Physico-Chemical Characteristics of Rice Bran Processed by Dry Heating and Extrusion Cooking

Abstract

Rice bran was stabilized by dry heat and extrusion cooking method. Physico-chemical, functional, and storage characteristics of stabilized rice bran were evaluated. The rice bran stabilization affected its crude fat and crude ash contents significantly, whereas the other constituents remained almost unchanged. The content of reducing, nonreducing, and total sugars remained unaffected and did not differ significantly from raw rice bran. The neutral detergent fiber (NDF) contents of raw and dry-heat treated rice bran did not differ significantly. However, after extrusion stabilization, the NDF increased significantly. Similar effects were observed in composition of gum fiber and total fiber. The stabilization of rice bran had a significant reduction in lysine and phytic acid content. Bulk density and water absorption were higher in extruded stabilized rice bran than dry-heat treated bran. Protein solubility was maximum in raw bran, whereas damaged starch content was higher in stabilized bran. Color of the extruded bran was dark brown, whereas that of dry-heat stabilized was light brown. It was observed that dry-heat treated bran was stored up to 30 days, whereas extruded bran was stored up to 60 days without any changes in the free fatty acid content.

Keywords:

• Rice bran stabilization
• Composition
• Functional properties
• Nurtitional properties
• Storage

Introduction

Rice is a major food commodity throughout the world. India is the largest producer of rice in the world outside main land of China in productionwise. For value addition, low grade rice have been used for extrusion cooking to manufacture expanded crunchy snacks.1 Based on production, the potential availability of rice bran, a by-product of rice milling industry was estimated to be about 5 million metric tonnes per annum.2 The rice bran refers to the coating removed from brown rice during the process of milling. The bran constitutes nearly 8.5% of the total grain and is highly nutritious.3 Raw rice bran has ac characteristic bland flavor, slightly bitter, and sweet taste. Bran flavor is frequently descried as incipient rancid, musty, and sour because of its readily deterioration in commercial lots.4 It contains 12–25% fat, 10–16% protein, 10–20% starch, 3–8% reducing sugars, 8–11% hemicelluloses, 10–12% celluloses, 6–15% crude fiber, and 6.5–10% ash content. Rice bran is abundant in vitamins of the B group and tocopherols and is poor in vitamins A and C.5 Several human and animal studies conducted to date suggest that rice bran has interesting health benefits. It appears to be as effective as wheat bran as a laxative aid, effective hypocholesterolmic agent as well as several other health benefits reported in literature.6 7 Since rice bran contains less soluble fiber, its cholesterol lowering and other health enhancing properties may be related to associated compounds. These include rice bran oil, plant sterols, tocopherols, oryzanol, and β-sitosterol.8 Although the nutritional and food potential of rice bran have been recognized, the consumption of rice bran in human foods has been limited to a very small quantity. This may be partially due to the result of lipid deterioration by the enzymes such as lipase; growth inhibitors; microorganisms, and insects in rice bran.6 Several types of heat stabilization procedures have been mentioned which involve the application of dry or moist heat treatments for a sufficient time to arrest the activity of spoilage agencies such as lipases and destroy the antinutritional factors.9 Baked foods provide one of the most potential possibilities of utilizing rice bran in foods. Uses of rice bran are advocated in making breads, muffins, pan cakes, cookies, cakes, pies, extruded snacks, and breakfast cereals. The stabilization of rice bran by using extrusion cooking and dry heat treatment is practical and has the commercial potential, therefore, these two method of stabilization were compared.10 The stabilization of rice bran has been reported to improve nutritional, functional and keeping quality of rice bran.11 Considering the nutritional and food potential and associated health benefits of consuming rice bran the present investigation was planned to compare the effects of dry heat treatment with that of extrusion cooking on the physico-chemical characteristics of rice bran and their stability during storage.

Materials and Methods

Fresh rice bran was procured from M/S Shakti Rice Mills, Rudurpur (UA), India. Rice bran was stabilized immediately after procurement using dry-heat and extrusion cooking as described below.

Dry Heat Stabilization

Rice bran was transferred to open shallow pans and spreaded uniformly in thin layers. The pans were then placed in the oven maintained at 120°C and dried for 30 min. The dried bran was ground to pass through screen (0.32 mm) of the Fitz (Fitz company, USA) mill to produce particle size close to wheat flour. The bran was stored at an ambient temperature in polyethylene bags for further use.

Extrusion Stabilization

It was preformed using a Wenger X-5 Laboratory Extruder (Wenger Mfg. Co., Sabetha, KS, USA) with the following conditions: Water flow rate: 0.000038 m³/s, feed rate: 27 kg/hr, steam supply: 275.80 kPa, die opening: 0.0078 m, temperature: 135–140°C. Extrudates were air dried at 50°C for 24 h and ground in a Fitz Mill to pass through 0.32 mm screen of the mill as in dry heat method. The bran was stored at an ambient temperature in polyethylene bags for further use.

Chemical Composition

The samples of bran were analyzed for moisture, protein (N × 5.95), crude fat, total ash, crude fibre, calcium, and sugars according to AACC12 methods. The percent values of moisture, protein, fat, ash, and crude fiber were subtracted from 100 and the remainder was expressed as carbohydrates (by difference). Calorific value (kJ/100 g) was calculated from the physiological fuel values of protein, fat, and carbohydrates. While the color of different bran samples was noted visually. Neutral detergent fiber was determined according the method described by the worker.13 The method is based on the extraction of the food with hot neutral detergent solution. The residues (NDF) contain lignin, cellulose, hemicellulose, and cell wall protein.

Gum fiber was determined according to the procedure described by Sharma.14 One gram sample was extracted with 25 mL of acetic acid solution by stirring at room temperature for 30 min. The contents were centrifuged for 20 min at 6500 rpm and the supernatant recovered. The process of extraction and centrifugation was repeated four times and supernatants were pooled. Gum was precipitated from pooled acetic acid extract by the addition of ethanol with stirring till 50% concentration of ethanol was reached. The suspension was centrifuged and the supernatants were decanted. The precipitate was again washed with ethanol and acetone and dried at 50°C to a constant weight. Total fiber was roughly calculated as the sum of neutral detergent fiber and gum fiber according to the method described by worker.14

Available lysine was determined by Carpenter method,15 which is based on the reaction of fluorodinitrobenzene (FDNB) with the epsilon-NH₂ group of lysine in the proteins and the colorimetric determination of the DNP-lysine obtained by subsequent acid hydrolysis. Method of Haug and Lantzsch16 was used to estimate the phytic acid content in rice bran sample. In this method, sample extract was heated with acidic iron (ferric) solution of a known iron content. The decrease in iron (determined colorimetrically with 2–2 bipyridine) in supernatant is a measure of phytic acid content. Free fatty acid contents of raw and stabilized rice bran were determined by AACC method.12

Functional Characteristics

Bulk density of raw and stabilized bran were determined by method of Narain et al.17 Water absorption capacities of raw and stabilized rice bran were measured by a modified method reported by Sosuliski.18 Sample of 2.5 g was dispersed with 25 mL of water in a 50 mL centrifuge tube and allowed the mixture to stand for 15 min at 25°C, then centrifuged the sample at 3200 rpm for 15 min. The amount of water retained by the solids was measured. Fat absorption of rice bran were estimated by mixing 5 g sample with 50 mL corn oil allowing the mixture to stand for 15 min at room temperature and then centrifuging at 3200 rpm for 15 min. The fat retained by the solids was measured.19

Fortuin method20 was followed to determine the protein solubility of bran. Weighed sample (2.5 g) was dispersed in 100 mL distilled water and stirred for 1 h at a controlled temperature of 50°C. The solution was then centrifuged for 30 min at 3200 rpm. The supernatant was collected in a flask and the nitrogen content was determined by Kjeldahl method.12 Damaged starch in rice bran was determined according the AACC method.12 The method determines the percentage of starch granules which are susceptible to hydrolysis by amylase. Reducing sugars resulting from the enzymatic action on damaged starch are measured by ferrycyanide method and converted to damage starch by using a factor of 0.082.

Result and Discussion

Proximate compositions and calorific value of dry heat treated and extruded rice bran comparable to raw rice bran are presented in Table 1. From the table it can be reveled that only the crude fat and crude ash contents were affected significantly (p < 0.05) by the method of stabilization whereas the other constituents and calorific value remained almost unchanged after stabilization. The extruded rice bran had significantly (p < 0.05) higher ash content (8.62%) than did the dry heat treated rice bran (8.1%). A slight increase in ash content after extrusion may be due to the contamination of extrudates with mineral present in water used in wet extrusion processing of bran. Crude fat content of extrusion stabilized rice bran was significantly (p < 0.05) higher than raw rice bran. This indicated that stabilization process of rice bran cause fat cells to coalesce into oil droplets and rapture cell structure, thereby improving the speed of oil extraction.21 22 However, extrusion cooking appeared to be more effective than the dry heat method and the process may have great economic benefits for rice growing region in India. The results of the proximate compositions of rice bran are in agreement with those reported by worker.6 8

Table 1 Effect of method of stabilization on the proximate composition and calorific value of rice bran

Methods of stabilization	Crude protein (kg/100 kg bran)	Crude fat (kg/100 kg bran)	Crude fiber (kg/100 kg bran)	Crude ash (kg/100 kg bran)	Carbohydrates (kg/100 kg bran)	Calorific value (KJ/100 g bran)
RRB	16.05	14.72	7.20	8.40	39.72	1486
DHTRB	15.90	15.06	7.19	8.17	39.80	1502
ERB	16.01	15.69	7.33	8.62	39.95	1504
CD at P ≤ 0.05	NS	0.484	NS	0.22	NS	NS

Data reported are the average of triplicate determinations.

NS: Nonsignificant.

RRB: raw rice bran, DHT: dry heat treated rice bran, ERB: extruded rice bran.

Table 2 displays the effects of method of stabilization on sugars, fiber, available lysine and phytic acid contents of rice bran. The contents of reducing, nonreducing, and total sugars remained unaffected and did not differ significantly (p > 0.05) from raw rice bran. It has been reported that starch and nonreducing sugars such as sucrose might be degraded during extrusion to form reducing sugars.23 But in the present investigation it appeared that the extent of heat processing applied on rice bran was not severe enough to cause this change or some reducing sugars might have been formed and took part in the browning reaction as observed from the color of the stabilized rice bran which were significantly more than the raw rice bran. The neural detergent fiber (NDF) contents of raw and dry heat treated rice bran did not differ significantly. However, after extrusion stabilization, the NDF was significantly increased. Similar effects were observed in gum fiber and total fiber. Earlier workers24 25 also reported that neutral detergent fiber increased after heat processing due to the formation of compounds, which were insoluble in detergents. Heat treatment also exhibited a significant effect on the contents of gum fiber. Extrusion stabilized rice bran showed highest content of gum fiber followed by dry heat stabilized rice bran. Increased gum fiber in extruded rice bran could be the result of disruption of covalent and noncovalent bonds of the complex moeties leading to the formation of more water soluble and alcohol precipitable fragments. Almost similar results were also reported by various26 27 28 researchers. As the contents of NDF and gum fiber increased, the increase in total fiber calculated as the sum of NDF and gum fiber was also evident.29 Although some differences existed in the fiber constituents of rice bran, the values observed in this study were in close agreement with those reported by other workers.8 30 The stabilization of rice bran had a significant effect (p < 0.05) on lysine response. FDNB-reactive lysine contents of dry heat treated rice bran and extruded rice were 3.69 g and 3.95 g/16 g N compared to 4.09 g/16 g N in raw bran indicating a greater loss of lysine availability by dry heat method. Higher temperature short time extrusion processing when carried out under wet conditions has been reported to be favorable in terms of lysine retention.31 Heat stabilization of rice bran significantly (p < 0.05) reduced the phytic acid content of rice bran from 29.3 mg/g in raw rice to 26.50 mg/g in dry heat treated rice bran and 24.66 mg/g in extruded rice bran, respectively. Earlier work32 33 showed a significant reduction in phytic acid during extrusion processing of foods.

Table 2 Effect of method of stabilization on sugar and fiber compounds, available lysine, and phytic acid contents of rice bran

	Sugars			Fiber
Methods of stabilization	Reducing sugars (kg/100 kg bran)	Nonreducing sugars (kg/100 kg bran)	Total sugars (kg/100 kg bran)	NDF (kg/100 kg bran)	Gum fiber (kg/100 kg bran)	Total fiber (kg/100 kg bran)	Available lysine g/16 g N (not clear)	Phytic acid (mg/g bran)
RRB	0.90	2.31	3.11	23.63	0.95	24.58	4.09	29.33
DHTRB	0.89	2.12	3.00	24.86	1.53	26.25	3.69	24.50
ERB	0.87	2.12	3.01	27.2	1.82	28.98	3.95	24.66
CD at P ≤ 0.05	NS	NS	NS	1.38	0.23	1.20	0.24	2.95

Data reported are the average of triplicate determinations.

NS: Nonsignificant.

RRB: raw rice bran, DHT: dry heat treated rice bran, ERB: extruded rice bran.

Functional characteristics of rice bran as summarized in Table 3 showed that the bulk density of stabilized rice bran increased significantly (p < 0.05). Earlier workers21 22 also reported similar findings and the higher bulk density of extruded rice bran was observed to increase percolation rate of the solvent through the extrudates and hence better efficiency of oil extraction.

Table 3 Effect of method of stabilization on the functional characteristics of rice

Methods of stabilization	Bulk density (g/mL)	Water absorption (g/100 g)	Water solubility (g/100 g)	Fat absorption (g/100 g)	Protein solubility (g/100 g)	Damaged starch (g/100 g)	Visual color
RRB	0.500	143.68	7.57	96.32	7.32	0.82	Light tan
DHTRB	0.548	156.91	9.34	81.28	4.44	3.25	Brown
ERB	0.593	170.93	9.33	72.60	3.03	5.07	Dark brown
CD at P ≤ 0.05	0.026	3.696	0.608	2.756	0.635	0.300	—

Data reported are the average of triplicate determinations.

RRB: raw rice bran, DHT: dry heat treated rice bran, ERB: extruded rice bran.

Extruded rice bran had the highest water absorption (170.9 g/100 g) followed by dry-heat treated rice bran (143.78 g/100 g) these figure are lower than, whereas water absorptions in the range of 213.1–182.2% of edible stabilized rice bran as reported earlier.34 The variation may be attributed to the source of bran and their processing conditions. Similar trend was observed for water solubility. Increase in water solubility of stabilized rice bran may be due to the solubilization of the starch component during heat processing of rice bran by dry heat and extrusion stabilization. A significant decrease in fat absorption, characteristics of rice bran, after heat treatment was observed which may be due to the changes in bed porosity and pore size of rice bran.

Protein solubility was highest (7.32 g/100 g), in raw rice bran and lowest (3.03) in extruded rice bran indicating the occurrence of protein denaturation during heat stabilization of rice bran as has been reported earlier31 35 also. Heat stabilization of significantly (p ≤ 0.05) increased the damaged starch (enzyme susceptible starch) content in rice bran as compared to raw rice bran. Result of worker35 also indicated higher content of damaged starch in extruded wheat bran than the raw bran. The formation of damaged starch during extrusion/heat processing may be important in yeast fermentation and can act as a functional agent in bran dough.36 Color of the bran as observed visually extruded bran was dark brown in color, whereas the dry heat treated bran was just brown and the raw bran was light tan in color. The changes in color upon heating may be attributed to the formation of Maillard reaction compounds during extrusion and partly to the solubilization of color pigments of raw bran.

To evaluate the effectiveness of rice bran stabilization methods (dry heat and extrusion processing), a storage study was carried out. The changes in free fatty acid (FFA) contents recorded during storage are presented in Table 4. Stabilization methods and storage period significantly (p < 0.05) affected the rate of production of free fatty acids. A highly significant increase in FFA was observed in raw rice bran during storage period. FFA content in raw rice bran increased from an initial level of 4.05–64.60% at 60 days of storage. This phenomena is typical of raw rice bran and results in the development of hydrolytic rancidity in unstabilized rice bran.11 This has been attributed to the presence of active inherent lipase system of rice bran which hydrolyses the oil free fatty acids and glycerol and renders bran unfit for oil extraction and human consumption. Dry heat method of stabilization restricted the development of FFAs up to 30 days of storage and thereafter a significant increase was observed. This increase in FFAs content of bran may be attributed to the presence of residual lipolytic activity of enzyme lipase, which increased under favorable conditions during storage. Bran might have absorbed some moisture during storage, thereby favoring the lipolytic activity. This type of behavior with dry-heat treated rice bran was attributed to the incomplete destruction of lipase by dry heat method and increase in moisture content of rice bran during storage.37 By extrusion stabilization method the free fatty acid content of rice bran did not increase significantly (p > 0.05) even at 60 days of storage, indicating that the extrusion processing of rice bran is more effective in destroying the lipolytic system of rice bran than the dry heat method. The capability of the extruder in inactivating the fat hydrolyzing enzymes of rice bran has also been reported earlier.9 11 Less then 5% FFAs are stated to be desirable in economic refining of oil.38 Sensory analysis11 revealed that experienced panelists could not distinguish rancid bran from nonrancid bran until the level of free fatty acids exceeded 15%. However, the level of fatty acids in bran that is considered acceptable for human consumption is 4% according to US Standards.8

Table 4 Effect of the methods of stabilization and storage period on the development of free fatty acids (FFAs) in rice bran

	Free fatty acids kg/100 kg bran storage period (days)
Methods of stabilization	0	15	30	45	60
RRB	4.05	13.58	20.46	37.75	64.60
DHTRB	3.66	3.75	3.79	5.70	9.15
ERB	3.85	4.08	4.22	4.20	4.10

CD at P ≤ 0.05 methods of stabilization = 0.399.

CD at P ≤ 0.05 storage period = 0.515.

CD at P ≤ 0.05 methods × storage period = 0.893.

RRB: raw rice bran, DHT: dry heat treated rice bran, ERB: extruded rice bran.

Conclusion

From the present investigation it can be concluded that the functional properties such as water absorption, water solubility, bulk density, and enzyme susceptible starch of rice bran improved significantly after stabilization. However the increases were more pronounced in extrusion stabilized rice bran. On the other hand extrusion stabilized rice bran exhibited lower fat absorption and fat solubility than dry heat stabilized rice bran. The improvement in shelf life was also much higher for the rice bran stabilized by using extrusion technology than by dry heat treatment.