooking time and losses during dehulling were respectively 185 s, 10 min and 15.1 % as compared to 492 s, 20 min and 31.5 % for control respectively. It is concluded that a dosage of about 972 J/g was the best for the black gram dehulling at a rate of 630 W or higher power level.
Keyword Black gram . Microwave exposure . Dehulling time . Dehulling yield . Dhal yield . Dehulling loss J. J. Joyner : B. K. Yadav (*) Department of Food Engineering, Indian Institute of Crop Processing Technology, Pudukkottai Road, Thanjavur 613005, Tamilnadu, India e-mail: [email protected]
Introduction Black gram (Vigna mungo L) or urid is reported to be originated in India and being cultivated in Burma, Bangladesh, Sri Lanka and to a lesser extent in Thailand, Australia and other Asian and South Pacific countries (Poehlman 1991). Black gram is a rich protein food containing around 24 % protein, 59.6 % carbohydrate and 1.4 % fat (Gopalan et al. 1989). Black gram is extensively used in South Asian subcontinent for direct consumption as dehulled grain and as flour in various fermented foods, in Japan to make “moyashi” and its flour for making fresh skinless beef sausages in some European nations as protein substitute. The pulses are reported to contain antinutrients in seed coat. Also, the seed coat is hard and imparts a bitter taste, reducing the palatability of the pulses. In order to remove the antinutrients and improve the palatability, it is common practice to dehull the pulses before using. Dehulling is a process of removal of hull from the cotyledon of pulses. It reduces the fibre content and improves appearance, texture, cooking quality, palatability and digestibility of grain legumes (Tiwari et al. 2007). Dehulling of black gram has been a difficult operation due to the presence of vitreous layer of gums and mucilages, which makes bond between hull and cotyledon stronger. Pulse dehulling constitutes two major steps, viz., loosening of the hull followed by its removal in suitable milling machine (Narasimha et al. 2003). Loosening the hulls during dehulling is traditionally achieved either by wet or dry methods (Kurien 1977). Dry method involves application of oil and water. In this method, cleaned and graded grains are subjected to pitting to increase soaking. Then these pitted grains are mixed thoroughly with 1 % edible oil and spread for sun drying in thin layer for 2–3 days. At the end of drying, 2.5 % water is sprayed and mixed thoroughly. For tempering, grains are heaped overnight. Then the grains are dehulled
J Food Sci Technol
with roller machine. The wet method is now a day obsolete and rarely used. It has been reported by several researchers that heat treatment significantly improves the protein quality in pulses by destruction or inactivation of the heat labile anti-nutritional factors (Jenkins et al. 1982; Deosthale 1982; Khokhar and Chauhan 1986; Wang et al. 1997; Vijayakumari et al. 1998). Therefore, it may be useful to apply treatments for loosening the hulls by thermal means. The traditional thermal processing takes longer and exhibit all associated risk. Based on preliminary studies it was found that microwave can be used as an effective pre-treatment for dehulling pulses, which is hygienic and easy to apply, with the benefit of heat treatment. Also, the thermal treatment is expected to increase the pore sizes in the pulses which in turn reduce the water penetration time as well as reduce the cooking time (processing time for use as component in food product development) of the pulses. Microwave can also provide disinfestations of the grains and improve the shelf life of the dehulled grains (Vadivambal et al. 2007). Heating by microwave would disrupt the bondage between the hull and cotyledon of pulses by denaturing the protein and gums present between them. The information on the rate of microwave power application, dosage required to get the desired effect are essential in order to assess the suitability of the process. So far, there is no study explaining the effect of microwave treatment on the dehulling of black gram and its effect on quality parameters of the dehulled pulses. Keeping this in view, the present work has been undertaken with an overall objective to study the effect of microwave application, viz., energy input (power level), exposure and dosage, on the dehulling of black gram as well as its effect on the quality, viz., colour and cooking time of dehulled pulses.
Materials & methods Materials Samples Black gram ADT5 variety was purchased from the local market and was first cleaned well to remove the extraneous matter before conducting the experiments. The moisture content of the sample was 12.7 % (w.b.). Normally, in the commercial market about 12–13 % moisture content is common which comes to dehulling to the pulse processor and hence only single initial moisture content was considered in this study. Microwave oven A domestic microwave oven (Model: IFB 30SC2) having the operation frequency of 2,450 MHz and maximum input power of 1,400 W was used to conduct the experiment. The rated power output from the microwave oven is 900 W corresponding to 100 % power level. The oven has facility to be used at 10 to 100 % power level at an interval of 10 %. Methodology Microwave treatment A total weight of 100 g of cleaned seeds were taken in a glass dish of 155 mm diameter and placed on the turn table in the microwave oven. Three levels of microwave energy input, viz., 450, 630 and 810 W, were applied each at 7 different exposure times ranging from 60 to 150 s at an interval of 15 s. The depth of grain layer was approximately 5 mm. The maximum exposure time was set through preliminary study corresponding to the beginning of the emergence of roasted smell. The microwave dosage supplied to the black gram is calculated by Eq. 1.
J Microwave PowerðWÞ Microwave Exposure timeðsÞ Microwave dosage ¼ g Initial weight of black gramðgÞ
ð1Þ
Average surface temperature of microwave treated samples
Dehulling procedure
The surface temperature of the microwave treated grains was monitored immediately after the samples were taken out from the microwave oven, using an Infrared thermometer (Model: Center 350, Center Technology Corporation, Shu-Lin, Taiwan). The temperature was measured by holding the IR thermometer at a horizontal distance of 15 cm above the grain surface. Five temperature measurements at surface of the grains were taken, one at the centre and four around the perimeter of the petridish and the average value was calculated.
Dehulling of black gram was performed in laboratory model emery roll polisher (Model: TM05, Satake Corporation, Japan). Dehulling was continued till complete removal of husk was achieved in all grains and corresponding dehulling time was noted. After dehulling, the different fractions were collected and graded as dehulled whole and split, broken, powder and fine brokens. They were weighed separately and noted for further analysis. All samples were studied in triplicates.
J Food Sci Technol
Analysis of dehulling properties of black gram
Dehulling yield
The dehulling properties of black gram were assessed in terms of dehulling time, dehulling yield, dhal yield and losses.
Dehulling yield was defined as the quantity of total whole dehulled kernels and broken kernels (except fine brokens, powder and husk) that are produced in the dehulling of pulses. Head kernels are defined as the kernels retained over the sieve no. 10 (BSS mesh). Broken is defined as cotyledon retained on sieve no. 30 and pass through sieve no. 10 (BSS mesh). Fine broken is defined as the broken that passed through sieve no. 30 (BSS mesh). The dehulling yield is calculated using the formula given in Eq. 2.
Dehulling time Dehulling time was defined as the duration of dehulling of black gram for complete removal of husk from the cotyledons, i.e., 100 % dehulling of the grains.
Dehulling yieldð%Þ ¼
Weight of dehulled kernelsðgÞ þ weight of brokensðgÞ 100 Initial weight of black gramðgÞ
ð2Þ
(dhal) as a percentage of original seed weight. It is calculated by the relationship given in Eq. 3.
Dhal yield Dhal yield (from APQ Method 104.1, Burridge et al. 2001) was defined as the yield of dehulled whole and split kernels
Dhal yieldð%Þ ¼
Mass of dehulled whole and split seedsðgÞ 100 Initial weight of black gramðgÞ
ð3Þ
grain used for dehulling. It is calculated using the relationship given by Eq. (4) (Goyal et al. 2007).
Dehulling loss Dehulling loss is calculated as the weight fraction of the powder and fine broken relative to the total weight of the
Lossð%Þ ¼
Weight of powder obtainedðgÞ þ Weight of fine brokenðgÞ 100 Initial weight of black gramðgÞ
Analysis of the quality of dehulled dhal Colour Colour of seeds was determined by Hunter colour lab colorimeter (Model: Colour Quest XE, USA). Calibration of this machine was done using black and white standardizing templates. The sample was placed over the eye and the CIELAB L*, a*, and b* values were monitored for each sample as the average of six readings. The colour difference between the control sample (grains without any treatment) and the dhal samples subjected to microwave treatment was calculated using Eq. 5 in terms of ΔE*(Francis and Clydesdale 1975).
ΔE ¼
h 2 2 2 i1=2 L0 −L þ a0 −a þ b0 −b
ð5Þ
ð4Þ
Where L0*, a0*, and b0* are the colour parameters of the control sample. It has been reported that if the value of ΔE
ORIGINAL ARTICLE
Microwave assisted dehulling of black gram (Vigna mungo L) J. Jerish Joyner & B. K. Yadav
Revised: 17 September 2013 / Accepted: 26 September 2013 # Association of Food Scientists & Technologists (India) 2013
Abstract This article summarises the results of the investigation of application of microwave exposure on the dehulling characteristics of the black gram and the properties of the dehulled grains. Black gram was exposed to 3 microwave power levels, viz., 450, 630 and 810 W for 7 different exposure, ranging from 60 to 150 s at an interval of 15 s with a view to determine the suitable combination of dosage in order to get the maximum yield with little change in colour. Related changes in properties were also studied. It was observed that the surface temperature of the grain increased with the increase in microwave power level from 450 to 810 W as well as exposure time from 60 to 150 s in the range from 58 to 123 °C while the dehulling time reduced from 445 to 170 s. The dehulling yield increased with increasing microwave dosage in the beginning and reached to the maximum value followed by decreasing trend. The colour of the dehulled grain changed slowly up to a microwave dosage of 972 J/g after that it changed vividly darker than the control. The dehulling and dhal yields and colour change were polynomial functions of microwave dosage. The highest yield of 73.7 % was achieved at about 972 J/g with a little change in colour (CIELAB ΔE* value of 2.58). The corresponding dehulling time, cooking time and losses during dehulling were respectively 185 s, 10 min and 15.1 % as compared to 492 s, 20 min and 31.5 % for control respectively. It is concluded that a dosage of about 972 J/g was the best for the black gram dehulling at a rate of 630 W or higher power level.
Keyword Black gram . Microwave exposure . Dehulling time . Dehulling yield . Dhal yield . Dehulling loss J. J. Joyner : B. K. Yadav (*) Department of Food Engineering, Indian Institute of Crop Processing Technology, Pudukkottai Road, Thanjavur 613005, Tamilnadu, India e-mail: [email protected]
Introduction Black gram (Vigna mungo L) or urid is reported to be originated in India and being cultivated in Burma, Bangladesh, Sri Lanka and to a lesser extent in Thailand, Australia and other Asian and South Pacific countries (Poehlman 1991). Black gram is a rich protein food containing around 24 % protein, 59.6 % carbohydrate and 1.4 % fat (Gopalan et al. 1989). Black gram is extensively used in South Asian subcontinent for direct consumption as dehulled grain and as flour in various fermented foods, in Japan to make “moyashi” and its flour for making fresh skinless beef sausages in some European nations as protein substitute. The pulses are reported to contain antinutrients in seed coat. Also, the seed coat is hard and imparts a bitter taste, reducing the palatability of the pulses. In order to remove the antinutrients and improve the palatability, it is common practice to dehull the pulses before using. Dehulling is a process of removal of hull from the cotyledon of pulses. It reduces the fibre content and improves appearance, texture, cooking quality, palatability and digestibility of grain legumes (Tiwari et al. 2007). Dehulling of black gram has been a difficult operation due to the presence of vitreous layer of gums and mucilages, which makes bond between hull and cotyledon stronger. Pulse dehulling constitutes two major steps, viz., loosening of the hull followed by its removal in suitable milling machine (Narasimha et al. 2003). Loosening the hulls during dehulling is traditionally achieved either by wet or dry methods (Kurien 1977). Dry method involves application of oil and water. In this method, cleaned and graded grains are subjected to pitting to increase soaking. Then these pitted grains are mixed thoroughly with 1 % edible oil and spread for sun drying in thin layer for 2–3 days. At the end of drying, 2.5 % water is sprayed and mixed thoroughly. For tempering, grains are heaped overnight. Then the grains are dehulled
J Food Sci Technol
with roller machine. The wet method is now a day obsolete and rarely used. It has been reported by several researchers that heat treatment significantly improves the protein quality in pulses by destruction or inactivation of the heat labile anti-nutritional factors (Jenkins et al. 1982; Deosthale 1982; Khokhar and Chauhan 1986; Wang et al. 1997; Vijayakumari et al. 1998). Therefore, it may be useful to apply treatments for loosening the hulls by thermal means. The traditional thermal processing takes longer and exhibit all associated risk. Based on preliminary studies it was found that microwave can be used as an effective pre-treatment for dehulling pulses, which is hygienic and easy to apply, with the benefit of heat treatment. Also, the thermal treatment is expected to increase the pore sizes in the pulses which in turn reduce the water penetration time as well as reduce the cooking time (processing time for use as component in food product development) of the pulses. Microwave can also provide disinfestations of the grains and improve the shelf life of the dehulled grains (Vadivambal et al. 2007). Heating by microwave would disrupt the bondage between the hull and cotyledon of pulses by denaturing the protein and gums present between them. The information on the rate of microwave power application, dosage required to get the desired effect are essential in order to assess the suitability of the process. So far, there is no study explaining the effect of microwave treatment on the dehulling of black gram and its effect on quality parameters of the dehulled pulses. Keeping this in view, the present work has been undertaken with an overall objective to study the effect of microwave application, viz., energy input (power level), exposure and dosage, on the dehulling of black gram as well as its effect on the quality, viz., colour and cooking time of dehulled pulses.
Materials & methods Materials Samples Black gram ADT5 variety was purchased from the local market and was first cleaned well to remove the extraneous matter before conducting the experiments. The moisture content of the sample was 12.7 % (w.b.). Normally, in the commercial market about 12–13 % moisture content is common which comes to dehulling to the pulse processor and hence only single initial moisture content was considered in this study. Microwave oven A domestic microwave oven (Model: IFB 30SC2) having the operation frequency of 2,450 MHz and maximum input power of 1,400 W was used to conduct the experiment. The rated power output from the microwave oven is 900 W corresponding to 100 % power level. The oven has facility to be used at 10 to 100 % power level at an interval of 10 %. Methodology Microwave treatment A total weight of 100 g of cleaned seeds were taken in a glass dish of 155 mm diameter and placed on the turn table in the microwave oven. Three levels of microwave energy input, viz., 450, 630 and 810 W, were applied each at 7 different exposure times ranging from 60 to 150 s at an interval of 15 s. The depth of grain layer was approximately 5 mm. The maximum exposure time was set through preliminary study corresponding to the beginning of the emergence of roasted smell. The microwave dosage supplied to the black gram is calculated by Eq. 1.
J Microwave PowerðWÞ Microwave Exposure timeðsÞ Microwave dosage ¼ g Initial weight of black gramðgÞ
ð1Þ
Average surface temperature of microwave treated samples
Dehulling procedure
The surface temperature of the microwave treated grains was monitored immediately after the samples were taken out from the microwave oven, using an Infrared thermometer (Model: Center 350, Center Technology Corporation, Shu-Lin, Taiwan). The temperature was measured by holding the IR thermometer at a horizontal distance of 15 cm above the grain surface. Five temperature measurements at surface of the grains were taken, one at the centre and four around the perimeter of the petridish and the average value was calculated.
Dehulling of black gram was performed in laboratory model emery roll polisher (Model: TM05, Satake Corporation, Japan). Dehulling was continued till complete removal of husk was achieved in all grains and corresponding dehulling time was noted. After dehulling, the different fractions were collected and graded as dehulled whole and split, broken, powder and fine brokens. They were weighed separately and noted for further analysis. All samples were studied in triplicates.
J Food Sci Technol
Analysis of dehulling properties of black gram
Dehulling yield
The dehulling properties of black gram were assessed in terms of dehulling time, dehulling yield, dhal yield and losses.
Dehulling yield was defined as the quantity of total whole dehulled kernels and broken kernels (except fine brokens, powder and husk) that are produced in the dehulling of pulses. Head kernels are defined as the kernels retained over the sieve no. 10 (BSS mesh). Broken is defined as cotyledon retained on sieve no. 30 and pass through sieve no. 10 (BSS mesh). Fine broken is defined as the broken that passed through sieve no. 30 (BSS mesh). The dehulling yield is calculated using the formula given in Eq. 2.
Dehulling time Dehulling time was defined as the duration of dehulling of black gram for complete removal of husk from the cotyledons, i.e., 100 % dehulling of the grains.
Dehulling yieldð%Þ ¼
Weight of dehulled kernelsðgÞ þ weight of brokensðgÞ 100 Initial weight of black gramðgÞ
ð2Þ
(dhal) as a percentage of original seed weight. It is calculated by the relationship given in Eq. 3.
Dhal yield Dhal yield (from APQ Method 104.1, Burridge et al. 2001) was defined as the yield of dehulled whole and split kernels
Dhal yieldð%Þ ¼
Mass of dehulled whole and split seedsðgÞ 100 Initial weight of black gramðgÞ
ð3Þ
grain used for dehulling. It is calculated using the relationship given by Eq. (4) (Goyal et al. 2007).
Dehulling loss Dehulling loss is calculated as the weight fraction of the powder and fine broken relative to the total weight of the
Lossð%Þ ¼
Weight of powder obtainedðgÞ þ Weight of fine brokenðgÞ 100 Initial weight of black gramðgÞ
Analysis of the quality of dehulled dhal Colour Colour of seeds was determined by Hunter colour lab colorimeter (Model: Colour Quest XE, USA). Calibration of this machine was done using black and white standardizing templates. The sample was placed over the eye and the CIELAB L*, a*, and b* values were monitored for each sample as the average of six readings. The colour difference between the control sample (grains without any treatment) and the dhal samples subjected to microwave treatment was calculated using Eq. 5 in terms of ΔE*(Francis and Clydesdale 1975).
ΔE ¼
h 2 2 2 i1=2 L0 −L þ a0 −a þ b0 −b
ð5Þ
ð4Þ
Where L0*, a0*, and b0* are the colour parameters of the control sample. It has been reported that if the value of ΔE
