Quality attributes of paddy rice as affected by frozen storage

ABSTRACT

After harvest, paddy rice may encounter frozen conditions during the storage and transportation periods. However, the effect of frozen storage on quality of paddy rice remains unclear. In this research, two moisture contents of paddy rice (13.5% and 15.1%) were stored at two frozen temperatures (−18°C and −30°C) for two months and then evaluated for quality attributes, including germination rate, peroxidase and polyphenol oxidase activities, fat acidity, milling quality, pasting property, cooking property, and texture of the cooked rice. The results showed that the frozen storage generally decreased the germination rate, activities of peroxidase and polyphenol oxidase, husked rice yield, head rice yield of paddy rice, reduced the water uptake ratio and volume expansion rate of milled rice during cooking, and lowered the hardness of cooked rice. At frozen condition, the decrease of temperature and the increase of moisture content led to a more pronounced reduction in germination rate and peroxidase activity. Frozen treatment of paddy rice did not significantly affect the pasting temperature of milled rice flour, but decreased the peak viscosity. Fat acidity of paddy rice slightly increased after frozen storage.

KEYWORDS:

• Rice
• frozen storage
• milling quality
• cooking property

Introduction

Rice (Oryza sativa L.) is a grain crop that provides staple food for nearly half of the global population.^[1] Rice is grown in over 100 countries, with 90% of the global production from Asia, principally China, India, Bangladesh, and Indonesia. According to Food and Agriculture Organization of the United Nations, in the year of 2022, about 517 million tons of rice (milled basis) was produced worldwide. Asia, Sub-Saharan Africa, and South America are the largest consumers of rice.

After harvest, rice should be stored to fulfill the consumption of humans all the year round. With the development of economic globalization and regional specialization, recent decades have witnessed a high-speed growth of global grain trade.^[2] As an important trade commodity, massive amounts of rice will be preserved for a certain period of time in transportation facilities and transit warehouses. In some countries, rice serves as a reserve grain with storage duration more than one year so as to guarantee the food safety.^[3] For example, China’s grain inventory has been kept at a high level and reserved rice can be stored for a longer duration for three years.

During storage of paddy rice, changes of biochemical and physicochemical properties may occur and result in the alteration of milling, cooking, and eating qualities. Kim et al.^[4] reported that storage at 20°C for seven months obviously decreased germination rate of paddy rice. Jungtheerapanich et al.^[5] found that storage at ambient temperature (30°C) for nine months resulted in an increase in head rice yield of paddy rice. Zhang et al.^[6] reported that storage at 20°C for 1.5 years significantly increased the fat acidity of paddy rice and would affect the contents of starch and non-starch lipids. Zhao et al.^[7] reported that storage of rice promoted protein oxidation and increased the surface hydrophobicity of protein. Sun et al.^[8] found that cooked rice hardness of stored paddy rice gradually increased with the prolonged storage duration.

Quality attributes of stored paddy rice are influenced by various factors, among which storage temperature is a crucial factor. Zhao et al.^[9] reported that elevation of storage temperature (temperature range 15–37°C) can promote the hydrolysis of fat, inhibit the catalase activity, and cause rough surface of rice endosperm. Kim et al.^[10] found that higher storage temperatures (temperature range 10–30°C) led to more rapid degradation rates and oxidative reactions of lipids during the storage of rice. Till now, most of the storage research has been carried out under the temperature conditions above 0°C. In many rice producing regions, the atmosphere temperature may fall below freezing point after the harvest of rice. For rice trade, the environment temperature of transportation facilities and transit warehouses may also be below freezing point. Accumulated researches showed that the frozen storage can cause freezing injury to stored grains. Odio et al.^[11] reported that storage of maize seed at −18°C might decrease the percentages of normal seedlings. Calvo-Brenes and O´Hare^[12] found that storage at −20°C, soon after the harvest, decreased the carotenoid concentration of yellow sweet-corn. However, the effect of frozen condition on quality of paddy remained unclear. In this research, two frozen conditions were simulated to investigate the characteristics of paddy rice after frozen storage. This research can provide preference for scientific storage of rice and logistic route design of rice trade.

Materials and methods

Materials

Paddy rice (variety Ezhong-5), harvested in Hubei, China, was obtained from a local farmer (Hubei, China). The assay kits of peroxidase (POD) and polyphenol oxidase (PPO) activities were purchased from Nanjing Jiancheng Bioengineering Institute (Jiangsu, China). Chemicals were of analytical grade unless otherwise stated.

Storage of paddy rice

Paddy rice samples (5 kg each) with moisture contents of 13.5% and 15.1% were placed in plastic bags. Bagged samples were stored at three different temperatures (4°C, −18°C and −30°C) in different refrigerators/freezers (4°C: BCD-452WDPF refrigerator, Haier Smart Home Co., Ltd., Qingdao, China; −18°C: BD/BC-219E freezer, Zhejiang Xingxing Refrigeration Co., Ltd., Taizhou, China; −30°C: DW-40L508 freezer, Qingdao Haier Biomedical Co., Ltd., Qingdao, China). After storage for two months, paddy rice samples were analyzed for quality attributes.

Determination for germination rate of paddy rice

The germination rates of paddy rice samples were determined according to the procedure of Genkawa et al. ^[13] with slight modification. Grains were washed four times with distilled water. They were then placed on wet filter paper in a Petri dish. The Petri dishes were incubated at 25°C. The germinated grains were counted after seven days and used to calculated the germination rate.

Peroxidase (POD) and polyphenol oxidase (PPO) activities

POD and PPO activities of paddy rice were determined using the commercial assay kits (Nanjing Jiancheng Bioengineering Institute, Jiangsu, China).

Husked rice yield (HURY) and head rice yield (HERY) of paddy rice

The HURY of paddy rice was determined according to the Chinese National Standard GB/T 5495–2008. In brief, paddy rice (20–25 g) was weighed (accurate weight M), and the included sprouted kernels was picked out, weighed, and recorded as M1. The paddy rice was dehulled using a rice dehuller (JLGJ4.5, Taizhou Grain Instrument Factory, Taizhou, China). Brown rice and unsound rice were individually weighed and recorded as M2 and M3, respectively. The HURY was calculated according to Formula (1).

(1) $\mathrm{H}\mathrm{U}\mathrm{R}\mathrm{Y}=\frac{\left(M_1+M_2\right)-\left(M_1+M_3\right)/2}{M}\times 100\mathrm{\%}$(1)

The HERY of paddy rice was determined according to the Chinese National Standard GB/T 21,719–2008. The obtained brown rice was milled using a rice polishing machine (JNM-3, Sinograin Chengdu Storage Research Institute, Sichuan, China), with the mill time set at 40 s. The head rice was weighed and recorded as M4. The HERY was calculated according to Formula (2).

(2) $\mathrm{H}\mathrm{E}\mathrm{R}\mathrm{Y}=\frac{M_4}{M}\times 100\mathrm{\%}$(2)

Fat acidity of paddy rice

Fat acidity of paddy rice was determined according to the Chinese National Standard GB/T ISO 7305–1998. Briefly, rice grains were dehulled with a hulling machine (THU35C, Satake Co., Ltd., Riichi, Japan) and then milled with a hammer mill (JXFM, Shanghai Jiading Grain & Oil Instrument Co., Ltd., Shanghai, China) to brown rice flour. The flour was passed through a 40-mesh sieve. Fat of flour was extracted with anhydrous ethanol and then titrated with potassium solution (KOH), with phenolphthalein used as an indicator. The fat acidity of the rice was expressed as mg KOH per 100 g of sample.

Pasting property measurement of milled rice flour

Paddy rice were dehulled using a THU35C hulling machine (Satake Co., Ltd., Riichi, Japan) and milled using JNM-3 milling machine (Sinograin Chengdu Storage Research Institute, Sichuan, China). The milled rice was made to flour using a hammer mill (JXFM, Shanghai Jiading Grain & Oil Instrument Co., Ltd., Shanghai, China). A Rapid Visco Analyzer (Newport Scientific Pty, Ltd., Warriewood, Australia) was used to determine the pasting property of the milled rice flour. Using a programmed heating and cooling cycle, the flour suspensions (12%, w/w) were maintained at 50°C for 1 min, heated to 95°C at a rate of 12°C/min, held at 95°C for 2.5 min, then cooled to 50°C at a rate of 12°C/min, and finally held at 50°C for 2 min.

Cooking quality measurement of milled rice

The cooking quality of milled rice (section 2.7) was measured according to the method of Zhou et al.^[13] with slight modification. Seven grams of milled rice were placed in a metal mesh bowl for washing and cooking. The rice was washed three times with distilled water and then immersed in 120 mL water. Afterwards, the water was heated for boiling. After boiling for 20 min, the cooked rice was withdrawn from the water and cooled. The water uptake ratio (WUR) of rice was expressed as the weight ratio of water absorbed to raw sample in dry basis. The volume expansion ratio (VER) of rice was expressed as the volume ratio of cooked sample to raw sample.

Texture measurement of the cooked rice

Ten grams of milled rice (section 2.7) was rinsed twice with distilled water. Water was added to the rice at a volume ratio of 1.5:1. After soaking for 30 min, rice was cooked in a steamer (Guangdong Midea Electric Appliances Co., Ltd., Guangdong, China) for 40 min and simmered for 20 min. Textural attributes of the cooked rice were determined using a TA.XT2i Texture Analyzer (Stable Micro System, Surrey, UK). A two-cycle compression program was conducted at a compression ratio of 70% and a test speed of 1 mm/min.

Statistical analysis

Data were presented as means ± standard deviation (SD). Differences of means were evaluated by one-way analysis of variance (ANOVA) followed by Duncan’s multiple range test. A significance level was set at 0.05. All statistical analyses were performed using a commercial statistical package (SPSS Inc., Chicago, IL, USA).

Results and discussion

Germination rate

The germination rates of paddy rice stored at different temperature conditions are shown in Figure 1. Unstored paddy rice exhibited a high level of germination rate (94.0–95.5%), indicating the excellent viability of the grain. At all three storage temperature conditions, paddy rice showed a decrease in germination rate after storage. Paddy rice stored at 4°C only had a slight decrease in germination rate, but the decrease ratio of germination rate was much greater at two frozen conditions. These results suggested that frozen storage deteriorated the vigor of paddy rice grains. During the freezing period, the water within and among the embryo’s cells freezes, and the formed crystals may puncture the membranes of cells; when the temperature rises, the ice melts and cell contents leak out, leading to the damage of the embryo cell structure.^[14,15] The vitality of paddy rice under frozen condition was affected by moisture content and storage temperature. For paddy rice stored at −18°C, germination rates of paddy rice at moisture contents of 13.5% and 15.1% were decreased to 64.3% and 52.5%, respectively. However, when the storage temperature was further decreased to −30°C, paddy rice at the two moisture contents dramatically declined to 11.6% and 6.5%, respectively, suggesting that paddy rice lost most of the germination capacity after storage at this temperature. It was reported that the loss of vigor would accelerate the aging process of paddy rice.^[16] For paddy rice used for grain reserve, frozen condition with temperature below −18°C should be avoided to ensure the quality of rice after storage. More frozen conditions will be considered in further research to fully understand the effect of frozen treatment on the vigor of paddy rice.

Figure 1. Germination rate of paddy rice stored at different temperature conditions. At a specific moisture content, bars with the same letter are not significantly different (α = 0.05).

POD and PPO activities

The POD and PPO activities of paddy rice stored at different temperature conditions are shown in Figure 2. At temperature of 4°C, storage did not arouse significant change in POD and PPO activities of paddy rice. However, paddy rice exhibited significant change in activities of these two antioxidant enzymes after storage at frozen conditions. These results suggested that the frozen storage would affect the physiological activity of the paddy rice after the removal of frozen condition. The activity of enzyme is related to the spatial conformation, which can be affected by the freezing and thawing processes.^[17,18] For agriculture products, antioxidant enzymes can eliminate reactive oxygen species, and enhancing the activities of these enzymes can prolong the storage period.^[19,20] The frozen treatment-induced inactivation of POD and PPO would cause unfavorable effect to the storability of paddy rice, which was consistent with the result of germination rate. At frozen condition, the decrease of temperature generally decreased the POD and PPO activities of the stored paddy rice. Paddy rice samples of two moisture contents did not exhibit significant difference in PPO activity. However, greater decrease of POD activity was observed at moisture content 15.1% than at moisture content 13.5%. These results suggested that the increase of moisture content and the decrease of temperature would aggravate the freezing injury to paddy rice.

Figure 2. Peroxidase (POD) and polyphenol oxidase (PPO) activities of paddy rice stored at different temperature conditions. At a specific moisture content, bars with the same letter are not significantly different (α = 0.05).

Fat acidity

Fat acidity is an excellent index for judging quality deterioration of stored rice.^[21] During storage, fat of rice can be hydrolyzed by the endogenous lipase to free fatty acids, which undergo peroxidation under the action of the lipoxygenase, giving rise to an undesirable off-flavor.^[22] Fat acidities of paddy rice stored at different temperature conditions are shown in Figure 3. At temperature of 4°C, only slight change of paddy rice in fat acidity were observed after storage, suggesting that the aging of rice can be retarded at this storage temperature. This result was generally consistent with Park et al.^[23] At both frozen conditions, rice exhibited a significant increase in fat acidity. Similar results have been reported for the frozen storage of dairy products.^[24] At moisture of 13.5%, paddy rice stored at −30°C had significantly higher fat acidity than at −18°C. However, at moisture 15.1%, there was no significant difference between paddy rice stored at −18°C and −30°C in fat acidity. These results suggested that frozen treatment can accelerate the hydrolytic degradation of lipid in paddy rice, and the effect was related to the moisture content of paddy rice.

Figure 3. Fat acidity of paddy rice stored at different temperature conditions. At a specific moisture content, bars with the same letter are not significantly different (α = 0.05).

Milling quality of paddy rice

The husked rice yield (HURY) and head rice yield (HERY) of paddy rice stored at different temperature conditions are shown in Figure 4. Before storage, paddy rice had high HURY and HERY at both moisture contents. At storage temperature of 4°C, the paddy rice only showed slight decrease in HURY and had no significant change in HERY. This result suggested that short-term storage at 4°C only had limited impact on milling quality of paddy rice. For paddy rice stored at two frozen conditions, significant decrease of HURY and HERY were observed. These results suggested that frozen treatment have a pronounced effect on the milling quality of paddy rice. The milling quality of rice is related to the temperature history. Jaiboon et al.^[25] reported that increase of drying temperature could elevate the HERY when using fluidized bed to dry high-moisture waxy paddy. Zhao et al.^[26] found that the changes of temperature and moisture content caused the glass transition behavior of rice and may lead to the fissuring of rice and the change of milling performance. At frozen condition, the variation of temperature only made slight change in milling quality of paddy rice.

Figure 4. Husked rice yield (HURY) and head rice yield (HERY) of paddy rice stored at different temperature conditions. At a specific moisture content, bars with the same letter are not significantly different (α = 0.05).

Pasting property

The RVA curves of paddy rice stored at different temperature conditions are shown in Supplement Figure 5 and RVA parameters are shown in Table 1. For both grain moisture contents, storage at all three temperature conditions did not arouse significant change in pasting temperature (PT) of rice. At storage temperature of 4°C, rice only exhibited slight change in pasting viscosity parameters at both moisture content conditions. This result was consistent with Zhou et al.,^[27] which suggested that storage at 4°C can effectively retard the aging process of rice. Paddy rice stored at frozen conditions also showed slight change in pasting viscosity parameters. For rice of moisture content 13.5%, peak viscosity (PV), trough viscosity (TRV), and final viscosity (FV) significantly decreased when stored at −18°C; while at −30°C, PV and TRV decreased. For rice of moisture content 15.1%, PV, TRV, and FV decreased after storage. There was slight difference in pasting viscosity parameters between rice stored at 4°C and frozen conditions.

Figure 5. Rapid visco-analyzer curves of milled rice flour of paddy rice stored at difference temperature conditions. (A) moisture content 13.5%; (5) moisture content 15.1%.

Table 1. Rapid visco-analyzer (RVA) parameters of flour milled from paddy rice stored at different temperature conditions*.

Moisture content	Storage temperature (ºC)	PT (ºC)	PV (cp)	TRV (cp)	BDV (cp)	FV (cp)	SBV (cp)
13.5%	Unstored sample	76.4 ± 0.1a	2228 ± 3a	1047 ± 2a	1176 ± 8a	1920 ± 11a	881 ± 1a
	4	76.2 ± 0.5a	2057 ± 57b	987 ± 55ab	1070 ± 45b	1908 ± 100a	921 ± 47a
	−18	76.7 ± 0.2a	1948 ± 10c	880 ± 42c	1068 ± 35b	1762 ± 55b	882 ± 13a
	−30	76.2 ± 0.5a	2095 ± 11b	966 ± 10b	1129 ± 13a	1884 ± 19a	918 ± 10a
15.1%	Unstored sample	76.4 ± 0.1a	2230 ± 8a	1052 ± 1a	1171 ± 2a	1979 ± 39a	880 ± 1b
	4	76.1 ± 0.4a	2095 ± 11b	966 ± 10b	1129 ± 13ab	1884 ± 19a	918 ± 10a
	−18	76.1 ± 0.4a	1955 ± 30c	892 ± 59c	1063 ± 87b	1742 ± 93b	850 ± 35b
	−30	76.2 ± 0.4a	1910 ± 40c	826 ± 29d	1084 ± 33ab	1620 ± 46c	795 ± 18c

*Data were expressed as means ± SD. Means within a column that had the same letter were not significantly different (α = 0.05). PT: pasting temperature; PV: peak viscosity, TRV: trough viscosity; BDV: breakdown viscosity; FV: final viscosity; SBV: setback viscosity.

Cooking quality of milled rice

The cooking quality attributes of rice stored at different temperature conditions are shown in Figure 6. For paddy rice stored at 4°C, cooked rice did not have significant change in water uptake ratio (WUR) and volume expansion ratio (VER) after storage. This result was consistent with Pearce et al.^[28] Different from storage at 4°C, frozen storage aroused significant some change in cooking quality of rice. At moisture content of 15.1%, storage at frozen condition significantly decreased the WUR and VER of rice during cooking. The freeze-thaw treatment can damage the conformational structure of protein, resulted in the alteration of function characteristics. Li et al.^[29] reported that free-thaw treatment induced the protein aggregation of myofibrillar protein and reduced the water-holding ability of its heating-induced gel. For paddy rice with moisture of 15.1%, variation of frozen temperature did not significantly alter the cooking quality attributes of paddy rice. At moisture content of 13.5%, paddy rice did not exhibit significant change in WUR when stored at −18°C, but showed significantly declined WUR when temperature were decreased to −30°C; at this moisture, VER of paddy rice significantly decreased after frozen storage, but the alteration of temperature did not arouse significant change in VER.

Figure 6. Water uptake ratio (WUR) and volume expansion ratio (VER) of milled rice from paddy rice stored at different temperature conditions. At a specific moisture content, bars with the same letter are not significantly different (α = 0.05).

Texture attributes of cooked rice

The texture attributes of cooked products of paddy rice stored at different temperature conditions are shown in Table 2. When paddy rice was stored at 4°C, the cooked rice did not have significant change in texture attributes. This result was consistent with Park et al.^[23] However, the frozen treatment of paddy rice slightly decreased the hardness of the cooked rice. This result might be partly associated with the pasting viscosity change of rice. Chao et al.^[30] reported that brown rice varieties with high FV tend to result in harder texture in cooked grain. Peng et al.^[31] reported that hardness of cooked rice positively correlated with the FV of rice flour. No statistical difference of adhesiveness and springiness were observed in rice stored at frozen condition and the unstored rice.

Table 2. Texture attributes of cooked rice of paddy rice stored at different temperature conditions*.

Moisture content	Storage temperature (ºC)	Hardness	Adhesiveness	Springiness
13.5%	Unstored sample	1657 ± 35^a	−17.6 ± 2.5^a	.66 ± 0.01a
	4	1590 ± 94^ab	−16.9 ± 4.1^a	.60 ± 0.11a
	−18	1487 ± 13^ab	−20.4 ± 1.7^a	.62 ± 0.01a
	−30	1467 ± 77^b	−20.0 ± 1.7^a	.62 ± 0.06a
15.1%	Unstored sample	1632 ± 14^a	−17.6 ± 1.1^a	.63 ± 0.02a
	4	1588 ± 40^ab	−16.5 ± 1.0^a	.64 ± 0.03a
	−18	1493 ± 15^b	−15.9 ± 3.9^a	.58 ± 0.04a
	−30	1516 ± 59^b	−21.1 ± 1.2^a	.56 ± 0.04a

*Data were expressed as means ± SD. Means within a column that had the same letter were not significantly different (α = 0.05).

Conclusion

Frozen storage decreased the germination rate of paddy rice and led to a reduction in POD and PPO activities. The frozen treatment-induced decrease of germination rate and POD activity can be aggravated by decreasing temperature and elevating moisture content. For paddy rice, frozen storage generally caused a decrease in HURY, HERY, PV, WUR and VER during cooking, and cooked rice hardness, and brought about an increase in fat acidity. There results suggest that the frozen storage has detrimental effect on the vigor, antioxidant enzyme activity level, and milling quality of paddy rice, and can also affect the pasting and cooking properties of the rice. In further research work, more frozen conditions will be designed to fully understand the effect of frozen storage on the quality of paddy rice. For rice storage enterprises, much attention should be paid to the freezing-induced change of storage characteristic during the long-term storage of paddy rice.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the Open Fund of Key Laboratory for Deep Processing of Major Grain and Oil (Wuhan Polytechnic University), Ministry of Education [2020JYBQGDKFB09].