Cooking and Eating Qualities of Phka Rumduol, A Leading Variety of Cambodian Rice

Abstract

The cooking and eating qualities of Phka Rumduol, a leading rice variety in Cambodia, were examined in comparison to Phka Malis and Koshihikari. Per the results, Phka Rumduol showed greater elongation after cooking than Phka Malis. Further, Phka Rumduol tended to absorb a greater amount of water. Alkali degradation of Phka Rumduol was stronger than Koshihikari but weaker than Phka Malis. Physical properties determined by multiple bite measurements with a Tensipresser showed that Phka Rumduol and Phka Malis differed significantly from Koshihikari in values (H3, H5, A3, A5 and −A5). Sensory evaluation by a Cambodian panel showed that Phka Rumduol and Phka Malis were significantly different from Koshihikari in grain dimension and favorability. A questionnaire conducted before the sensory evaluation revealed that Cambodian panelists enjoyed eating cooked rice more than 2 times per day. These results confirm the importance of rice in the diet of Cambodians.

Introduction

Phka Rumduol, a type of fragrant long grain rice, is one of the leading varieties in Cambodia. It was developed from a local variety and released by the Cambodian Agriculture Research Development Institute (CARDI). With its aromatic smell, soft texture and pleasing appearance, it is considered to be one of the most delicious rice varieties in Cambodia. Moreover, Phka Rumduol won the World's Best Rice Award in 2012, 2013 and 2014.

We previously reported on the quality and safety of Cambodian rice (Ek et al., 2018) using four varieties from six local markets of Phnom Penh, and rice quality in Cambodian markets (Ek et al., 2020) using Phka Rumduol and Somali from three local markets of Takeo Province, Phnom Penh City and Battambang Province. Market survey results in the above reports showed that Phka Rumduol and Somali were of high quality.

The cooking and eating properties of rice have been investigated from the 1950s to the present time. However, the results of those studies were not always consistent with the results of sensory evaluations, especially in the case of Indica rice (Concepcion et al., 2015). Studies of Cambodian rice reported results for starch structure (Sar et al., 2014) and textural properties using a viscometer (Concepcion et al., 2018). Low- (25%) and high- (90%) compression tests with single cooked milled-rice grain using a Tensipresser were reported previously (Okadome et al., 1996, Okadome, 2005). According to the study of Okadome et al. (2005) on Japonica rice, the surface hardness and overall hardness were affected by protein and amylose, respectively. In Japonica rice, amylose and protein contents were reported to be reliable indicators of palatability (Inatsu, 1988).

In general, rice is not always sold in Cambodian markets based on variety. It has been sold by a kind of brand based on qualities such as color, shape, aroma, favorability and market price, for instance, fragrant rice premium jasmine (wet or dry season), white rice premium white, ginger rice, white rice and pearl riceⁱ⁾. Fragrant rice premium jasmine (wet season) includes Phka Malis, Phka Rumduol, Phka Romeat, Phka Rumdeng, Somali and Neang Malis. Chhourn O. (2014) evaluated the genetic variability among local and modern rice varieties in Cambodia; a lower genetic diversity value (0.46) was observed for 35 modern domestic varieties developed in Cambodia. Subsequently, CARDIⁱⁱ⁾ reported that Cambodia obtained the DNA fingerprints of three premium jasmine rice varieties, namely Phka Rumduol, Phka Romeat and Phka Rumdeng. Based on 18 distinct markers for alleles expressing the fragrance of jasmine rice, it was determined that Phka Romeat shows 100% similarity with the famed Thai rice Hom Mali. Phka Rumduol and Phka Rumdeng are each congruent with Thai Hom Mali in 17 markers and differ in the position of the 18^th marker, indicating that they are indigenous Cambodian varieties of the same premium quality jasmine rice. The above reports did not specify whether Phka Malis has an identical or indigenous genetic pattern with Thai Hom Mali. Phka Malis is often sold as jasmine rice or Khao Mali.

The purpose of this paper was to investigate the cooking quality of Phka Rumduol using the simplest method available. Further, the cooking and eating qualities of Phka Malis and Koshihikari were assessed for comparison purposes.

Materials and Methods

Materials    Phka Rumduol (cultivated in 2017; 90% milling degree) was obtained from CARDI, Phnom Penh, Cambodia. Phka Malis was purchased at a supermarket in Phnom Penh, Cambodia in 2017 (unknown milling degree) and Koshihikari (produced in Minami-Uonuma, Niigata Prefecture, Japan in 2017; milling degree not shown) was purchased at a supermarket in Tokyo.

Methods

Measurement of moisture content    Moisture content was measured using a moisture meter (Kett PM 650) as previously reported (EK et al., 2020).

Weight measurement of 1 000 grains    A sample of 100 grains was counted out, weighed and multiplied by 10.

Measurement of changes in grain weight and length following cooking    Eight grams of milled rice was placed in an empty tea bag. The sample was presoaked for 30 min in 160 mL water in a tall beaker. The beaker was placed in an electric rice cooker (Hitachi RZ-ES 10) and cooked normally. A bag without milled rice was also treated as the control.

Water absorption by cooking    The weight of the bag without rice after cooking (W1) and that of the bag with rice after cooking (W2) were measured. Water absorption rate by cooking = W2–W1 / the weight of milled rice before cooking.

Grain elongation by cooking    The length of 10 white rice grains was measured before (L1) and after cooking (L2) was measured. The elongation ratio = average of L2 / average of L1.

Assessment of alkali degradation    Six grains of milled rice (Phka Rumduol, Phka Malis and Koshihikari) were immersed in 10 mL of 1.7% KOH in a laboratory dish for 24 hr at room temperature (Battacharya and Sowbbagyo, 1972). The extent of spreading and degradation of the grains was then photographed.

Measurement of physical properties of cooked rice    A 25-g portion of milled rice washed three times lightly (Phka Rumduol, Phka Malis and Koshihikari) was placed in an aluminum vessel and 30 mL of water was added. The sample was cooked in an electric rice cooker as normal. Additionally, a sample containing 25 g of Koshihikari and 40 mL of water was also prepared. Each cooked rice sample was placed in a laboratory dish, covered tightly with polyethylene film and kept for 60 min at room temperature. A 10-g portion of cooked rice was placed in an aluminum cup and subjected to measurement using a Tensipresser (My Boy II, Taketomo Co.). The 4 kinds of samples (Phka Rumduol, Phka Malis, Koshihikari 30 mL and Koshihikari 40 mL) were each measured in quadruplicate. The data of cooked rice was obtained in duplicate and that of compression of the cooked rice in triplicate or quadruplicate.

The multiple bite measurement was used. H1, H3 and H5 (−H1, −H3 and −H5) indicate the heights (heights in minus direction) measured with 23%, 46% and 92% compression, respectively. A1, A3 and A5 (−A1, −A3 and −A5) indicate the areas with 23%, 46% and 92% compression ratio, respectively. A typical pattern of multiple bite measurement is shown in Figure 1.

Fig. 1.

Typical pattern of multiple bite measurement with a Tensipresser

The cooking method of 25 g of milled rice with 30 mL water was proposed by Cambodian researchers as the authors of this paper as the optimal method for cooking Cambodian rice using a rice cooker.

Sensory evaluation    Cooked rice prepared in the same manner as for the Tensipresser measurement was divided into 3 parts and served to each Cambodian panelist (9 in total; 5 males, 4 females). The panelists' interest in rice was determined before the test. The evaluation was conducted by scoring samples using a preference scale (1 to 5, indicating best to worst) with respect to grain dimension, aroma, hardness, color (whiteness), and favorability.

Statistical analysis of data    The data of measurements of grain elongation, physical values determined by a Tensipresser and sensory evaluation were reported as the average ± SD. One-way analysis of variance was performed using Excel-Tokei (Social Survey Research Information Co., Ltd.) for the data of grain elongation and physical values with a Tensipresser, followed by Tukey's post-hoc test. Two-way analysis of variance using Excel-Tokei 2015 was used to examine the sensory evaluation data, followed by Tukey-Kramer's post-hoc test.

Results and Discussion

Moisture content of Phka Rumduol, Phka Malis and Koshihikari    The moisture content of Phka Rumduol, Phka Malis and Koshihikari was 12.37, 13.22 and 12.93%, respectively. The moisture content did not show a remarkable difference between the three rice samples.

Weight of 1 000 grains of the three varieties    The weight of 1 000 grains was 18.6 g for Phka Rumduol, 19.6 g for Phka Malis, and 21.8 g for Koshihikari.

Measurement of cooking qualities

Grain elongation and water absorption of the three rice samples following cooking    Grain elongation of the 3 rice samples is shown in Table 1 and Figure 2. Water absorption is also shown in Table 1.

Table 1. Elongation and water absorption of rice grains by cooking

	elongation			water absorbance
	L1 (mm)	L2 (mm)	L2/L1	W1(g)	W2(g)	W2/W1
Phka Rumduol	6.892 ± 0.381a	10.198 ± 1.265a	1.486 ± 0.223a	8.01	34.97	4.37
Phka Malis	7.199 ± 0.773a	9.166 ± 0.693a	1.285 ± 0.162b	8.01	26.99	3.36
Koshihikari	4.990 ± 0.241b	7.045 ± 0.526b	1.416 ± 0.138a	8.00	29.99	3.75

Different alphabet small letters in the same column show significant difference (P<0.05)

Fig. 2.

The grain shape of elongated 3 rice samples by cooking Five grains in 10 grains were taken photographs

Measurements of L1 and L2 for Cambodian samples (Phka Rumduol and Phka Malis) and Koshihikari showed significant differences, respectively (P<0.01). L2/L1 values were significantly different between Phka Rumduol and Phka Malis (P<0.05). The results suggested that the grain length of Phka Rumduol and Phka Malis was significantly different from that of Koshihikari. On the other hand, the grain elongation of Phka Rumduol tended to be greater than that of Phka Malis.

The water absorbance values before and after cooking (W2/W1) could not be statistically analyzed because of insufficient replicates; however, the values were observed to be the highest in Phka Rumduol, followed by Koshihikari and Phka Malis.

Alkali degradation of the three varieties    The pattern of alkali degradation of the three varieties after 24 hr is shown in Figure 3. The speed of degradation by exposure to 1.7% KOH was the fastest in Koshihikari, followed by Phka Rumduol and Phka Malis. After 48 hr, the shape of Koshihikari was mostly lost and that of Phka Rumduol was cottony. On the other hand, 3/10 grains of Phka Malis retained their shape.

Fig. 3.

Patterns of degradation in alkali of the 3 rice samples The diameter of a laboratory disk in Figure 3 was 89 mm

Physical properties of the 3 cooked rice samples    The multiple bite measurement data obtained by a Tensipresser are shown in Table 2, Table 3-1 and 3-2, and Figures 4, 5-1 and 5-2.

Table 2. Four elements of multiple bite elements with a Tensipresser

	Hardness	Toughness	Adhesiveness	Stickiness
	[+H5]	[A1/A5]	[−A5]	[−H5]
Phka Rumduol	65.45 ± 4.56a	46.97 ± 1.76a	41.92 ± 4.23a	50.77 ± 4.27a
Phka Malis	59.28 ± 3.25a	43.90 ±4.33a	43.76 ± 9.51a	50.11 ±2.23a
Koshihikari-30mL	42.40 ± 1.30b	35.80 ± 1.65b	47.86 ± 3.99a	44.89 ±4.31b
Koshihikari-40mL	27.00 ± 3.36c	29.81 ± 1.98c	46.51 ± 12.95a	39.11 ±5.37b

Different alphabet small letters in the same column show significant difference (P<0.01 or 0.05)

Table 3-1. H values of multiple bite measurement with Tensipresser

	[+H1]	[−H1]	[+H3]	[−H3]	[+H5]	[−H5]	[−H5]/[H5]
	[gw/cm^2]	[gw/cm^2]	[gw/cm^2]	[gw/cm^2]	[gw/cm^2]	[gw/cm^2]
Phka Rumduol	250.58 ± 25.09a	32.06 ± 4.61a	759.73 ± 9.24a	45.46 ± 5.06a	2089.25 ±78.24a	434.90 ± 54.82a	0.21 ±0.02a
Phka Malis	270.53 ± 52.90a	33.86 ±8.51a	753.70 ±37.65a	20.20 ± 3.27a	1984.50 ±61.54a	428.83 ± 84.44a	022 ± 0.05a
Koshihikari-30mL	245.70 ±41.57a	29.13 ±4.24a	600.30 ±35.04b	14.33 ±6.77a	1702.75 ±57.94b	391.88 ± 40.33a	0.23 ± 0.03a
Koshihikari-40mL	213.50 ± 22.43a	26.51 ±4.24a	462.98 ±54.82b	6.12 ± 6.28a	1339.00 ± 85.57b	304.63 ± 63.55a	0.23 ±0.06a

Different alphabet small letters in the same column show significant difference (P<0.01 or 0.05)

Table 3-2. A values of of multiple bite measurement with Tensipresser

	[+A1]	[−A1]	[+A3]	[−A3]	[+A5]	[−A5]	[A5]/[A1]
	[gw·cm/cm^2]	[gw·cm/cm^2]	[gw·cm/cm^2]	[gw·cm/cm^2]	[gw·cm/cm^2]	[gw·cm/cm^2]
Phka Rumduol	38.64 ± 3.20a	5.49 ± 1.31a	174.73 ± 4.19a	14.04 ± 2.10a	677.03 ± 15.10a	50.00 ± 3.81a	14.32 ± 1.31a
Phka Malis	40.91 ± 7.20a	5.56 ± 1.20a	174.43 ± 9.05a	14.04 ± 2.46a	646.33 ± 14.43a	48.24 ± 2.19a	14.73 ± 2.80a
Koshihikari-30mL	36.64 ± 5.41a	4.32 ± 1.19a	139.08 ± 8.31b	10.87 ± 2.01a	516.75 ± 10.33b	37.36 ± 4.62b	14.11 ± 2.02a
Koshihikari-40mL	30.83 ± 3.12a	4.11 ± 1.07a	109.08 ± 11.43b	10.00 ± 3.19a	384.28 ± 34.15b	28.89 ± 2.67c	13.23 ± 0.95b

Different alphabet small letters in the same column show significant difference (P<0.01 or 0.05)

Fig. 4.

Comparative relationships of H5 between cooked rice

Asterisks shows significant deviation based on Tukey's test (* P<0.05 and ** P<0.01)

Fig. 5-1.

Comparative relationships of A5 between rice samples

Fig. 5-2.

Comparative relationships of −A5 between rice samples

Asterisks show significant deviation based on Tukey's test (*P<0.05 and ** P<0.01)

Table 2 shows the typical analysis of the following 4 parameters: hardness, springiness, adhesiveness and stickiness. There were no significant differences between Phka Rumduol and Phka Malis in hardness, springiness and stickiness; however, significant difference were shown between Cambodian samples and Koshihikari in the above parameters (P<0.01). Moreover, significant differences were shown between Koshihikari (30 mL) and Koshihikari (40 mL) (P<0.05). However, no significant differences were shown in adhesiveness for all samples.

Koshihikari showed generally high stickiness (data not shown). In order to clarify the textural difference between samples, the values measured with 23%, 46% and 92% compression with a Tensipresser are shown in Tables 3-1 and 3-2.

As shown in Tables 3-1 and 2 and Figures 4 and 5, Phka Rumduol and Phka Malis were evaluated as harder (H3 and H5) and tougher (A3 and A5) than Koshihikari (30 mL and 40 mL), with significant differences observed. Phka Rumduol and Phka Malis showed higher −5 values than Koshihikari (30 mL and 40 mL); however, the −A5 data seemed to be influenced by H and/or A values. The −A5 value of Koshihikari 30 mL was greater than that of Koshihikari 40 mL, although not significantly so. Phka Rumduol tended to show higher values than Phka Malis for H and A.

In the preliminary data of Koshihikari cooked with only 30 mL water, the observed texture and/or eating quality of the rice was harder than that consumed by the Japanese on a daily basis. In this experiment, the values obtained by adding 30 mL or 40 mL water to Koshihikari are shown.

The cooking method, in which 30 mL water was added to Koshihikari, is considered to contain less water than the typical method used in Japan. Moreover, the method employed in this experiment was considered to be the most suitable for Cambodian rice, i.e., the rice was cooked immediately after washing and without soaking in water for a period of time.

Sensory evaluation of the three varieties by a Cambodian panel    Cambodian panelists (9 members) completed a questionnaire prior to the sensory evaluation. Table 4 indicates that the panelists enjoyed eating cooked rice more than 2 times per day, and both short and long grain rice were consumed (short rice in Table 4 means shorter-grain rice produced in Cambodia, both are the Indica variety). The panelists indicated that they are proud of Cambodian rice. The questionnaire results are supported by the fact that rice is a staple food and accounts for at least 65% of the energy intake of Cambodians, who have a daily average consumption of 462 g/person (paddy, uncooked weight).

Table 4. Cambodian's interest in rice by 9 participants

Do you like cooked rice?	How many times do you eat cooked rice in a day?	Do you eat long rice or short rice in daily meals?	Do you think Cambodian rice is superior to other countries' rice?
1=Yes, 2=So So, 3=No	1=One time, 2=Two time, 3=Three time, 4=More than three time	1=Long, 2=Short	1=Yes, 2=No, 3=Other idea
All are 1	2 times:4,3times:5	long:5, short:4	Yes:7, No:2

The number of panelists was too few to draw significant conclusions in this experiment. In our previous study (EK et al., 2020), the same questionnaire was administered to 120 Cambodian panelists and highly similar results were obtained.

In the sensory evaluation, 1 indicates the positive direction of the scale (very long, very aromatic, very hard, very white and very favorable) and 5 indicates the negative direction (short, less aroma, soft, dark and dislike).

Results of the sensory evaluation are shown in Figure 6. Grain dimension and favorability of Phka Rumduol and Phka Malis were significantly different from Koshihikari 30 mL and 40 mL (P<0.01). Phka Rumduol was observed to be longer and tended to score higher in grain length and favorability than Phka Malis. However, there were no significant differences between rice samples in aroma, hardness and whiteness. Phka Malis was observed to be somewhat whiter than the other samples. This is because commercial milled rice in Cambodia is typically polished after normal milling, which makes it appear whiter. Sensory evaluation of hardness seemed to indicate not only textural hardness but also a feeling of the sample being dried out.

Fig. 6.

Comparative pattern of sensory evaluation between 3 rice varieties

Asterisks show significant difference by Tukey-test (** P<0.01)

Fig. 7.

Comparative relationship in favorability between 3 rice varieties

Asterisks show significant deviation based on Tukey's test (** P<0.01)

The data indicated that Cambodian panelists prefer cooked rice of their own country.

Conclusion

Phka Rumduol show significantly greater elongation after cooking than Phka Malis (P<0.05). Phka Rumduol tended to absorb a greater amount of water after cooking than Phka Malis and Koshihikari. The alkali degradation results revealed that Phka Rumduol was more resistant than Koshihikari but less so than Phka Malis.

The Tensipresser measurement of physical properties showed that Phka Rumduol and Phka Malis showed higher values in hardness, springiness and stickiness than Koshihikari. Furthermore, Phka Rumduol and Phka Malis showed significant differences in H3, H5, A3 and A5 compared to Koshihikari (P<0.01), which indicated the hardness and/or toughness of cooked rice. In the present study, the most suitable cooking method was employed for the Cambodian rice samples. Sensory evaluation by Cambodian panelists showed that Phka Rumduol and Phka Malis were significantly different from Koshihikari in grain dimension and favorability (P<0.01). Panelists tended to score the favorability of Phka Rumduol higher than Phka Malis (not significantly different).

Grain elongation, water absorption and alkali degradation can be used as parameters for the simple evaluation of rice quality; however, it is necessary to examine many Cambodian varieties in the future. Physical measurement of rice using a Tensipresser and sensory evaluation will enable the detailed examination of many Cambodian varieties.

Notably, Cambodian panelists enjoyed eating cooked rice more than 2 times per a day. The results of this study support the importance of rice in Cambodian dietary life.

Acknowledgements    The authors sincerely thank Dr. Ngo Bunthan, Rector of Royal University of Agriculture, Cambodia (RUA), Dr. Sok Kunthy and Dr. Huon Thavrak, Vice Rectors of RUA for their valuable advice. We also thank Dr. Tadayoshi Tanaka and Prof. Yoko Yoneyama of Aikoku Gakuen Junior College for their suggestions.

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