Impact of milling and water-to-rice ratio on cooked rice and wastewater properties

Abstract

Due to the environmental concerns and expense associated with the disposal of wastewater after industrial cooking of rice, the purpose of this study was to evaluate the extent of leaching, water uptake and volumetric expansion of rice during cooking at various milling durations and water-to-rice ratios. Two cultivars of Arkansas rice, a long grain (Francis) and a medium grain (Jupiter), were milled for 10, 20, and 30 s with a laboratory mill. Samples were cooked in aluminum foil covered beakers at water-to-rice ratios of 10:1, 15:1, and 20:1. After 20 min, rice was weighed for water uptake and measured for volumetric expansion using hexane displacement. The excess cooking water was evaluated for total solids, amylose, and protein to determine the extent of leaching. Water uptake and volumetric expansion significantly increased with milling duration in both cultivars. Leached solids increased as the water-to-rice ratio increased in Francis and increased with milling duration in Jupiter; however, the amylose content of leached solids was unaffected by the water-to-rice ratio in both cultivars. Thus, shorter milling durations may limit the extent of leaching during cooking, as observed with Jupiter, while a lower water-to-rice ratio may reduce wastewater contamination for cultivars similar to Francis.

Keywords

Rice cooking water-to-rice ratio leaching water uptake expansion milling

Introduction

In home cooking, rice is cooked either in excess water or in a predetermined amount of water (with an approximate rice-to-water ratio of 1:2 (v/v)) that is completely absorbed during the cooking process (Bett-Garber, 2007; Juliano, 1984). On an industrial scale, however, rice cooking often involves the use of variable levels of excess water (Boykin et al., 2005; Stover, 1992). At the end of the cooking process, the excess water often contains significant levels of starch, protein, and lipids (Nierle et al., 1981) as well as various inorganic compounds (Queiroz et al., 2007). This poses a significant problem in industry due to the costly disposal of industrial wastewater with a high biological oxygen demand (BOD) and chemical oxygen demand (COD) (Boykin et al., 2005).

Studies indicate that leaching of starch, proteins and lipids can be affected by several factors, including water-to-rice ratio (WRR) and degree of milling. Degree of milling is a measurement of the amount of bran removed from the surface of the rice kernel (Perdon et al., 2001). Nierle et al. (1981) observed less solids when cooking brown rice than milled white rice indicating that the degree of milling likely affects the extent of solids lost during cooking. Similarly, Morawicki et al. (2009) observed that the extent of leaching of rice cooked in excess water was affected by the degree of milling and the WRR.

Other factors that may contribute to the loss of solids during cooking include the grain length (commonly characterized as short, medium or long grains) and cooking duration. Due to differences in gelatinization temperature, the long-grain rice cultivars were less likely to leach amylose which resulted in reduced total solids in the effluent (Metcalf and Lund, 1985). In addition, increasing the cooking duration in excess water resulted in a greater loss of solids (Yadav and Jindal, 2007).

The rice kernel undergoes significant changes during cooking. These changes can be partially defined by water uptake and volumetric expansion. Water uptake of rice is the amount of water absorbed by a quantity of rice when cooked in a boiling bath for a defined period of time (Bergman et al., 2004) while volumetric expansion, related to the water uptake, is the extent of swelling of the kernel. The water uptake is affected by cooking duration (Yadav and Jindal, 2007), amylose content (Metcalf and Lund, 1985), and degree of milling (Mohapatra and Bal, 2006). Volumetric expansion is affected by both grain properties (Ge et al., 2005) and cooking temperatures (Ramesh, 2001; Sowbhagya et al., 1994). Understanding how water uptake and volumetric expansion are influenced by excess water may contribute to the optimization of cooking conditions during operations.

Since wastewater contamination is an industrial concern during cooking of rice in excess water, it is important to evaluate changes in rice and cooking water as affected by various processing and pre-processing conditions. Thus, the goal of this cooking study was to evaluate simultaneously the effects of grain length, milling duration, and excess water (at various WRRs) on cooked rice water uptake, volumetric expansion, and leaching of solids such as starch and protein.

Materials and methods

Materials

Two cultivars of rice, Francis (a long-grain cultivar) and Jupiter (a medium grain) were harvested in the fall of 2007 from Stuttgart, AR and Newport, AR, respectively. Petroleum ether, hexane, ethanol, sodium hydroxide, acetic acid, iodine, potassium iodide and dimethyl sulfoxide were obtained from VWR International (West Chester, PA). The standards (amylose, amylopectin and bovine serum albumin) were purchased from Sigma-Aldrich (St Louis, MO).

Methods

Experimental design

The effects of cultivar (long vs. medium grain), milling duration, and WRR were studied simultaneously using a 2 × 3 × 3 full factorial experimental design in triplicate. The three milling durations were 10, 20, and 30 s and the three WRRs were 10:1, 15:1, and 20:1.

Rice harvest and pre-study treatment

The long-grain cultivar (Francis) was harvested at 18.5% harvest moisture content (MC) as determined by an individual kernel moisture meter (GAC2000, Dicky-John, Auburn, IL), while the medium-grain cultivar (Jupiter) was harvested at a 16.7% harvest MC. The rough rice was transported to the University of Arkansas rice processing laboratories where it was cleaned (Carter-Day Dockage Tester, Seedburo Equipment Co., Chicago, IL; MCI Kicker Grain Tester, Seedburo Equipment Co., Chicago, IL) and then spread on drying trays. The drying trays (wooden frame strung with wire mesh to allow air flow through the rice layer) were then placed in a chamber in which the equilibrium MC was set at 12.5% (relative humidity 62.1%, temperature 26.5 °C) using an environmental control unit (Parameter Generation and Control, Black Mountain, NC). During the slow drying process, rice MC was monitored periodically using an oven drying method modified from Jindal and Siebenmorgen (1987) during which duplicate 16 g samples of rough rice were dried for 24 h in a convection oven (1370 FM, Sheldon Inc., Cornelius, OR) set at 130 °C. Once the rice reached 12.4% MC, samples (150 g) were sealed in Ziploc® bags, stored at 4 °C and milled within a week.

Milling

Samples were removed from cold storage and allowed to equilibrate, while still sealed, at room temperature for 24 h. Each of the bagged samples (with 150 g rough rice) was dehulled using a laboratory huller (Type THU, Satake, Tokyo, Japan). The resulting brown rice was then milled using a laboratory milling machine (McGill No.2, RAPSCO, Brookshire, TX) for 10, 20, or 30 s, and the total milled mass was measured and recorded. The milled rice was separated into head rice (rice kernels 3/4 original length or longer) and broken kernels using a sizing device (Rice Sizing Device 61, Grain Machinery MFG Corp. Miami, Fl.). Excess bran and chaff was removed from the whole kernels by forcing air through the rice sample for 30 s (Grainblower, Seedburo Equipment Co,. Chicago, IL). The head rice (or whole rice) kernels were then weighed and the head rice yield (HRY) was calculated using equation (1).

(1)

Fractions of the same cultivar and milling duration were pooled together and placed in an equilibration chamber set at 29 °C and 58% RH long enough to ensure a 12% MC in the kernels before cooking. After moisture content equilibration, the samples were divided into 20 g aliquots, bagged, sealed, and stored at 4 °C until the cooking tests were performed.

Surface lipid content

The surface lipid content was determined by lipid extraction with a Soxtec Avanti 2055 (Foss North America, Eden Prairie, MN) according to the methods described in Matsler and Siebenmorgen (2005) and AACC International (2000). Prior to extraction, samples of whole rice kernels (4–5 g) were placed in Soxtec thimbles, covered with defatted cotton and dried for one hour at 103 ± 2 °C. After cooling in a desiccator, boiling beads were added and the samples were placed in the Soxtec. Lipids were extracted with petroleum ether (approximately 70 mL). Following the evaporation of the solvent, the Soxtec thimbles were dried, allowed to cool in a desiccator, and subsequently weighed. The surface lipid content was calculated as the percent weight loss following lipid extraction from whole rice kernels.

Cooking

Milled rice samples from each cultivar and milling duration were removed from 4 °C storage and placed, while sealed, at room temperature for 1 h prior to cooking tests. In a beaker, 200, 300, or 400 mL water was brought to a boil and 20 g of rice was added immediately (representing WRRs of 10:1, 15:1, and 20:1, respectively). The samples were stirred thoroughly and then covered tightly with aluminum foil to prevent evaporation and maintain a constant volume. The foil was pressed around the beaker opening while leaving a small depression in the center where an ice cube was placed to promote condensation inside the beaker (Figure 1). Once covered, heat was reduced to assure a gentle boil for 20 min. The cooked rice was then strained and the cooking water was collected in a pre-weighed glass beaker. The cooked rice was analyzed for water uptake and volumetric expansion, while the cooking water was analyzed for total solids, amylose and protein content.

Water uptake

Immediately after cooking, the rice samples were weighed and water uptake was calculated as the percentage of weight gained during cooking (based on the original weight of the uncooked rice) using equation (2).

(2)

Volumetric expansion

Volumetric expansion was determined through volume displacement of hexane. After the cooked rice was drained and weighed for water uptake, the cooked rice was placed in 150 mL hexane and the change in volume was recorded. The displacement of 20 g uncooked rice sample of the same cultivar/milling duration/WRR in 50 mL of hexane was also determined. The percent volumetric expansion was calculated using the following equation.

(3)

Solids leached

Immediately after the rice was cooked, the cooking water was collected in a pre-weighed glass beaker and placed in a laboratory oven set at 115 °C for 24 h. The beakers were cooled in a dessicator, weighed, and the mass of leached solids per 20 g rice sample was calculated. Then the dry solids were scraped out of the beaker into Ziploc^TM bags, sealed and stored at 4 °C until the amylose and protein content could be determined.

Apparent amylose content

Apparent amylose content of the leached solids was determined using the method described by Juliano et al. (1981). From each sample, 100.0 ± 2.0 mg of dried leached solids was transferred into a 50 mL test tube. To this, 1 mL of 95% ethanol and 9 mL of 1 N NaOH were added. The test tubes were sealed and the samples were incubated overnight in a 50 °C water bath rotating at 100 rpm. After cooling to room temperature, the samples were rinsed 5 times with 10 mL of de-ionized (DI) water. The rinse water was added back and the final volume was brought to 100 mL with DI H₂O. A 0.5 mL aliquot was mixed with 9.2 mL DI H₂O, 0.1 mL 1 N acetic acid, and 0.2 mL of IKI solution (0.2% I₂ in 2% KI) and incubated for 30 min at room temperature. The absorbance was measured at 620 nm with a spectrophotometer (PharmaSpec 1700, Shimadzu, Japan). The results were compared to standards solutions composed of different concentrations of amylose (potato starch) and amylopectin (waxy rice). The percentage of amylose (on a dry weight basis) was determined from the standard regression equation of amylose in the standards.

Bio-rad protein assay

The samples were prepared by dissolving 0.01 g of leached solids in 2 mL of 90% dimethyl sulfoxide. The samples were held overnight in a 95 °C hot water bath rotating at 100 rpm. Protein concentration was determined using a protein assay kit (BIO-RAD, Hercules, CA) with bovine serum albumin as the protein standard prepared in the same manner as the samples.

Statistical analysis

JMP v.7 (SAS Institute, Cary, NC) was used for multivariate analysis of variance (MANOVA) and analysis of variance (ANOVA) to determine the significance of treatments. Tukeys least squares adjustment method was used to compare treatment means (α < 0.05).

Results and discussion

Head rice yield

The HRY for Francis was 79% after milling for 10 s and 75% after milling for 20 s and 30 s. A decrease in HRY with increased milling duration was also observed by Saleh and Meullenet (2007). In contrast, however, the HRY for Jupiter ranged from 90% to 91% for all milling durations with no differences between means.

Surface lipid content

The surface lipid content of the rice cultivars as affected by milling duration are shown in Table 1. The surface lipid content decreased in Francis from 0.97% (with 10 s of milling) to 0.30% (with 30 s) while the lipid content of Jupiter rice decreased from 0.79% (with 10 s of milling) to 0.44% (with 30 s). Surface lipid content is a relative indicator of the extent of bran removal since the lipid content of the bran is significantly greater than the endosperm. The typical surface lipid content of white rice ranges from 0.3% to 0.5% according to Lu and Luh (1991); therefore, milling for 30 s was sufficient to remove most of the bran from the surface of the rice and simulate the production of white rice.

Table 1.

The surface lipid content (%) of the rice kernels after milling (mean ± SD)

Cultivar	Milling time (s)
Cultivar	10	20	30
Jupiter	0.97 ± 0.02	0.68 ± 0.03	0.44 ± 0.03
Francis	0.79 ± 0.02	0.43 ± 0.03	0.30 ± 0.00

Water uptake

The effects of WRR, DOM and cultivar on the water uptake of cooked rice are shown in Tables 2 and 3. As expected, the water uptake of Francis was greater than Jupiter. Bergman et al. (2004) reported that water uptake was greater in long-grain rice cultivars than medium-grain when cooked in excess water. This may be attributed to differences in amylose content between the medium and long-grain cultivars (Bergman et al., 2004). Water uptake remained unaffected by WRR at all milling durations except in Francis at a DOM of 20 s where the water uptake decreased (from 240% to 231%) as WRR increased. There was, however, a significant interaction between cultivar and WRR that affected water uptake (p = 0.0407) as shown in Table 2. This is likely due to differences in surface area, gelatinization temperature, amylose content and protein content of the two cultivars because these properties can influence the extent of water uptake (Bett-Garber et al., 2007). The interaction of WRR and cultivar on water uptake indicates that changes in the amount of cooking water may affect the cooking of each cultivar differently and the optimum industrial WRR for water uptake may also differ for each cultivar.

Table 2.

The probabilities following multivariate analysis of variance (MANOVA) to determine the influence of rice variety, degree of milling (DOM), and water-to-rice ratio (WRR) on the water and rice properties after cooking

	Volumetric expansion	Water uptake	Solids leached	Amylose leached
Variety	<0.0001^a	<0.0001^a	0.2616	<0.0001^a
DOM	<0.0001^a	<0.0001^a	0.0208^a	<0.0001^a
WRR	0.1044	0.9650	<0.0001^a	0.9864
DOM*Variety	0.4823	0.9901	0.0066^a	0.0743
WRR*Variety	0.2501	0.0407^a	0.5702	0.5628
DOM*WRR	0.3783	0.7760	0.2056	0.9664
VarietyDOMWRR	0.2838	0.2912	0.1582	0.6493

Statistically significant (p < 0.05).

Table 3.

The effects of milling duration and water-to-rice ratio (WRR) on water uptake and volumetric expansion of rice after cooking

	WRR	Milling Duration (s)
		10	20	30
Cultivar		10	20	30	Mean
		Water Uptake (%)
Francis	10:1	227 aB	240 aA	241 aA	236 a
	15:1	216 aB	234 abA	239 aA	231 a
	20:1	217 aB	231 bB	246 aA	230 a
	Mean	220 B	235 A	242 A
Jupiter	10:1	184 aB	203 aAB	208 aA	199 a
	15:1	192 aB	202 aAB	217 aA	204 a
	20:1	192 aA	208 aA	211 aA	204 a
	Mean	190 B	205 A	212 A
		Volumetric Expansion (%)
Francis	10:1	348 aA	357 aA	367 aA	357 a
	15:1	338 aA	352 aA	357 aA	349 a
	20:1	338 aC	355 aB	371 aA	355 a
	Mean	341 B	355 A	365 A
Jupiter	10:1	295 aA	305 aA	319 aA	306 a
	15:1	286 aB	300 aAB	329 aA	305 a
	20:1	293 aA	338 aA	329 aA	320 a
	Mean	291 B	314 A	325 A

Values within a cultivar followed by the same lowercase letter in the same column are not significantly different (p > 0.05). Values followed by the same uppercase letter in the same row are not significantly different (p > 0.05).

When averaged across WRRs, water uptake of both cultivars significantly increased as milling duration increased (Table 3). As milling duration increased from 10 to 20 s, the water uptake increased from 220% to 242% and 190% to 212% for Francis and Jupiter, respectively. Saleh and Meullenet (2007) also showed that increased milling duration resulted in increased cooked rice water uptake. More extensive milling may also increase water diffusion during cooking due to removal of the outer bran layers which reduces the thickness of the grain (Mohapatra and Bal, 2006), the amount of bran remaining on the kernel and, consequently, the surface lipid content (Morawicki et al., 2009; Saleh and Meullenet, 2007).

Volumetric expansion

Volumetric expansion significantly increased with milling duration for both cultivars (Tables 2 and 3). The volumetric expansion (at all WRRs) of the long-grain cultivar (Francis) increased from 341% to 365% as milling duration increased (from 10 to 30 s) while the volumetric expansion of the medium grain cultivar (Jupiter) increased from 291% to 325%. In contrast, the WRR did not have a significant effect on the percent volumetric expansion (Tables 2 and 3). The volumetric expansion was only dependent on the amount of bran that remained on the kernel, not on the WRR; therefore, optimizing the WRR to reduce wastewater contamination would not adversely affect the volumetric expansion of the rice kernel.

Volumetric expansion was significantly affected by cultivar (p < 0.0001) as indicated in Table 2 with greater expansion in Francis than Jupiter (Table 3). Sowbhagya et al. (1994) demonstrated that volumetric expansion is linearly correlated with amylose content; therefore, it was expected that the volumetric expansion of the long-grain cultivar would be greater than the medium grain due to the greater amylose content (Webb, 1985) and amylose affects the ability of the rice kernel to swell while cooking (Ramesh, 2001).

Total leached solids

As milling duration increased, the total leached solids of Jupiter increased (from 9.4% to 11.2% of uncooked rice) but there was no change in Francis (Table 4); however, there was an interaction between milling duration and cultivar that affected the total leached solids (p = 0.0066, Table 2). Other studies have also observed that leaching varied between cultivars (Cameron and Wang, 2005; Cho et al., 2009) and that the effect of milling duration on total leached solids varied with cultivar (Cho et al., 2009).

Table 4.

The effects of milling duration and water-to-rice ratio (WRR) on the solids content of the water after rice cooking

	Leached Solids (% dry basis)
		Milling Duration (s)
Cultivar	WRR	10	20	30	Mean
Francis	10:1	9.6 bA	10.2 aA	9.2 aA	9.7 b
	15:1	10.9 aA	10.4 aA	10.2 aA	10.5 b
	20:1	11.0 aA	11.7 aA	11.6 aA	11.4 a
	Mean	10.5 A	10.7 A	10.4 A
Jupiter	10:1	8.9 aA	8.9 aA	10.2 aA	9.3 a
	15:1	9.5 aA	9.9 aA	12.3 aA	10.5 a
	20:1	9.7 aA	11.8 aA	11.2 aA	10.9 a
	Mean	9.4 B	10.2 AB	11.2 A

Total leached solids was affected by WRR (p < 0.001) as indicated in Table 2; however, when the total solids were evaluated by cultivar, the change in leached solids with increasing WRR was only significant in Francis (Table 4). The total leached solids (for all milling durations) in Francis increased from 9.7% to 11.4% as WRR increased from 10:1 to 20:1. A positive relationship between WRR and amount of leached solids confirms that leached solids increase when rice is cooked in increasingly excess water (Nierle et al., 1981). According to Vidal et al. (2007), the internal temperature of the rice increases as the WRR increases which leads to greater amounts of gelatinization within the kernel and increased gelatinization could lead to an increase in leached solids. Overall, to minimize wastewater contamination from leached solids, the optimum WRR was 10:1 (for Francis) and the optimum milling duration was 10 s (for Jupiter).

Amylose content of leached solids

The amylose content of the leached solids was significantly affected by milling duration (p < 0.0001) and by cultivar (p < 0.0001) as indicated in Table 2. As milling duration increased from 10 to 30 s, the amylose content increased from 13.8% to 29.0% and 1.7% to 9.9% for Francis and Jupiter, respectively (Table 5); thus, to dramatically reduce the percent amylose in the leached solids in the cooking water, the milling duration should be limited to 10 s. The increase in leaching with more extensive milling was most likely due to the greater amount of exposed starchy endosperm on the surface of the granule. In addition, differences in leaching between cultivars was attributable to differences in cell arrangement and protein content of the cultivars, which affects the rate of water diffusion into the kernel (Lee, 1987), starch dissolution and amylose leaching.

Table 5.

The effects of milling duration and water-to-rice ratio (WRR) on the amylose content of the water after cooking

	Leached Amylose (% dry basis)
		Milling Duration (s)
Cultivar	WRR	10	20	30	Mean
Francis	10:1	14.7 aB	23.9 aAB	28.9 aA	22.5 a
	15:1	14.1 aB	23.9 aAB	27.5 aA	21.8 a
	20:1	12.5 aB	19.9 aAB	30.6 aA	21.0 a
	Mean	13.8 C	22.6 B	29.0 A
Jupiter	10:1	0.9 aB	6.5 aAB	8.7 aA	5.4 a
	15:1	1.4 aB	4.9 aB	10.9 aA	5.7 a
	20:1	2.9 aA	8.3 aA	10.0 aA	7.1 a
	Mean	1.7 B	6.5 A	9.9 A

There were no significant differences in leached amylose as WRR increased (p = 0.9864, Table 2). The lack of observable differences in amylose leaching as WRR increased is consistent with a study by Nierle et al. (1981) where they found the amylose content of rice cooked in excess water remained constant. This is also supported by Metcalf and Lund (1985) in which the authors stated that amylose content cannot be used to predict the amount of solids lost.

Protein content in leached solids

The protein content of the leached solids ranged from 0.0 to 8.6 mg BSA/g leached solids for Francis and 0.0 to 9.3 mg BSA/g leached solids for Jupiter; however, none of the differences were significant with an increase in WRR (p = 0.4066) or milling duration (p = 0.0994). There was also no significant difference in the protein content of the leached solids of the two cultivars (p = 0.6217). It is possible that the large variation and the lack of observable differences could be the result of the samples having such low concentrations (below the limits of detection of the assay). Overall, there were detectable amounts of protein in the excess water indicating that there are losses in protein when rice is cooked in excess water. This was also observed by Nierle et al. (1981) where they showed, in several of their rice samples, a decrease in protein content when rice was cooked in excess water.

Conclusions

Leached solids are an adverse effect of cooking rice in excess water. The solids leached consist of valuable nutrients such as proteins, starches, and lipids that contaminate industrial waste. Reducing solids leached in cooking water will result in effluent with lower BOD and aid in reducing the environmental impact of many food companies. As the water-to-rice ratio increased, the total solids increased in the Francis cultivar, indicating that lower water-to-rice ratios may reduce the BOD of the wastewater. Furthermore, shorter milling durations may also limit the amount of solids leached. Less milling leaves traces of bran on the kernel that would add nutritional value to the product and save in industrial costs by reducing the energy required to mill the kernel. Since consumers prefer a white, polished rice kernel, however, shorter milling duration may not be long enough to remove the brown tinge that the bran layer leaves on the rice kernel.

Greater milling time increased the water uptake and volumetric expansion indicating a greater ability to absorb water with increased milling time. Thus, increasing milling time may reduce industrial cooking times; however, the energy saved in cooking may be lost in extended milling times.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Conflict of Interest Statement

The authors declare that there is no conflict of interest.

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