HARDNESS OF COOKED RICE AS AFFECTED BY VARIETIES, COOLING METHODS AND CHILL STORAGE

HARDNESS OF COOKED RICE AS AFFECTED BY VARIETIES,COOLING METHODS AND CHILL STORAGE

YING MA1 and DA-WEN SUN2,3

1College of Food Science and EngineeringHarbin Institute of Technology

Harbin, China

2Food Refrigeration and Computerised Food Technology (FRCFT)University College Dublin

National University of IrelandEarlsfort Terrace, Dublin 2, Ireland

Accepted for Publication October 5, 2007

ABSTRACT

Rice is an important carbohydrate source in daily catering, and also oneof the main components in some ready meals. Ready meals containing cookedrice should be cooled quickly for storage. In this research, air blast coolingand cold room cooling were used to cool cooked rice. The effect of coolingcondition, rice variety and chill storage on the textural property, especiallyhardness, of cooked rice was investigated. Results show that air blast coolingcan significantly reduce the cooling time and weight loss as compared withcold room cooling for the three varieties (Thai jasmine rice, long grain riceand Japanese rice). The cooling methods can also significantly affect thehardness of cooked rice depending on the rice variety. Thai jasmine rice haslower amylose content; therefore, hardness was generally lower than that ofthe other two varieties. During the chill storage at 4C, the hardness wasincreased for all samples except for cooked Thai jasmine rice cooled by airblast cooling, indicating that fast cooling time can inhibit or slow down thestarch retrogradation during chill storage for low-amylose-content rice.

PRACTICAL APPLICATIONS

In the food industry, ready meals are usually kept at 4 or -18C for longershelf life. However, the retrogradation of rice starch takes place very easilyduring the storage, and storage temperature can significantly affect the

3 Corresponding author. TEL: +353-1-7165528; FAX: +353-1-4752119; EMAIL: [email protected]

Journal of Food Process Engineering 32 (2009) 161–176. All Rights Reserved.© Copyright the AuthorsJournal Compilation © 2008 Wiley Periodicals, Inc.DOI: 10.1111/j.1745-4530.2007.00206.x

161

mailto:[email protected]

behavior of cooked rice. In both domestic and international markets, theend-use quality of rice has significant impact on its market value and accept-ability to consumers. Therefore, the behavior of cooked rice is interesting tofood scientists and technologists because it profoundly affects the quality,acceptability and shelf life of the rice-containing dishes. The current researchwill provide further information for the food industry on the effect of coolingcondition, rice variety and chill storage on the textural property, especiallyhardness, of cooked rice.

INTRODUCTION

The trend of replacing regular mealtime by quick snack food and readymeals is becoming popular in the world. For example, the average U.K. mealpreparation time has gone down from 2 h, two generations ago, to 20 mintoday. Similarly, in the U.S.A., 44% of all weekly meals are prepared in lessthan 30 min (Fletcher 2006). Ready meals are generally composed of differentcomponents such as meat, poultry, fish, seafood, pasta, cooked rice and veg-etables. Most ready meals only need reheating prior to serving. Among thecomponents, rice is an important one in some ready meals. There are severalattributes that affect the texture of cooked rice such as rice variety, chilling andstorage condition, moisture content, cooking method, and precooking andpostcooking processing (Mohapatra and Bal 2006).

In the industrial production of ready meals containing cooked rice, thecooked rice must be quickly cooled down to less than 10C immediately aftercooking for food safety reason (Anon 1989, 1991). Several cooling methodscan be applied to cool cooked rice such as vacuum cooling, air blast cooling,plate cooling and cold room cooling (Zhang and Sun 2006b). Among them, airblast cooling is the most common method used in the food industry (Janz et al.2001; Wang and Sun 2002; Zhang and Sun 2006a). After cooling, the readymeals usually keep at 4C for storage or for sale in supermarkets, during whichstarch retrogradation normally occurs.

The cooking and eating quality of rice mainly depends on starch charac-teristics, especially on amylose content, gel consistency and gelatinizationtemperature (Tian et al. 2005), among which amylose content is the maininfluencing factor. Different varieties of rice possess different amylose con-tents, thus producing different qualities of cooked rice. The low-amylose type(less than 20% amylose) always has a soft texture and is sticky when cooked.The intermediate-amylose type (21–25%) produces rather soft cooked rice,whereas the high-amylose type (>25%) has a hard texture (Cheaupun et al.2004). In the world rice market, rice is classified into six basic types. Amongthem, predominantly indica with high quality and long grain rice, japonica

162 Y. MA and D.-W. SUN

short or medium grain and aromatic rice play a significant role in the interna-tional market (Cheaupun et al. 2004). Therefore, in the current study, threevarieties of rice, i.e., long grain rice (indica rice), Japanese rice (japonicamedium grain) and Thai jasmine rice (aromatic rice), were selected for experi-ments based on their amylose content and consumer preferences.

Starch retrogradation usually results in harder and firmer rice. Baik et al.(1997) found that starch retrogradation occurs quicker in rice stored at 4Ccompared with rice stored at room temperature for 5 days. Narpinder et al.(2006) pointed out that amylose aggregation and crystallization is completedwithin the first few hours of storage, while amylopectin aggregation andcrystallization occurs during later stages. Ong and Blanshard (1995a) studiedthe final texture of cooked parboiled rice from 11 different cultivars of indicanonwaxy rices, and Ramesh et al. (1999) studied the relationship of finaltexture of cooked rice and starch structure from seven different indica culti-vars, and both found that amylose content had great effect on the texture ofcooked rice.

There are many textural attributes for describing the eating quality ofcooked rices, among which hardness is the most important attribute. Formeasuring the textural change of cooked rice, several methods can be used(Karim et al. 2000). A very popular one is the texture profile analysis (TPA),which can provide a number of textural parameters. These parameters can bedivided into the primary parameters of hardness, cohesiveness, springiness andadhesiveness, and the secondary parameters of fracturability, chewiness andgumminess (Bourne 1978; Ramesh et al. 1999; Chuang and Yeh 2006; Belloet al. 2006; Mohapatra and Bal 2006). Although texture is multidimensional,hardness and stickiness are critical, with hardness being the most importantand most commonly measured parameter (Meullenet et al. 1998).

Although air blast cooling and cold room cooling have been used to coolcooked rice (Zhang and Sun 2006b), little research has been conducted tofocus on the effect of cooling process, rice variety and chill storage on cookedrice textural properties. Therefore, the objectives of the current study were to(1) compare the effects of rice variety on weight loss (WL) and cooling timebased on different cooling methods; and (2) monitor the textural property,especially hardness, and shelf life of cooled rice during chill storage asaffected by cooling methods and rice varieties.

MATERIALS AND METHODS

Rice

Three varieties of rice were used in the study. The long grain ricewas produced in Surinam, with a moisture content of about 13% w.b. and

HARDNESS OF COOKED RICE 163

containing high amylose content (26.9–29.7%) (Mundy et al. 1989). The Thaijasmine rice (Vudhichai Produce Co Ltd, Bangkok, Thailand) was purchasedfrom a local supermarket, with a moisture content of about 14% w.b. andcontaining low amylose content (13–18%) (Cheaupun et al. 2004; Pongthornand Aluck 2006). The Japanese rice (Riseria Monferrato S.P.A., Casale Mon-ferrato, Italy) was also purchased from a local supermarket, with a moisturecontent of 14% w.b. and containing intermediate amylose content (average20.3%) (Kazuyoshi and Tadao 1986).

Cooking

Five hundred grams of rice was washed three times with tap water, thenthe rice was placed into a pot and soaked with about 750 mL water (about 60C)for about 20 min. The rice was cooked to boiling with full power until thewater in the pot was dried, then the rice was simmered for another 20 min withthe lowest power. After cooking, about 1,200 g cooked rice was obtained.Finally, the cooked rice was removed from the pot and was placed on astainless steel tray for air blast cooling and cold room cooling. The surface ofthe rice in the steel tray was flattened, and the rice was cooled from a tem-perature of 80–90 to 4C.

Air Blast Cooling

The air blast chiller (Model CBF20, Foster Refrigerator Ltd., Norfolk,U.K.) was applied to cool rice. The outside dimension of the chiller is120 ¥ 80 ¥ 75 cm, and the dimension of the chilling chamber is35 ¥ 70 ¥ 60 cm. The air blast chiller had been running for at least 30 minbefore the cooling. About 2,000 g cooked rice was put into the stainless steeltray, and the thickness of the rice layer in the tray was about 5–6 cm. The traywas then placed into the chiller chamber at about 70 mm in front of the fans(Zhang and Sun 2006b). A data acquisition program based on LabView (v4.2,National Instruments, Austin, TX) was used to record the cooling temperature,cooling time and relative humidity of the air. Five T-type thermocouples(Radionics Ltd., Dublin, Ireland) were used for temperature measurement,which were inserted at the center and near four corners of the tray. The inserteddepth of the thermocouples was about 2.5–3.0 cm.

Cold Room Cooling

In order to study the effect of slow air cooling in a cold room, a 250-Lrefrigerator (Unit 136a, AGB Scientific Ltd., Dublin, Ireland) was used tosimulate the cold room cooling. About 2,000 g cooked rice was put into thestainless steel tray, and the thickness of the rice layer in the tray was about


5–6 cm. Then the tray was immediately put into the refrigerator at 4C untilit was cooled to 4C. A data acquisition program based on LabView (v4.2,National Instruments) was used to record the cooling temperature andcooling time. As described previously, five T-type thermocouples (RadionicsLtd.) were inserted at the center and near the four corners to measure thetemperatures. The inserted depth of the thermocouples was about 2.5–3.0 cm.

TPA

The cooled cooked rice was then stored at 4 � 1C for 12 days. During thestorage, TPA parameters of cooled cooked rice were measured every day. Forcomparison, TPA parameters of fresh cooked rice were also measured. TheTPA of the samples was performed by using a texture analyzer (Model 5544,Instron Corporation, High Wycombe, U.K.) with a 500-N load cell and two-cycle compression test (Bello et al. 2006). Before the TPA test, about 40-gsample was placed into a small plastic bag and was heated for 40 s in amicrowave oven (750 W), and then the sample was placed into a small heatinsulation box. When the sample preparation was finished, 10 grains of cookedrice were selected from the center of the box. These grains were carefullyplaced under the rod-type probe (35 mm), and the sample was compressed to60% at 0.5 mm/s. The time between chewing was 3 s. Textural parameters,such as hardness, springiness, cohesion force resilience, cohesion energy resil-ience, gumminess and chewiness, were determined from the two-cycle curvesusing Bluhill software for Windows (Instron Corporation). The texture profileanalysis was repeated 8–10 times per replicate.

WL and Moisture Determination

WL was calculated as the percentage weight difference between theweights of cooked rice before and after cooling. The WL was calculatedaccording to the following equation:

WL % %a b

a

( ) =−

×W W

W100

where Wa and Wb are weights of the cooked rice before and after the cooling,respectively.

Moisture content was determined by the oven method (Plus II Oven,Dublin Ind. Est., Glasnevin, Ireland) at 104C for 48 h (Association of OfficialAnalytical Chemists 1990). Two replicates were performed for each sample.


Statistical Analysis

Data for cooling time, moisture content, WL and TPA parameters wereanalyzed using Version 8.0 of the SAS System (Copyright 1999–2000 by SASInstitute Inc., Cary, NC).

RESULTS AND DISCUSSION

Cooling under Different Air Blast Conditions

In order to determine the suitable cooling condition for subsequentexperiments, cooked long grain rice was cooled by air blast to 4C in thefollowing cooling conditions: (1) under cooling temperature of 0C, threecooling air velocities of 1.0, 1.3 and 1.5 m/s were used; and (2) under coolingair velocity of 1.5 m/s, three cooling temperatures of 0, -1 and -2C wereemployed.

Table 1 shows the cooling time, WL and moisture content of cooked longgrain rice. It can be seen from Table 1 that air velocity strongly affected thecooling time. With the increase in velocity, cooling time was reduced. This isdue to the increase in convective heat transfer coefficient. A similar situationwas found in air blast cooling of cooked meat (Wang and Sun 2002). However,the effect of air velocity on WL and final moisture content was not so obvious,although there was a general trend of increase in WL and thus decrease in finalmoisture content with air velocity; this is probably due to more evaporationthat occurred at higher cooling velocity. Table 1 also shows that the effect ofair velocities of 1.3 and 1.5 m/s on cooling time, WL and moisture content wasnot significant (P > 0.05). Therefore, for cooling cooked rice by air blast, thehigher velocity of 1.3–1.5 m/s is recommended.

TABLE 1.COOLING TIME, WEIGHT LOSS (WL) AND MOISTURE CONTENT OF COOKED LONG

GRAIN RICE AT DIFFERENT AIR VELOCITIES AND AIR BLAST COOLINGTEMPERATURES (RELATIVE HUMIDITY, 86%)

Velocity (m/s) Temperature (C) Cooling time (min) WL (%) Moisture content (%)

1.0 0 87.77 � 16.09a 4.89 � 0.32b 63.79 � 1.07a

1.3 0 64.45 � 5.17b 5.05 � 0.23ba 62.08 � 2.48b

1.5 0 61.24 � 3.80bc 5.34 � 0.26ba 62.54 � 0.57ba

1.5 -1 52.50 � 5.19cd 5.98 � 1.46a 63.11 � 1.01ba

1.5 -2 46.27 � 2.64d 5.47 � 0.23ba 62.28 � 1.16b

Different letters within the same column indicate significant difference (P < 0.05) using Duncan’smultiple range tests. Three replicates were performed for each sample.


The effect of three cooling temperatures of 0, -1 and -2C is also shownin Table 1. Similar to air velocity, different cooling temperature also has anobvious effect on cooling time, and the cooling time is significantly decreasedwith decreasing cooling temperature; however, its effect on WL and moisturecontent is not so obvious. This is because the cooling time was shortenedmainly by the increase in temperature gradient within the rice bed when alower cooling temperature was used, thus a higher heat conduction rate wasachieved. By combining the results from Table 1 in the following experiments,the cooling condition was set at 1.5 m/s and –1C.

Comparison between Air Blast Cooling and Cold Room Cooling

Cooling Time and WL. Air blast cooling and cold room cooling are themost common methods to precool cooked food products (Desmond et al.2000; McDonald et al. 2000; Sun and Wang 2000; Zhang and Sun 2006b).Table 2 compares the results of the cooked rice (three varieties) cooled by airblast and cold room. It can be seen that cooling time is much shorter for airblast than for cold room. In air blast and cold room cooling, cooling takes placemainly due to heat convection on the surface of the product and heat conduc-tion in the product; therefore, cooling efficiency is closely related to the airflow velocity. Therefore, forced airflow increased heat convection between thecooked rice and the environment, contributing to shorter cooling time. In themeantime, the WL by cold room cooling is higher than that by air blast, whichis caused by the longer cooling time needed for the former. Zhang and Sun(2006b) once cooled parboiled long grain rice using air blast cooling at asimilar cooling condition, and similar results on WL and cooling time wereobtained. However, the cooling time and WL by cold room cooling are higherthan those reported by Zhang and Sun (2006b), which is mainly because of thedifference in rice variety, as heat and moisture transfer within the rice areaffected by the thickness, viscosity and surface properties of cooked rice (Huand Sun 2001). Unlike the results in Table 1, the final moisture contents of therice cooled by cold room cooling are higher than those by air blast, as higherinitial moisture contents were obtained after cooking for the batches used forcold room cooling.

TPA of Fresh and Cooled Cooked Rices. Usually, rice variety, amylosecontent, starch type, water to rice ratio and cooking method can affect thetextural characteristics of cooked rice such as hardness, thickness and loose-ness (Ramesh et al. 1999; Mohapatra and Bal 2006). Although texture ismultidimensional, hardness is critical and it mainly governs the palatability ofthe cooked rice (Bello et al. 2006). Figure 1 compares the hardness of cookedrices as affected by cooling methods. It can be seen from Fig. 1a that there is


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no significant difference found between the fresh cooked rice and that cooledby air blast cooling for all three rice varieties (P > 0.05). However, the hard-ness of the long grain rice was significantly increased due to cold room cooling(P < 0.05), although no significant difference existed in the other rice varieties(P > 0.05). This indicates that a fast cooling speed (i.e., air blast) has less effecton rice hardness, while slow cooling (i.e., cold room cooling) has some

a

b

Fresh

Cooled

Fresh

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b

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b

c c

c c

e e

a

a

a

0

0

5

10

15

20

25

30

35

40

45

50

1020

3040

50

Long Japanese

Rice VarietyThai

Long Japanese

Rice VarietyThai

TPA

Har

dnes

s (N

)T

PA H

ardn

ess

(N)

FIG. 1. COMPARISON OF THE HARDNESS BETWEEN FRESH AND COOLED COOKEDRICES BY (a) AIR BLAST COOLING AND (b) COLD ROOM COOLING


negative effect on cooked rice quality. It can also be observed that the hardnessvalues for the samples used in air blast cooling (Fig. 1a) are higher than thoseused for cold room cooling (Fig. 1b), as higher initial moisture contents wereobtained for the cooked batches used for cold room cooling (Table 2). Figure 2compares the cohesiveness of fresh cooked rices and cooled ones, and asignificant increase in cohesiveness was found for long grain rice due to fastcooling (Fig. 2a) (P < 0.05) although not for the other varieties; however, forslow cooling, cohesiveness was significantly increased after cooling for both

0.50

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Long JapaneseRice Variety

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a

b

FIG. 2. COMPARISON OF COHESIVENESS BETWEEN FRESH AND COOLED COOKEDRICES BY (a) AIR BLAST COOLING AND (b) COLD ROOM COOLING


long grain rice and Japanese rice (P < 0.05) although not for the other(P > 0.05), which again indicates the effect of cooling speed on rice quality.Actually, the effect on cohesiveness is very complicated as the extent ofcohesive behavior depends on the nature of the food and external factors likemoisture and temperature (Adhikari et al. 2001). Figures 1 and 2 also showthat only for Thai Jasmine rice, cooling methods had no effect on both hard-ness and cohesiveness, as no significant difference was found between thefresh cooked rice and the cooled one (P > 0.05), which also indicates that ricevariety plays an important role in the cooling effect. Results also show thatcooling and rice variety have an effect on other TPA parameters such asgumminess, chewiness and springiness (data is not shown). However, theireffects are complicated, and further studies are needed to clarify these effects,and therefore are not discussed here.

Based on these results, it can be seen that the textural properties of cookedrice are significantly affected by cooling methods depending on the ricevariety. In general, cooling treatment can affect the quality of cooked rice suchas TPA parameters (Zhang and Sun 2006b), moisture content (Chuang and Yeh2006) and rice grain structure (Navdeep and Narpinder 2003). However, forthe three varieties of rices, only the textural properties of long grain rice weresignificantly affected by cooling method. Usually, the amylose contentgoverns the texture of cooked rice. Cooked rice from high-amylose rice typesare usually firm and dry, while those of low amylose content are more moistand soft (Ong and Blanshard 1995a). For rice, amylose contents are classifiedas waxy (0–2% amylose), very low (5–12%), low (12–20%), intermediate(20–25%) or high (25–33%) (Vandeputte and Delcour 2004). Thai jasmine ricebelongs to low-amylose-content rice (Pongthorn and Aluck 2006). Japaneserice belongs to intermediate amylose content (Mizukami et al. 1999), whilelong grain rice belongs to high amylose content (Mundy et al. 1989). Figures 1and 2 show that cooling methods have different effects on their hardness andcohesiveness. Iturriaga et al. (2006) have proved that different texture existedamong cooked rices from different cultivars. Usually, retrogradation can easilyhappen for rice with higher amylose content at lower temperature (0–4C) (Ongand Blanshard 1995b), therefore, only Thai jasmine rice was not affected bythe cooling methods (Figs. 1 and 2) because of its low amylose content.

Comparison of TPA Variation during Storage

After cooling, the cooled rices were chill stored at 4 � 1C for 12 days.Figure 3 shows the variation of hardness during storage for the three varietiesof cooked rices cooled by air blast and cold room cooling. It can be seen thatthe hardness of all samples increased significantly (P < 0.05) during storage,except Thai jasmine rice cooled by air blast. However, unlike hardness, the


changes in cohesiveness were not obvious for all the samples, which seems tobe contradictive to the results in Fig. 2, again indicating the complexity of thecohesive behavior, which depends on many factors such as nature of the food,moisture and temperature (Adhikari et al. 2001). The significant increase ofthe hardness means that the textural behavior of the rices has changed.

10

15

20

25

30

35

40

45

50

55

60

1 2 3 4 5 6 7 8 9 10 11 12

Days

TP

A H

ardn

ess

(N)

Long Japanese Thai

10

15

20

25

30

35

40

45

50

55

60

1 2 3 4 5 6 7 8 9 10 11 12

Days

TP

A H

ardn

ess

(N)

Long Japanese Thai

a

b

FIG. 3. COMPARISON OF HARDNESS OF COOLED COOKED RICES DURING CHILLSTORAGE BY (a) AIR BLAST COOLING AND (b) COLD ROOM COOLING


Therefore, the cooling speed can significantly affect the textural behaviorfor Thai jasmine rice during its subsequent storage, and rapid cooling(49.25 � 2.68 min for air blast cooling versus 221.33 � 14.15 min for coldroom cooling) can inhibit or reduce the starch retrogradation at 4C.

Riva et al. (2000) studied cooked rice stored at 4C for 5 days, and notedthat the degree of retrogradation was quite small in the first 24 h and beganto increase rapidly on subsequent storage. In the current research, a similarchanging tendency of the rice texture was also observed. Studies from Ong andBlanshard (1995a) and Ramesh et al. (1999) indicated that amylose contentshad a great impact on the texture of the cooked rice than did physical attributes.Results from the current study also confirmed that for the cooked rice with lowamylose content, i.e., Thai jasmine rice, it could keep a good texture during thewhole storage period if it was rapidly cooled after cooking. For rice withmiddle amylose contents, i.e., Japanese rice, it could maintain a good texturein the first few days (4–5 days) of storage, while for rice with high amylosecontent, i.e., long grain rice, it could only maintain a good texture in 2–3 daysof storage; however, their texture changed significantly when the storageperiod was prolonged.

CONCLUSIONS

The textural property, especially hardness, of cooked rice is significantlyaffected by cooling methods depending on the rice variety. For the three ricevarieties, hardness was affected by cooling speed, and slow cooling tended tochange the textural behavior of the rice as compared with fresh cookedsamples (P < 0.05). As Thai jasmine rice has low amylose content, its hardnesswas lower than those of the other two rice varieties.

During chill storage at 4C for 12 days, except for Thai jasmine cookedrice cooled by air blast, the hardness of the other cooked rice samples wasgradually increased during storage; therefore, starch retrogradation of cookedrice depends on not only cooling speed but also rice variety. Rapid cooling andlow amylose content are two key factors to retard starch retrogradation duringchill storage.

ACKNOWLEDGMENTS

This project was conducted by the FRCFT Group, UCD, and Prof. YingMa would like to acknowledge the valuable assistance provided for the workby the staff in FRCFT, particularly Dr. Adriana Delgado.


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HARDNESS OF COOKED RICE AS AFFECTED BY VARIETIES, COOLING METHODS AND CHILL STORAGE

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