Influence of Acid Treatment on Physicochemical Properties of Aged Rice Flour

Abstract

This study investigated the influence of citric acid and ascorbic acid solutions (0.0–0.9% w/w) on the physicochemical properties of 12 month aged rice flour (aged) using 0.7 month rice flour (fresh) as the reference. The results showed that rice storage did not affect the chemical composition. However, 12 month aged rice flour showed an increase in formation of disulfide bonds and amylose-lipid complexes which could restrict starch granules from swelling during gelatinization. Moreover, 12 month aged rice flour showed an increase in gelatinization temperature and enthalpy. Acid treatment did not break the formed disulfide bond but reduced the protein content in the acid-treated 12 month aged rice flour producing a significant increase in granular swelling and thus increased peak viscosity (p ≤ 0.05) but caused a reduction in retrogradation. The excess concentration (0.9% w/w acid solution) induced granule rupture. Aging of rice also affected the textural properties of freeze-thaw gels, increasing the hardness value but this was reduced by the addition of acids.

Keywords:

• Aged rice flour
• Citric acid
• Ascorbic acid
• Frozen rice flour gel
• Gel electrophoresis

INTRODUCTION

Storage of rice, a staple food in many countries,^[1] is a normal step between harvest and consumption. During rice storage, a number of physicochemical properties of the rice are subject to change^[2–6] causing increases in hardness and decreases in stickiness when the aged rice is cooked perhaps because the disulfide bonds and amylose-lipid complexes in the aged rice restrict starch granules from swelling during gelatinization.^[7,8] Chrastil^[2] reported that the number of disulfide bonds and the average molecular weight of oryzenin, which is a major protein in rice, increased during aging of the rice grains.^[2] Such changes in protein properties contribute to the effect of aging on the pasting properties of rice.^[3] Several researchers also reported that the peak viscosity and breakdown of fresh rice flour were higher than in aged rice flour.^[3–5,9] The gelatinization temperature and gelatinization enthalpy increased with increasing rice aging times.^[4]

The texture of cooked aged rice was found to be harder and less sticky than when the same type of rice was freshly harvested and cooked. Some people dislike rice that has a hard and dry texture. Therefore, some researchers have mitigated changes in the texture of cooked rice by the addition of acid or a reducing agent to the cooking water such as Ohishi and coworkers^[10] reported that the addition of acetic acid at 0.05–0.2 M in rice flour can remove proteins and lipids on the surface of the starch granule leading to more swollen and fragile granules than in untreated starch. These changes produce the soft texture of cooked rice. Moreover, Ohno and coworkers^[11] reported that the addition of reducing agents such as sodium sulfate, cysteine, and dithiothreitol into cooking water could decrease the hardness of cooked aged rice because the reducing agents cleaved disulfide bonds in the aged rice causing the starch granules to be easily gelatinized.

The current study aimed to improve the textural properties of rice-based food made from aged rice flour. Therefore, the influence of citric acid and ascorbic acid solutions (0.0–0.9% w/w) on the physicochemical properties of 12 months rice flour (aged) were investigated. Newly harvested rice grains, which were then kept for 0.7 months before milling, were used as fresh rice. This study also applied gel electrophoresis to compare protein pattern of 12 month aged rice flour before and after treated with citric acid and ascorbic acid solutions. There have been no previous reports using this technique to look at the protein pattern of 12 month aged rice treated with acids. Furthermore, the acids were applied to improve the textural properties of frozen 12 month aged rice flour gel which was used as the model for a rice-based food product. We expect that the results obtained in the present study will enhance understanding of the influence of acid on aged rice and be useful for improving the quality of aged rice product or frozen products made with aged rice.

MATERIALS AND METHODS

Materials

Dry milled rice cultivar samples of “Khao Dawk Mali 105” (KDML 105) with two aging periods of 0.7 (fresh) and 12 (aged) months were examined in this study. Both fresh and aged milled rice samples were obtained from a rice mill in Kalasin province, Thailand. Samples were packed in nylon pouches under vacuum and stored at 10°C. The samples were then ground and sieved for analysis. Citric acid and ascorbic acid used were food grade.

Acid-Treated Rice Flour Preparation

The 12 month aged rice flour samples treated with citric acid and ascorbic acid solutions were prepared by mixing the flour (100 g) in 400 mL citric acid solution or ascorbic acid solution (0.1, 0.2, 0.3, and 0.9% w/w) with continuous stirring for 1 h at an ambient temperature and then centrifuging at 11,000 × g, 25°C for 25 min. The precipitated flour was washed three times with distilled water and dried at 40°C for 24 h, then ground and stored at 4°C for analysis. The 0.7 and 12 months rice flour samples that were prepared using an equal amount of distilled water and stored at 4°C were considered as the controls. This study selected 0.9% (w/w) acid concentration to show the effect of high acid concentration on 12 month aged rice flour.

pH Value, Total Acidity, and Chemical Composition

pH value and total titratable acidity as citric acid and ascorbic acid were determined using AOAC methods.^[12] All measurements were done in duplicate. The moisture, protein, fat, and ash contents were determined using AOAC methods.^[12] All measurements were performed in triplicate.

Water Absorption Index (WAI) and Water Solubility Index (WSI) of Rice Flour

The WAI and WSI were determined using the modified method of Anderson and coworkers.^[13] Rice flour samples (2 g, dry basis) were dispersed in 25 mL distilled water at 30°C and stirred every 5 min for 30 min. Then, they were separated using a centrifuge at 3000 × g for 20 min. The supernatant was placed into an aluminum can and dried at 90°C for 4 h. The WSI and WAI were determined as follows:

(1) $\text{WAI}\left(\text{g/g}\right)\text{=weight of wet sediment/dry weight of flour sample}$(1)

(2) $\text{WSI}\left(\text{g/100g}\right)\text{=}\left(\text{weight}\text{of}\text{dry}\text{solid}\text{in}\text{supernatant/dry}\text{weight}\text{of}\text{flour}\text{sample}\right)\times \text{100}$(2)

All measurements were performed in triplicate.

Swelling Power of Rice Flour

The swelling power was determined using the modified method of Konik-Rose and coworkers.^[14] The rice flour samples (1 g, dry basis) were dispersed in 10 mL distilled water. The samples were held in a 75°C water bath and stirred every 2 min for 30 min. The samples were cooled to room temperature and then centrifuged at 15,000 × g for 20 min. The sediments were weighed and determined as follows:

(3) $\text{Swelling}\text{power}\left(\text{g/g}\right)\text{=weight}\text{of}\text{swollen}\text{sample/dry}\text{weight}\text{of}\text{flour}\text{sample}$(3)

All measurements were performed in triplicate.

Pasting Properties of Rice Flour

The pasting properties of each suspension were determined using a rapid visco analyzer (RVA3D, Newport Scientific Instrument and Engineering, Australia) according to AACC.^[15] Suspensions of rice flour were prepared at 8% (w/w) in distilled water. The paste was held at 50°C for 1 min before heating to 95°C at a constant rate of 10°C/min, and then it was held at 95°C for 2.5 min. After that, it was cooled to 50°C at the same constant rate and held at 50°C for 1.5 min. All measurements were performed in duplicate.

Thermal Properties of Rice Flour

Thermal properties of the rice flour samples were determined using a differential scanning calorimeter (DSC; Pyris-1, Perkin Elmer, Norwalk, CT, USA). The gelatinization properties were determined using a modified method of Katekhong and Charoenrein.^[5] The samples were heated at a rate of 10°C/min from 25 to 160°C and cooled at a rate of 5°C/min to 25°C. For determination of the retrogradation properties, the gelatinized samples were stored at 4°C for 2 weeks and then reheated under the same conditions used in the gelatinization test. All measurements were performed in duplicate. To show the clearly different thermal property changes between the low and high concentrations, 0.2 and 0.9% (w/w) acid-treated rice flours were used in this experiment.

Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis (SDS-PAGE) of Rice Flour

Rice flour samples were determined for their protein pattern using SDS-PAGE^[16] with a stacking gel containing 4% acrylamide and a running gel containing 15% acrylamide concentration. The native rice flour was mixed with a sample buffer (pH 6.9) of 0.5 M Tris–HCl (pH 6.8), glycerol, 10% (w/v) SDS, with or without β-mercaptoethanol (β-ME) and 1% (w/v) bromophenol blue. The 0.2 and 0.9% (w/w) acid-treated rice flour were mixed with the pH adjusted sample buffer (pH 7.0 and 7.3, respectively). The acid treating caused rice flour samples to have low pH values and consequently lowered the pH of sample solution for SDS-PAGE to approach the isoelectic point (pI) of rice protein (glutelin: pH 4.8).^[17] Therefore, pH of sample buffers for acid-treated rice flour was adjusted to be higher than that of the native sample. The sample solutions were boiled for 4 min and centrifuged at 2700 × g for 10 min. A sample containing 10 µg of protein was loaded into each well. The electrophoresis was run at a constant voltage (150 V) for 50 min using a Bio-Rad Mini-Protean II cell (Bio-Rad Laboratories; Hercules, CA, USA). The gels were fixed and stained using Bio-Rad Coomassie blue R-250 stain solution for 30 min, and destained using destain solution for 19 h. The molecular weight of each band was determined using precision plus, protein, all-blue, molecular weight (MW) markers, MW ~10–250 kDa (Amersham Biosciences UK Ltd.; Buckinghamshire, UK), as the MW standards.

Texture of Frozen Rice Flour Gels

The rice flour gels were prepared following the method of Katekhong and Charoenrein.^[5] The samples were frozen in a cryogenic cabinet freezer (Minibatch 1000 L; Bangkok Industrial Gas Co., Bangkok, Thailand) at –20°C. After that, the samples were stored in a chest freezer at –20°C for 22 h and thawed at ambient temperature for 2 h. This freeze–thaw cycle was repeated for five cycles. The freezing experiments were carried out in two separate trials. The hardness of gel samples were measured using a compression test texture analyzer (TA-XT plus, Stable Micro System, UK) with a P/25 cylinder probe as described by Katekhong and Charoenrein.^[5] The samples were compressed by 40% of their initial height at the test speed of 1 mm/s. At least five samples were analyzed for each treatment. The 0.3 and 0.9% (w/w) acid concentration were eliminated for this test because the high concentrations induced the starch granule rupture.

Statistical Analysis

The experiments were ordered in a completely randomized design. The data were analyzed using an analysis of variance and the differences between means were determined using the Duncan’s new multiple range test. All statistical analyses were performed using the software package SPSS 16.0 for Windows (SPSS Inc., Chicago, IL, USA).

RESULTS AND DISCUSSION

To investigate the effect of citric acid and ascorbic acid on 12 month aged rice flour clearly, we used the 0.7 month rice flour as the reference and 0.7 and 12 months rice flour treated with water as the controls. The terms “native 0.7 month” and “native 12 months” rice flour are used to refer to the rice flour that was ground and sieved from 0.7 and 12 months rice grain, respectively.

pH Value, Total Acidity, and Chemical Composition of Rice Flour

The reduction in pH value and increase in total acidity of acid-treated rice flour (Table 1) were due to soaking rice flour with acid solutions. These changes become more pronounced with increasing concentration of acid. Lower pH values of acid-treated rice flour indicated that acid residue was presented in the sample. The reason the pH value of citric acid-treated rice flour was lower than that of ascorbic acid-treated rice flour is due to the lower pK_a value of citric acid. The pK_a of citric acid is 3.08, which is lower than the pK_a of ascorbic acid (4.10).^[18] For chemical composition, native 12 months rice flour was not significantly different in the protein, fat and ash contents compared to native 0.7 month rice flour. Similar results have been reported by some other investigators.^[19] However, the protein, fat, and ash contents of the control samples of 0.7 and 12 months rice flour were slightly decreased as compared to the native samples which were probably due to treating the flour samples in water for 1 h. Moreover, the 12 month aged rice flour samples treated with citric acid and ascorbic acid solutions showed clearly significant decreases in the protein, fat, and ash contents (p ≤ 0.05). Changes in the protein content become more pronounced with the increased acid concentration. The decrease in the protein content was due to the removal of protein from the rice flour by acid solutions. The groups in the protein fraction that are removed are glutelin and albumin, which are soluble in acid solution and water, respectively.^[11,20] However, the decrease in the protein content of citric acid-treated rice flour was greater than that of ascorbic acid-treated rice flour due to the lower pH value of citric acid. The decrease in the fat content might have been due to the degradation of triglyceride in the rice by the acid. Acid solutions would break down the ester bonds of triglyceride to make a short chain fatty acid^[21] that was removed into solution. The decrease in the ash content might have been due to losing some compounds such as mineral or chemical compounds that could have dissolved in the water or acid solutions during treatment.

TABLE 1 pH, total acidity, and chemical composition of rice flour without and with citric acid (C) and ascorbic(A) acid solutions

Sample	Treatment	pH	Total acidity (g/100 g)	Moisture**	Protein*	Fat*	Ash*
0.7 month	Native	6.38 ± 0.03^a	0.00 ± 0.00^b	10.32 ± 0.04^f	8.13 ± 0.03^a	0.34 ± 0.01^a	0.29 ± 0.02^a
	Control	6.08 ± 0.01^c	0.00 ± 0.00^b	11.20 ± 0.00^d	8.02 ± 0.00^a	0.31 ± 0.01^a	0.21 ± 0.01^b
12 months	Native	6.22 ± 0.00^b	0.00 ± 0.00^b	11.13 ± 0.11^d	8.04 ± 0.04^a	0.30 ± 0.01^a	0.28 ± 0.01^a
	Control	5.98 ± 0.01^d	0.00 ± 0.00^b	10.90 ± 0.00^e	7.81 ± 0.00^b	0.27 ± 0.00^b	0.20 ± 0.00^b
	C 0.1%	4.06 ± 0.00^f	0.01 ± 0.00^b	11.72 ± 0.11^ab	6.65 ± 0.02^d	0.17 ± 0.01^c	0.17 ± 0.01^bc
	C 0.2%	3.81 ± 0.00^h	0.01 ± 0.00^b	11.41 ± 0.02^c	5.95 ± 0.10^g	0.17 ± 0.00^c	0.17 ± 0.01^bc
	C 0.3%	3.69 ± 0.00^i	0.02 ± 0.00^b	11.84 ± 0.03^a	5.28 ± 0.03^h	0.16 ± 0.05^c	0.17 ± 0.01^bc
	C 0.9%	3.58 ± 0.00^j	0.05 ± 0.01^a	11.84 ± 0.05^a	4.55 ± 0.04^j	0.16 ± 0.03^c	0.18 ± 0.01^bc
	A 0.1%	4.23 ± 0.00^e	0.00 ± 0.00^a	11.44 ± 0.00^c	7.18 ± 0.01^c	0.14 ± 0.02^c	0.15 ± 0.01^bc
	A 0.2%	3.95 ± 0.01^g	0.01 ± 0.00^a	11.44 ± 0.02^c	6.49 ± 0.01^e	0.15 ± 0.00^c	0.14 ± 0.02^c
	A 0.3%	3.93 ± 0.01^g	0.01 ± 0.00^a	11.58 ± 0.01^bc	6.23 ± 0.02^f	0.15 ± 0.02^c	0.15 ± 0.01^bc
	A 0.9%	3.81 ± 0.00^h	0.02 ± 0.01^a	10.76 ± 0.02^e	4.88 ± 0.00^i	0.14 ± 0.01^c	0.16 ± 0.01^bc

Data are shown as mean ± standard deviation.

^a-lMeans with different letters in the same column are significantly different (p ≤ 0.05). The calculation of total acidity as citric acid for 12 month aged rice flour treated with citric acid solution and total acidity as ascorbic acid for 12 month aged rice flour treated with ascorbic acid solution.

^*,**Unit: % of dry basis and wet basis, respectively.

WAI, WSI, and Swelling Power of Rice Flour

Native 0.7 month rice flour had significantly higher WAI (Fig. 1a) and WSI (Fig. 1b) values than native 12 months rice flour (p ≤ 0.05). The low WAI and WSI values in the native 12 months rice flour indicated that the starch granules of the 12 months rice flour were more resistant to absorption and dissolving in water than those of 0.7 month rice flour^[3–5] which correlated with an increase in the disulfide bonding of proteins during the aging of rice grains.^[2,22] The WAI and WSI values of the control and acid-treated samples were higher than those of the native rice flour. As the concentrations of the citric acid and ascorbic acid solutions increase, the values of the WAI and WSI increased. Treatment with citric acid and ascorbic acid solutions can remove some proteins around the starch granules of 12 months rice flour (results on chemical composition) causing amylopectin molecules to absorb more water and the amylose molecules to be easily eluted into solutions and thus, the WAI and WSI increased.^[10]

FIGURE 1 A: WAI; B: WSI; and C: swelling power of 0.7 month rice flour (0.7M) and 12 month aged rice flour (12M) without and with citric acid (12M+C) and ascorbic acid (12M+A) solutions. The data are shown as means ± SD (bars).

After heating rice flour at 75°C for 30 min., the results show that the starch granules of the native 0.7 month rice flour swelled more than in the native 12 months rice flour (Fig. 1c) because the disulfide bonds and amylose-lipid complexes in the 12 months rice flour restricted the swelling of starch granules.^[3–5,8] However, the control samples of 0.7 and 12 months rice flour showed greater swelling power than the native rice flour which was probably due to treating the flour samples in water. Moreover, the starch granules of the 12 months rice flour samples treated with citric acid solution up to concentrations of 0.3% (w/w) swelled more than the untreated samples. The increase in swelling power might have been due to the loss of some protein around the starch granules. However, with 0.9% (w/w) acid solutions, the swelling power was reduced might be due to acid residual which facilitated the granule rupture during heating with frequent stirring. On the other hand, the swelling power increased in the 12 months rice flour treated with ascorbic acid solution at 0.1% (w/w) acid solution and then the value decreased when the concentration exceeded 0.2% (w/w). The obvious changes in swelling power of ascorbic acid-treated rice flour might have been due to a higher depolymerization of the starch by ascorbic acid. At the same concentration, ascorbic acid with a lower molecular weight would have a larger number of acid molecules than citric acid. Therefore, the treatment of aged rice flour with ascorbic acid leads to a structural breakdown and consequently to a loss of granule integrity.^[22] This could have been the reason for the drop in the swelling power with increasing acid concentration.

Pasting Properties of Rice Flour

The pasting properties of all rice flour samples are shown in Table 2. From the RVA analysis, the native 12 months rice flour showed significantly lower peak viscosity (156 to 139 Rapid Visco Units (RVU)) and breakdown (24 to 15 RVU) and a higher final viscosity (235 to 247 RVU) and setback (102 to 123 RVU) compared to the native 0.7 month rice flour (p ≤ 0.05). The lower peak viscosity of the 12 months rice flour indicated that the starch granules of the 12 months rice flour were more resistant to swelling than those of the 0.7 month rice flour which was due to an increase in the formation of protein disulfide bonds and amylose-lipid complexes during the aging of the rice grains. The increase in the setback was attributed to an increase in the retrogradation of the 12 months rice flour.^[2–5] However, the rice flour samples treated with citric acid solutions showed an increase in the peak viscosity (~158–166 RVU) and breakdown (~65–89 RVU). This effect increased with increasing concentrations of citric acid solution up to 0.3% (w/w). At 0.9% (w/w) acid solutions, the peak viscosity and breakdown of the acid-treated rice flour samples were reduced (~115 and 73 RVU, respectively). Similar findings have been reported by Jyothi and coworkers,^[23] who found that the addition of acetic and hydrochloric acid at 0.05 and 0.1% in cassava starch increased the peak viscosity and breakdown. However, at 0.5% and higher concentrations, a fall in viscosity was noticed. Therefore, the change in the pasting properties of the control and acid treatment samples was due to the removal of some protein molecules on the starch surface (see previous results on chemical composition) and the presence of acid residue, which led to more swollen starch granules at the high acid concentration (0.9% [w/w] acid solution) and resulted in the easy rupture of the granules.^[23,24] While the 12 months rice flour samples treated with ascorbic acid solution showed a significant decrease in the peak viscosity, there was an increase in breakdown with increasing concentrations of ascorbic acid solution (p ≤ 0.05). Our results were similar to Ohishi and coworkers,^[8] who found that the addition of 0.05–0.2 M acetic acid in rice flour samples decreased the peak viscosity but increased the breakdown. Ohishi and coworkers^[10] considered that proteins around the starch granules might indirectly disturb the gelatinization of the starch. Sriburi and Hill^[25] also found that the addition of ascorbic acid decreased the peak viscosity of extruded cassava starch with increasing acid concentrations because of the depolymerization of the starch by the ascorbic acid. They considered the reason for the drop in viscosity was due to the depolymerization of the starch and the reduction in proteins induced the collapse of the starch granules during heating. In the current study, the final viscosity and setback of all treated rice flour samples significantly decreased (p ≤ 0.05) which indicates that both citric acid and ascorbic acid solutions can interact with aged rice flour and induce changes in the rice’s behavior so that is similar to that of fresh rice flour.

TABLE 2 Pasting properties of the rice flour without and with citric acid (C) and ascorbic (A) acid solutions

Sample	Treatment	Viscosity (RVU)					Pasting temperature (°C)
		Peak	Tough	Breakdown	Final	Setback
0.7 month	Native	156.88 ± 2.30^c	132.42 ± 0.47^a	24.46 ± 2.77^h	235.09 ± 0.36^b	102.67 ± 0.84^b	86.15 ± 0.00^b
	Control	164.09 ± 1.38^ab	107.34 ± 1.91^c	56.75 ± 1.47^e	182.54 ± 0.65^e	75.21 ± 1.26^e	84.55 ± 1.19^bcd
12 months	Native	139.09 ± 1.53^e	123.82 ± 2.57^b	15.27 ± 1.03^i	247.80 ± 0.12^a	123.98 ± 1.69^a	88.20 ± 1.13^a
	Control	155.21 ± 0.30^d	104.21 ± 1.71^cd	51.00 ± 1.87^f	225.29 ± 1.62^c	121.08 ± 1.54^a	86.20 ± 1.13^b
	C 0.1%	158.79 ± 1.77^c	93.09 ± 1.50^e	65.71 ± 1.73^d	177.54 ± 1.81^f	84.46 ± 1.30^d	84.48 ± 0.74^bcd
	C 0.2%	161.75 ± 1.55^b	83.75 ± 1.45^f	78.00 ± 1.90^b	157.59 ± 1.87^h	73.84 ± 1.42^e	83.83 ± 0.49^cd
	C 0.3%	166.42 ± 1.14^a	76.50 ± 1.36^g	89.92 ± 1.50^a	133.21 ± 1.68^j	65.21 ± 0.30^f	83.20 ± 1.13^cde
	C 0.9%	115.54 ± 1.62^f	42.46 ± 1.66^i	73.08 ± 1.96^c	92.88 ± 1.36^l	50.42 ± 1.70^g	82.95 ± 0.07^e
	A 0.1%	140.34 ± 1.76^e	101.71 ± 1.49^d	38.63 ± 1.27^g	202.21 ± 0.31^d	100.50 ± 1.36^b	85.68 ± 0.46^bc
	A 0.2%	114.50 ± 1.51^f	75.75 ± 1.06^g	38.75 ± 1.45^g	168.79 ± 1.00^g	93.04 ± 1.06^c	85.23 ± 0.32^bc
	A 0.3%	106.54 ± 1.39^g	65.21 ± 1.37^h	41.33 ± 1.02^g	141.92 ± 1.43^i	76.71 ± 1.80^e	84.68 ± 0.95^bcd
	A 0.9%	96.88 ± 1.72^h	40.09 ± 0.12^i	56.79 ± 1.60^e	102.96 ± 1.24^k	62.88 ± 1.36^f	84.20 ± 1.70^bcd

Data are shown as mean ± standard deviation.

^a-lMeans with different letters in the same column are significantly different (p ≤ 0.05).

Thermal Properties of Rice Flour

DSC was used to clarify the effect of acid treatment on the thermal properties of rice flour. Native 0.7 and 12 months rice flour as well as the acid-treated rice flour with 0.2 and 0.9% w/w concentrations were used in this study. The gelatinization properties of 0.7 and 12 months rice flour without and with citric acid and ascorbic acid solutions are shown in Table 3. Native 12 months rice flour had a higher onset, peak and conclusion temperatures of gelatinization and gelatinization enthalpy than native 0.7 month rice flour. These results agreed with Tulyathan and Leeharatanaluk^[4] and Katekhong and Charoenrein.^[5] It was suggested that the increase in gelatinization temperature and enthalpy could be due to restricted starch granules caused by the disulfide bonds and amylose-lipid complexes. Several researchers also reported that the number of disulfide bonds significantly increased after rice aging.^[2,22] Furthermore, it was found that the onset, peak and conclusion temperatures of gelatinization of 12 months rice flour treated with citric acid and ascorbic acid solutions decreased while the ΔH increased compared to native 12 months rice flour, with the exception of the sample treated with 0.9% (w/w) citric acid. These changes might have been due to a decrease in the protein content of the rice flour and depolymerization of starch from the acid treatment and the presence of acid residue.

TABLE 3 Thermal properties of the rice flour without and with citric acid (C) and ascorbic (A) acid solutions

Sample	Treatment	Gelatinization				Amylose-lipid complex				Retrogradation
		To (°C)	Tp (°C)	Tc (°C)	ΔH (J g^–1 dry basis)	To (°C)	Tp (°C)	Tc (°C)	ΔH (J g^–1 dry basis)	To (°C)	Tp (°C)	Tc (°C)	ΔH (J g^–1 dry basis)
0.7 month	Native	62.4 ± 0.2^b	69.6 ± 0.0^b	76.6 ± 0.2^b	9.08 ± 0.27^d	91.4 ± 0.1^a	96.6 ± 0.1^a	103.8 ± 0.8^a	0.74 ± 0.05^b	45.5 ± 1.2^b	56.2 ± 0.5^b	62.1 ± 0.1^b	1.89 ± 0.01^b
12 months	Native	66.2 ± 0.4^a	73.8 ± 1.1^a	81.1 ± 1.1^a	10.47 ± 0.04^c	91.1 ± 0.7^a	97.3 ± 0.4^a	104.4 ± 0.7^a	1.53 ± 0.07^a	49.7 ± 0.9^a	57.9 ± 0.5^a	63.8 ± 0.0^a	3.03 ± 0.14^a
	C 0.2%	52.9 ± 0.0^cd	69.2 ± 0.1^b	76.7 ± 0.3^b	11.74 ± 0.05^b	91.4 ± 0.0^a	96.6 ± 0.4^a	104.2 ± 0.1^a	1.48 ± 0.01^a	45.8 ± 0.6^b	56.5 ± 0.3^b	61.5 ± 0.6^b	2.03 ± 0.14^b
	C 0.9%	52.8 ± 0.3^cd	69.2 ± 0.1^b	76.6 ± 0.5^b	10.66 ± 0.16^c	91.5 ± 0.5^a	96.8 ± 0.1^a	105.1 ± 0.4^a	1.46 ± 0.07^a	45.5 ± 0.4^b	56.2 ± 0.2^b	61.2 ± 0.5^b	1.58 ± 0.16^c
	A 0.2%	52.8 ± 0.1^cd	69.7 ± 0.4^b	77.5 ± 0.7^b	12.26 ± 0.11^a	90.6 ± 0.8^a	96.3 ± 0.1^a	104.4 ± 0.1^a	1.53 ± 0.01^a	46.7 ± 0.7^b	56.5 ± 0.5^b	63.1 ± 0.7^a	1.08 ± 0.17^d
	A 0.9%	52.3 ± 0.4^d	69.4 ± 0.2^b	77.4 ± 0.0^b	11.56 ± 0.14^b	90.4 ± 0.6^a	97.5 ± 0.4^a	104.5 ± 0.7^a	1.51 ± 0.07^a	45.8 ± 0.3^b	56.4 ± 0.6^b	61.8 ± 0.6^b	0.91 ± 0.05^d

Data are shown as mean ± standard deviation.

^a-lMeans with different letters in the same column are significantly different (p ≤ 0.05). T_o, T_p, T_c: Onset, peak, and conclusion temperature, respectively; ΔH: enthalpy.

Temperatures of melting amylose–lipid complex of all rice flour were not significantly different (Table 3). However, the 12 months rice flour showed an increase in enthalpy of the melting amylose–lipid complex as compared to 0.7 month rice flour. During the storage of rice, lipid hydrolysis was initiated by the action of lipases. This resulted in an increase of free fatty acids which could complex with amylose to amylose–lipid complex.^[21,26] Compared to untreated flour, acid-treated rice flour was not significantly different in the enthalpy of melting amylose–lipid complex.

The retrogradation properties of all rice flour gels after storage at 4°C for 2 weeks are shown in Table 3. The results showed that the DSC could detect the melting endotherm of the crystallized amylopectin occurring at a temperature of ~45–63°C. The retrogradation temperature and enthalpy of native 12 months rice flour were higher than for native 0.7 month rice flour. These results agreed with Zhu,^[27] who found that the retrogradation endotherm of low amylose rice was observed at 43.4–66.8°C. The acid-treated rice flour samples had reduced retrogradation temperature and enthalpy with increasing concentrations of the acid solutions. The acid residue could break down some glycosidic bonds of the starch molecules and so the starch molecules decreased in size during gelatinization, which made it difficult to re-associate and retrograde of the starch.^[28]

SDS-PAGE of Rice Flour

We used SDS-PAGE to elucidate the aging of rice grains and to induce protein disulfide bond formation and the removal of some glutelin proteins using acid treatment. Figure 2a shows that the 12 months rice protein consisted of five major protein bands with MW 10–15, 15–20, 50–75, 100–150, and over 250 kDa while the 0.7 month rice protein had four major protein bands with MW 10–15, 15–20, 25–37, and 50 kDa. The protein pattern of the 12 months rice flour showed subunits linked by disulfide bonds as determined by SDS-PAGE using β-ME which is a reducing agent (Fig. 2b).^[11,29] The protein pattern of the 0.7 month rice flour with β-ME was slightly different from the 0.7 month rice flour without β-ME. A similar change in protein patterns of the rice flour samples during storage was reported by Tulyathan and Leeharatanaluk^[4] who found that the number of high molecular weight protein bands increased as the rice was aged for 5 and 8 months. Chrastil^[2] also reported that the number of disulfide bonds and the average molecular weight of oryzenin (a major protein in rice) increased during the aging of rice grains.

FIGURE 2 SDS-PAGE of 0.7 month rice flour (0.7M) and 12 month aged rice flour (12M) without and with citric acid (12M+C) and ascorbic acid (12M+A) solutions. A: without β-ME; B: with β-ME. β: beta-mercaptoethanol.

Treating with citric acid and ascorbic acid solutions decreased the band intensity of some glutelin proteins (Fig. 2a). The change was pronounced with increasing concentration of acid solutions. However, no acid treatment was able to cleave the disulfide bonds to sulfhydryl groups, not even ascorbic acid which is a reducing agent (Fig. 2b). The SDS-PAGE results indicated that aging and treating with citric acid and ascorbic acid solutions affected the molecular weight and the intensity of the bands in the rice proteins.

Texture of Frozen Rice Flour Gels

The texture properties of all rice flour gels before and after the freeze–thaw cycle are shown in Table 4. The results showed that the hardness values of all samples increased after the repeated freeze–thaw cycles. However, the rice flour gels made from the 12 months rice flour had significantly (p ≤ 0.05) higher hardness values than the 0.7 month rice flour in all cycles. Katekhong and Charoenrein^[5] reported that freeze–thawed gels can induce a spongy structure in rice flour gels and the matrix around the pours of the rice flour gels made from the 12 months rice flour was thicker than that of the rice flour gels made from the 0.7 month rice flour. This could explain the harder texture noted in the freeze–thawed flour gels with longer aging durations and they became harder with repeated freeze–thaw cycles. The addition of citric acid and ascorbic acid solutions reduced the hardness values of the rice flour gels made from the aged rice flour. As the concentrations of citric acid and ascorbic acid solutions increased, the hardness values decreased. The direct relationship between a change in the hardness values from aging and the addition of citric acid and ascorbic acid solutions correlated well with the previously discussed changes in the retrogradation properties of rice flour samples.

TABLE 4 Hardness (N) of rice flour gels without and with citric acid (C) and ascorbic (A) acid solutions

Sample	Treatment		Hardness (N)
		Fresh	Cycle 1	Cycle 5
0.7 month	Native	0.72 ± 0.04^bcC	0.93 ± 0.08^bB	1.65 ± 0.03^bA
12 months	Native	1.02 ± 0.06^aC	1.57 ± 0.10^aB	3.15 ± 0.25^aA
	C 0.1%	0.71 ± 0.10^bcB	0.88 ± 0.00^bB	1.56 ± 0.04^bcA
	C 0.2%	0.44 ± 0.05^dB	0.55 ± 0.06^cB	1.09 ± 0.12^cA
	A 0.1%	0.82 ± 0.09^bcB	0.91 ± 0.07^bB	1.65 ± 0.09^bA
	A 0.2%	0.68 ± 0.05^cB	0.81 ± 0.05^bB	1.40 ± 0.37^bcA

Data are shown as mean ± standard deviation.

^a–cMeans with different letters in the same column are significantly different (p ≤ 0.05).

^A–CMeans with different letters in the same row are significantly different (p ≤ 0.05).

CONCLUSIONS

This study showed that rice aging increased the formation of disulfide bonds and amylose-lipid complexes both of which could restrict starch granules from swelling. It was also found that the use of citric acid and ascorbic acid solutions could change the physicochemical properties of the 12 month aged rice flour samples. The aging of rice led to changes in the WAI, WSI, swelling power, pasting properties, thermal properties, and protein patterns. However, the treatment with citric acid and ascorbic acid solutions removed protein, fat and ash molecules from the 12 month aged rice flour samples and induced swelling of the 12 month aged rice starch granules during heating. Therefore, the starch granules of acid-treated rice flour were easily gelatinized. Moreover, the changes in physicochemical properties of acid-treated rice flours might be resulted from the acid residue during measurement because the acid addition during treatment did not fully removed after washing with water. The changes in the 12 month aged rice flour treated with ascorbic acid solution were different from the 12 month aged rice flour treated with citric acid solutions which might have been due to a larger number of acid molecules in ascorbic acid solution thus higher in depolymerization of starch. However, the cause of these differences requires further study. In addition, repeated freeze–thaw cycles increased the hardness of those gels while the hardness was reduced by treating with citric acid and ascorbic acid solutions. From the current study, it is recommended that 0.2% (w/w) acid concentration is appropriate for improving the quality of 12 month aged rice flour. The higher acid concentrations induce the starch granules rupture causing the soft gel with low water holding capacity. These findings may be useful for improving the textural properties of aged rice products.

FUNDING

Financial supports from the Graduate School, Kasetsart University (Research Grant for International Publication) and from the Kasetsart University Research and Development Institute (KURDI) are gratefully acknowledged. We would also like to thank the Bangkok Industrial Gas Co., Ltd. for providing liquid nitrogen and a cryogenic freezer.

Additional information

Funding

Financial supports from the Graduate School, Kasetsart University (Research Grant for International Publication) and from the Kasetsart University Research and Development Institute (KURDI) are gratefully acknowledged. We would also like to thank the Bangkok Industrial Gas Co., Ltd. for providing liquid nitrogen and a cryogenic freezer.