Effects of Ageing on Selected Cooking Quality Parameters of Rice

Abstract

Ageing can improve cooking quality of rice by influencing major cooking quality parameters i.e., kernel expansion, water absorption, alkali digestion value, and gelatinization temperature along with changes in internal structure of rice grains. In this research, the effects of natural and artificial ageing on the selected cooking quality parameters of two Malaysian rice cultivars, named Mahsuri and Puteri, were studied. A relation was observed between water absorption and elongation ratio in both varieties under different aging conditions. Alkali digestion value and gelatinization temperature were also influenced by varieties and ageing conditions. This study revealed the potentiality of ageing for the improvement of rice cooking quality.

Keywords:

• Ageing
• Cooking quality
• Rice

INTRODUCTION

Ageing is a simple and useful method for the improvement of cooking quality of rice grains which can be practiced naturally or artificially. The efficacy and success of ageing depends on the rice varieties, storage environment, and treatments.^[1] Storage of rice for three to four months after harvesting is a simple natural ageing method which has great impact on the cooking quality of grains.^[1,2] Another example of natural ageing is being practiced in southern part of India where freshly harvested paddy is preserved in straw heaps for several days.^[2] In an artificial ageing process, Normand et al.^[3] treated fresh paddy with heat in closed trays and finalized that 90–110°C temperature for two to eight hours can give rice which is the same as rice kept for 14 months, but the chemical and physical properties were affected by heat treatment and changes were different from the natural ageing. Several studies confirmed that ageing can influence the cooking quality characters of rice and play a vital role in rice trading.^[1,4] Aged rice is popular in Asian countries for its taste and flavor. It shows higher kernel elongation, water absorption, volume expansion and less dissolved solid contents which make the cooked grains flaky.^[5] In addition, ageing of rice can influence the cooking and eating quality.^[6] Moreover, higher head yield can be obtained from aged rice.^[5,7] Thus, aged rice ensured good eating and cooking quality which is popular to the rice consumer. For example, in tropical Asia aged rice is more preferable than fresh rice except to those who consume japonica rice.^[8]

Since artificial ageing is the fast method compared to natural ageing, in this study it was tried to optimize the proper ageing time and temperature to improve some important parameters related to cooking quality of rice, i.e., length-breadth measurement, proportionate changes (PC), actual elongation, water uptake, gelatinization temperature, and anatomical structure of kernel for two important Malaysian rice varieties (Mahsuri and Puteri). The gathered information could be used by the rice breeders for using these varieties as parental materials to screen early generation plant materials and to improve cooking qualities for better consumers’ satisfaction aimed to potential marketing.

MATERIALS AND METHODS

Plant Materials

Polished white rice kernels of two Malaysian cultivars (Puteri and Mahsuri) were used for investigation.

Ageing Time and Temperature

Freshly harvested sample those were used within 30 days considered as non-aged kernels and sample from air tight containers at room temperature (25°C) for one to five months considered as natural aged kernels.^[1] For artificial ageing, three different temperatures (90, 100, and 110°C) and five different time periods (1, 3, 5, 7, and 9 h) were used as mentioned by Golam et al.^[9] which was a modified method of Norman^[10] and Tolson.^[11] Each sample was kept in 50 mL air tight test tube (Greiner, Germany) and treated under predefined time and temperature in an oven (Memmert, Germany) for artificial ageing. The aged materials were preserved in room temperature for one hour to be cooled and used for different measurements.

Kernel Expansion Measurement

Ten rice kernels were selected randomly from non-aged, natural-aged, and artificial-aged materials. The initial length and breadth of kernels were measured by using a digital slide calipers (stainless hardened, China). The measured kernels were soaked in 20 mL test tube with 5 mL of tap water for 20 min. After soaking, the test tubes were submerged in boiled water for approximately 30 min. Then the test tubes were removed from the water and preserved in a glass sheet for 50 min in order to evaporate the extra moisture and measured the final length and breadth using same digital slide calipers. PC were measured as stated by Sood and Siddiq:^[12]

where, L_F and B_F denote length and breadth of kernel after cooking, respectively; L₀ and B₀ indicate length and breadth of kernel before cooking. Actual elongation was measured by subtracting the average length of ten pre-cooked kernels from that of ten cooked kernels. Elongation ratio was measured according to the described method of Faruq et al.^[13] and Juliano:^[14]

Water Uptake Measurement

Twenty rice kernels were collected randomly from non-aged, natural-aged, and artificial-aged materials. The samples were cooked in a small beaker with 20 mL of water in a water bath. The cooked rice kernels were collected and placed on filter paper to soak up surface water. The water uptake (%) was then determined using the following formula:^[15]

where, W_C and W_UC is the weight of 20 cooked and uncooked kernels, respectively.

Alkali Spreading Value and Gelatinization Temperature Determination

Six rice kernels from non-aged, natural-aged, and artificial-aged materials were taken in petri dish and 10 mL of 1.7% KOH (potassium hydroxide) was added to it and then incubated at 30°C for 23 h. After incubation, the alkali spreading value was classified as low, intermediate, and high according to Perez and Juliano.^[16] After that gelatinization temperature was estimated from alkali spreading value as per IRRI^[17] guidelines.

Internal Structure Study (Paraffin Method)

Twenty dust-free rice kernels were taken from non-aged, natural-aged, and artificial-aged materials in 20 mL vial and soaked in spirit with ethyl alcohol solution (3:1) for 96 h followed by soaking consecutively in 50, 75, 85, 95, and 100% solutions of TBA for 2 h. Then the samples were soaked in fresh TBA solution for 24 h (every eight hours the TBA solution was replaced by fresh TBA solution). After that, the samples were soaked in absolute TBA and liquid paraffin solution (1:1) for 48 h and were transferred into new clean vials. Malted wax (at 60°C) was poured up to half of each vial and kept for 12 h. After that pure paraffin (at 60°C) was poured into the same vials and kept for another 12 h. The samples were merged into new pure paraffin at 60°C for 2 h. Then metal mould was used to prepare paraffin block. Six kernels were placed inside the metal mould and it was filled up to 50% with melted pure paraffin. The paraffin was steered by a hot steel spoon until the rice grains were placed inside the block and kept for 2 h. Then the blocks were ready for preparing microtom. A cutter was used to cut the block with grain into different thickness (5–15 micron) and the microtoms were studied under a compound microscope (Omano Microscopes, China) as described by Golam^[18] from a modified method of Ogawa et al.^[19]

RESULTS AND DISCUSSION

Ageing is a natural process but it can be induced artificially. Ageing can influence kernel expansion, water absorption, alkali digestion value, and gelatinization temperature which ultimately affects internal structure and cooking quality of rice grains.

Kernel Expansion by Natural Ageing

Kernel expansion of Mahsuri and Puteri rice varieties was studied under natural ageing conditions, where the kernels were kept for different time periods (1, 2, 3, 4, and 5 months). After first month of ageing, PC were 0.15 for Mahsuri and 0.42 for Puteri, followed by an increase to 0.17 and 0.70 at the 5th month. The highest actual elongation was found 4.2 mm at the 4th and 5th month in Mahsuri, while for Puteri it was 7.0 mm at the 5th month. Conversely, the lowest actual elongation for both Mahsuri and Puteri was found in the 1st month and the values were 3.5 mm and 4.3 mm, respectively. The elongation ratios were found 1.68 in Mahsuri and 1.65 in Puteri at the end of 1st month and were increased to 1.73 for Mahsuri and 2.08 for Puteri in the 5th month. Figure 1 represents higher actual elongation compared to PC and elongation ratio of Mahsuri and Puteri in different ageing conditions.

FIGURE 1 Kernel expansions of Mahsuri and Puteri in different ageing conditions.

Kernel Expansion by Artificial Ageing

In artificial ageing condition, proportionate change, actual elongation, and elongation ratio of Mahsuri and Puteri varieties were depended upon the temperatures (90, 100, and 110°C) and times (1, 3, 5, 7, and 9 h). In case of Mahsuri, the lowest proportionate change (0.14) was observed at 100°C with an hour treatment and highest proportionate change (0.23) was at 90°C with 5 h treatments. However, at three different temperatures and five different times, the ranges of proportionate change for Mahsuri were 0.14 to 0.23 which was categorized as poor to low degree of elongation according to Sood and Siddiq.^[12] In Putri, the lowest proportionate change (0.40) was observed at 90°C with an hour treatment and the highest proportionate change (0.90) was at 110°C with seven hour treatments. But the ranges of proportionate change at 90 and 100°C with different curing times were 0.40 to 0.61 indicated medium degree of elongation. In contrast, at 110°C temperature the degree of elongation was high (range of proportionate change 0.62–0.90). Sood and Siddiq^[12] found high degree of elongation (between this range) in five basmati varieties and Choudhury^[20] observed this range in 11 basmati type varieties.

Actual elongation also showed variations with different temperature (90, 100, and 110°C) and times (1, 3, 5, 7, and 9 h) treatments. The highest actual elongation (4.6 mm) was observed at 100°C with seven hour treatments while the lowest actual elongation (3.0 mm) at 90°C for an hour treatment in Mahsuri. In case of Puteri, the highest and lowest actual elongations were found 8.1 mm at 110°C with seven hour treatments and 3.8 mm at 90°C for an hour treatment, respectively. Thus, for both cultivars the lowest actual elongation was found at 90°C but the highest actual elongation for Mahsuri at 100°C and for Puteri at 110°C indicated positive influence of temperatures for actual elongation. Majorio^[21] also reported rice grain elongation throughout the ageing period.

In Mahsuri, elongation ratio ranged from 1.64–1.84 at different temperatures (90, 100, and 110°C) and times (1, 3, 5, 7, and 9 h) where the maximum elongation ratio (1.84) was obtained at 100°C for five hour treatment. Equally, the elongation ratio in Puteri ranged from 1.68–2.20 where the lowest value (1.68) was obtained from the treatment at 90°C for an hour and the highest value (2.20) from three hour treatment at 110°C. However, 110°C with three hour treatment was optimized for fine rice cultivar Puteri. Azeez and Shafi^[22] proposed that ageing of basmati type rice at 100°C for three hours would be excellent for good cooking quality as after 100°C tenderness started to decrease. Previously, Iwasaki and Tani^[23] mentioned that heating has similar effects on rice texture as ageing which increase kernel expansion by denaturation of protein.

Water Absorption

The water absorption index is a measure of the quantity of water absorbed during cooking. The higher water absorption index indicates more expansion and larger size of cooked rice. It was assumed that high water absorption through boiling is an indication of good cooking quality of rice.^[4] In this investigation, Mahsuri showed highest water absorption value (38.20%) at 90°C with five hours of ageing followed by natural ageing (26.17%) and non-aged (17.05%) rice kernels. Similarly in Puteri, highest water absorption value (45.63%) was observed at 100°C with three hours ageing followed by natural ageing (35.90%) and non-aged (25.67%) rice kernel (Fig. 2).

FIGURE 2 Water adbsorption (%) in non-aged, natural ageing, and artificial ageing kernel of Mahsuri and Puteri rice varieties.

Ageing can increase water absorption capacity of rice because water absorption showed positive relation with elongation ratio at different temperatures for both cultivars. In Mahsuri, the highest regression co-efficient (R² = 0.987**) was found at 90°C followed by 110°C (R² = 0.754) and 100°C (R² = 0.739) while in Puteri, the highest regression co-efficient was at 90°C (R² = 0.956**) followed by 110°C (R² = 0.832*) and 100°C (R² = 0.826*).

In Fig. 3 the regression lines representing the relation between water absorption and elongation ratio of Mahsuri and Puteri rice varieties where Fig. 3a, 3b, and 3c representing the ageing temperature (90, 100, and 110°C) of Mahsuri and Fig. 3d, 3e, and 3f are corresponding to 90, 100, and 110°C for Puteri.

FIGURE 3 The regression lines between water absorption and elongation ratio of Mahsuri and Puteri rice varieties. Where (a) regression line between water absorption and elongation ratio of Mahsuri at 90°C; (b) regression line between water absorption and elongation ratio of Mahsuri at 100°C; (c) regression line between water absorption and elongation ratio of Mahsuri at 110°C; (d) regression line between water absorption and elongation ratio of Puteri at 90°C; (e) regression line between water absorption and elongation ratio of Puteri at 100°C; (f) regression line between water absorption and elongation ratio of Puteri at 110°C.

In several studies^[24−27] it was observed that water absorption has positive correlation with kernel elongation of rice. However, Chauhan et al.^[27] reported that the relationships of cooking quality parameters viz., amylose content, milling recovery, water uptake, and kernel elongation of rice were different at different environments.

Alkali Digestion Value and Gelatinization Temperature

Ageing can influence gelatinization temperature as well as alkali digestion value which ultimately affects the cooking and eating quality^[28] of rice. Gelatinization temperature can be indexed by the alkali digestibility test and measured by alkali digestion value.^[29] In Mahsuri, low alkali digestion value (1.5) with high gelatinization temperature (75–82°C) and in Puteri, intermediate alkali digestion value (4.5) with intermediate gelatinization temperature (70–74°C) was observed in normal condition which is also supported by the findings of Juliano and Villareal.^[30] Champagne et al.^[31] mentioned that environmental factors such as temperature during grain development and grain ripening periods influence gelatinization temperature. Rosniyana et al.^[32] stated that rice with high gelatinization temperature becomes excessively soft and tends to disintegrate when overcooked and require more water and time for cooking compared to low or intermediate gelatinization temperature. In the present investigation, Mahsuri showed low alkali spreading value (1.25) in non-aged condition, low alkali spreading value (2.60) in natural ageing condition and intermediate alkali spreading value (4.30) in artificial ageing condition. For Puteri, alkali spreading value was intermediate (4.5) in non-aged condition, intermediated (4.80) in natural ageing condition and high (6.20) in artificial ageing condition. Figure 4 is representing the alkali spreading values (low, intermediate, and high) of Mahsuri and Puteri rice varieties in non-aged, natural-aged, and artificial-aged conditions while low alkali spreading value (1.25) of Mahsuri in non-aged condition (Fig. 4a), Intermediate alkali spreading value (4.5) of Puteri in non-aged condition (Fig. 4b), Low alkali spreading value (2.60) of Mahsuri in natural-aged condition (Fig. 4c), intermediate alkali spreading value (4.8) of Puteri in natural aged condition (Fig. 4d), intermediate alkali spreading value (4.30) of Mahsuri in artificial-aged condition (Fig. 4e) and high alkali spreading value (6.2) of Puteri in artificial-aged condition (Fig. 4b) were observed.

FIGURE 4 The alkali spreading values (low, intermediate, and high) of Mahsuri and Puteri rice varieties in different ageing conditions (non-aged, natural aged, and artificial aged) where (a) low alkali spreading value (1.25) of Mahsuri in non-aged condition; (b) intermediate alkali spreading value (4.5) of Puteri in non-aged condition; (c) low alkali spreading value (2.60) of Mahsuri in natural aged condition; (d) intermediate alkali spreading value (4.8) of Puteri in natural aged condition; (e) intermediate alkali spreading value (4.30) of Mahsuri in artificial aged condition; (f) high alkali spreading value (6.2) of Puteri in artificial aged condition.

The frequency distributions of alkali digestion value of both varieties in different ageing conditions are presented in Table 1. This observation indicated that when the natural ageing was practiced the alkali spreading values were almost similar to normal condition (1.25–2.60 for Mahsuri and 4.5–4.80 for Puteri) but in artificial ageing condition it was different from normal condition (1.25–4.30 in Mahsuri and 4.5–6.20 in Puteri). So, it is clear that the ageing condition can influence the alkali spreading value as well as gelatinization temperature of rice grains. Previously, Hamaker and Griffin^[33] mentioned that reducing agents had changed the gelatinization character of starch by altering the protein structure and Zhou et al.^[34] stated that storage condition were increased peak viscosity of rice. Wongdechsarekul and Kongkiattikajorn^[35] observed that these changes were dependent upon storage temperature.

Internal Structure of Rice Grains

Internal anatomical structure of rice kernel, cell shape, and their arrangement might influence the water uptake and the nature of swelling of kernel during cooking.^[25] So, internal structure of rice grains can represent the effects of ageing for cooking quality evaluation. In the present investigation, the internal cracks (vacuum like structures) in aged rice kernels were higher than non-aged kernels and the severity of cracks depends on the temperature, variety, and ageing time. The number and shape of internal cracks were also influenced by temperature, variety, and cooking time. Low internal cracks (<10%) at 90°C with 1 h ageing and high internal cracks (>30%) at 110°C with 3 h ageing (Table 2) were observed in both Mahsuri and Puteri but at 90°C temperature with 3 h ageing Mahsuri showed moderate (11–30%) and Puteri showed low (<10%) internal cracks. However, the number of broken rice was more at 110°C compared to 90°C for both varieties. Figure 5 shows transverse sections of non-aged and artificial aged (at 110°C with 3 h) Mahsuri and Puteri kernel representing the shape and size of internal cracks. Figure 5a shows transverse section of non-aged Mahsuri kernel represented fewer cracks than Fig. 5c, transverse section of aged Mahsuri kernel at 110°C with 3 h ageing. Similarly, Fig. 5b transverse section of non-aged Puteri kernel demonstrates smaller internal cracks than Fig. 5d, transverse section of aged Puteri kernel at 110°C with 3 h ageing. However, Li et al.^[36] mentioned that stress (drying) was induced internal cracks and the cracks were propagated along the seed capsule, endosperm tissue, and cell wall. Several researchers^[5,37−40] concluded that ageing can change the internal physical structure of rice kernel which ensures good cooking quality.

TABLE 1 Frequency distribution of alkali digestion value of Mahsuri and Puteri in different ageing conditions

		Alkali digestion value class
Parent	Ageing condition	0.5	1.5	2.5	3.5	4.5	5.5	6.5	Mean ± S.E
Mahsuri	Non-aged	−	1.25	−	−	−	−	−	1.23 ± 0.03
	Natural ageing	−	−	2.60	−	−	−	−	2.60 ± 0.03
	Artificial ageing	−	−	−	−	4.30	−	−	4.30 ± 0.03
Puteri	Non-aged	−	−	−	−	4.50	−	−	4.49 ± 0.03
	Natural ageing	−	−	−	−	4.80	−	−	4.80 ± 0.03
	Artificial ageing	−	−	−	−	−	−	6.20	6.20 ± 0.03

TABLE 2 Severity of internal cracks of Mahsuri and Puteri rice kernels at different temperature and ageing time

		Severity of internal crack
Temperature	Ageing time	Mahsuri	Puteri
90°C	1 hour	Low	Low
	3 hour	Moderate	Low
110°C	1 hour	Moderate	Moderate
	3 hour	High	High

Low = less than 10% surface area; moderate = 11-30% surface area; high = >30% surface area.

FIGURE 5 Transverse sections of non-aged and artificial aged (at 110°C with 3 h) Mahsuri and Puteri kernel. (a) Transverse section of non-aged Mahsuri kernel; (b) Transverse section of non-aged Puteri kernel; (c) Transverse section of aged Mahsuri kernel at 110°C with 3 h ageing and (d) Transverse section of aged Puteri kernel at 110°C with 3 h ageing.

CONCLUSIONS

Ageing process showed positive influence on major cooking quality parameters such as kernel expansion, water absorption, alkali spreading value, and gelatinization temperature of Mahsuri and Puteri rice varieties. The obtained results might be used in other rice varieties. Besides this, ageing can improve the quality of rice and their marketability will be widened. These studies also determined the ageing time and temperature of these two cultivars which can be used for domestic or commercial production as well as the rice breeders and geneticists can use these optimized results as screening parameters.

FUNDING

The authors are thankful to the School of Natural Resource and Environmental Sciences of National University of Malaysia, Malaysian Agricultural Research & Development Institute (MARDI), and the Institute of Biological Sciences, University of Malaya for providing support (Grant No. PV044-2011A) in these researches and publications.