Textural properties of Frozen stored Garaetteok supplemented with Agar and Casein

ABSTRACT

The effects of agar and casein on the textural characteristics of frozen stored Garaetteok, a traditional Korean rice cake, were investigated over a 20-d storage period. Garaetteok was made from rice flour with and without the addition of agar (GA), casein (GC), and a mixture of agar and casein (GAC). The moisture content of control Garaetteok (G) significantly (P < 0.001) decreased during frozen storage, whereas those of GA and GAC showed no significant decrease, although GC showed less of a decrease than G. Although the hardness and retrogradation rate of all uncooked Garaetteoks significantly (P < 0.05) increased during frozen storage, the retrogradation rate of GC did not significantly increase. After cooking, GC was the least hard and had relatively low adhesiveness and no significant (P < 0.05) difference in cohesiveness and springiness regardless of frozen storage time. These results indicated that casein was an effective additive for frozen stored Garaetteok with relatively minimal impact on textural properties compared with G. However, although agar maintained the moisture content, it was not an effective additive because of the increase in the hardness and adhesiveness of cooked Garaetteok regardless of frozen storage time.

KEYWORDS:

• Garaetteok
• Frozen stored rice cake
• Agar
• Casein
• Texture

Introduction

Garaetteok is a type of traditional Korean rice cake that is shaped into a long white cylinder. It is produced from nonglutinous rice flour by steam cooking and is screw-extruded into a chewy, white,and rod-shaped cake. Garaetteok is used in various Korean dishes, such as the spicy Tteokbokki and the traditional soup Tteokguk.^[1] The desirable qualities of Garaetteok include a low retrogradation, little loss of solids during cooking, and a good mouth feeling after cooking.^[2–5] The processing conditions, including moisture content,^[6]the number of extrusions,^[7] and flour milling,^[8,9] and rice variety^[10] are factors that affect the quality of Garaetteok. It has been reported that the processing conditions that yield Garaetteok of the most suitable quality are 40% moisture content (rice flour: water = 1: 0.4), rice soaking time of 8–10 h, and flour steaming time of 40 min.^[6,8] Rice flour is mainly heated in water to gelatinise the starch; thus, the texture of Garaetteok depends on the physical changes of the starch, which accounts for more than 70–80% of the rice content. Therefore, various surfactants, emulsifiers, and carbohydrates, including galactooligosaccharide, maltitol, and modified starch, have been used to retard the retrogradation of starch in Garaetteok.^[11–17]

Gums such as agar, carrageenans, and xanthan are added to food products mainly for their thickening and gelling properties and to improve mouth feel by changing the viscosity of solutions.^[18,19] Because agar holds water-soluble solids such as sugar without crystallisation, it is used widely in products such as bakery glazes, icings, and toppings.^[20] Owing to its enormous gelling power, agar is added into edible films to improve the puncture strength and water vapour permeability^[21,22] and gluten-free food formulations as polymeric substances mimicking the viscoelastic properties of gluten in bread dough. ^[23,24] The incorporation of dairy ingredients is fairly well established in the baking industry and has been associated with a variety of nutritional and functional benefits.^[25] Casein has been used as a structure-building ingredient or a thickening agent.^[26] Casein is used to improve the quality of frozen dough bread.^[27] A mixture of milk proteins and gums has been used to improve the freeze–thaw stability of frozen dough bread.^[28,29] A mixture of guar gum, casein, and egg white, together with rice semolina, works as a stabiliser and improves dough properties.^[30] A patent had been issued for combining agar and casein to reduce the caloric content and improve the nutritional balance of Garaetteok.^[31]

Garaetteok is produced commercially in Korea and distributed in the frozen state. However, little information is available regarding the textural properties of frozen Garaetteok. Therefore, the objectives of this research were: 1) to manufacture in the laboratory and characterise the frozen Garaetteok with the addition of agar and casein based on the previous patent^[31]; and 2) to investigate the effect of agar and casein on textural properties of frozen Garaetteok.

Materials and methods

Materials

Medium-grain polished rice imported from the US was obtained from Korean-government-controlled public rice stocks. Commercial food grade agar and casein were purchased from Samchun Pure Chemical Co. (Pyeongtaek, Korea) and Fonterra Co-operative Group (Auckland, New Zealand), respectively. The moisture contents of agar and casein were 17.92 ± 0.36% and 11.15 ± 0.52%, respectively. Other reagents were ACS reagent grade (Fisher Scientific, Fair Lawn, NJ).

Preparation of rice flour

The polished rice was steeped in ambient temperature (20 ~ 23°C) water for 4 h, drained for 2 h, and milled using a double-disk stone mill (Daehwa Co., Cheonan, Korea); the resulting flour was passed through an 18-mesh sieve. The flour moisture content, which was measured at 105°C for 60 min using an infrared moisture content meter (Moisture Analyzer MB45, Ohaus Co., USA), was 35.02 ± 1.23%. The flour was then packed in airtight plastic bags and stored at 4°C until used in experiments.

Preparation of Garaetteok

Different Garaetteoks were processed according to the protein-enriched low-calorie rice-cake and manufacturing method described in the previous patent.^[31]

Control Garaetteok

Rice flour (146 g, based on dry weight) was mixed with NaCl (1 g) in a Hobart dough mixer (N50, Hobart Co., Troy, OH, USA) for 30 s, and water according to the formula shown in Table 1 was added and mixed for 1 min. The dough was steamed by placing it in a bamboo steamer for 30 min. The steamed dough was kneaded three times in a small-scale cylinder-type rice-cake maker (MS2080, Osca electronic Co., Gimhae, Korea), fitted with a die 23 mm in diameter, by pushing the piston by hand. The final rice rod was pushed out into cold tap water through a die with a diameter of 11 mm by applying steady pressure using a piston. The rice rod, Garaetteok, was immediately scooped out of the cold water, drained over a wire sieve, laid individually on a rack, dried at ambient temperature (20 ~ 23°C) for 30 min, and wrapped with polypropylene film to prevent moisture loss. An analysis of unfrozen samples was performed on the same day of production. The frozen Garaetteok was stored at –20°C for 10 and 20 d packed in airtight plastic bags to prevent moisture loss. For analysis, frozen Garaetteok was defrosted at ambient temperature (20 ~ 23°C) for 1 h.

Table 1. Garaetteok formulas.

	Flour (g)	Salt (g)	Water (mL)	Agar (g)	Casein sol. (mL)
Control (G)	146	1	29	-	-
+ Agar (GA)	146	1	49	20.5	-
+ Casein (GC)	146	1	-	-	33
+ Agar + Casein (GAC)	146	1	16	20.5	33

Agar-containing Garaetteok

Garaetteok with the addition of agar (GA) was prepared using a mixture of rice flour, agar powder, and NaCl, as shown in Table 1. The dry ingredients, that is agar powder (20.5 g), rice flour (146 g), and salt, were mixed for 30 s in a Hobart dough mixer and passed through an 18-mesh sieve for homogeneous mixing. Because of the high water-absorption capacity of agar, it was mixed with the dry ingredients, and water was added at the end of the mixing procedure. The water (49 mL) and dry ingredients were mixed for 1 min, and the remainder of the procedure was the same as that used for the preparation of the control (G).

Casein-containing Garaetteok

Garaetteok with the addition of casein (GC) was prepared via the mixture of a casein solution with rice flour without the addition of formula water, as shown in Table 1. Rice flour (146 g, based on dry weight) was mixed with NaCl (1 g) in a Hobart dough mixer for 30 s, and then, the casein solution (33 mL) was added and mixed for 1 min. The remainder of the procedure was the same as that used for the preparation of the control. Casein solutions were prepared to achieve a final casein concentration in the Garaetteok of 4.5% (w/w). The casein solution was made according to the process of Mandala et al. ^[32] Casein (20 g, based on dry weight) was suspended in distilled water (80 mL), and 1 N NaHCO₃ was added so that the suspension pH was 6.9–7.0 at 40°C; the volume was made up to 100 mL with water. The adjustment of the pH of the solution increased the solubility of casein in water and facilitated its mixing with flour.

Agar- and casein-containing Garaetteok

Garaetteok with the addition of casein and agar (GAC) was prepared using the following procedure. First, a casein solution (33 g) and agar powder (20.5 g) were mixed in a food mixer (JW2003, Living-High-Tech., Daegu, Korea) for 1 min. The mixture of agar and casein was mixed with rice flour (146 g) and salt for 30 s and then passed through an 18-mesh sieve for homogeneous mixing. The remaining formula water (16 mL) was added, and the solution was mixed for 1 min. The remainder of the procedure was the same as that used for the preparation of the control (G).

Moisture content of Garaetteok

The modified vacuum-oven method, AACC Approved Method 44–40,^[33] was used to determine the moisture content of Garaetteok, using a vacuum dry oven (VOC-300SD, Eyela, Tokyo, Japan) at 760 mmHg, 60°C for 48 h.

Retrogradation rate of Garaetteok

The retrogradation of Garaetteok was determined using the methods of Tsuge et al.^[34] and Juliano, with modifications.^[35] Uncooked Garaetteok (1 g) was chopped into small pieces, buffered-enzyme solutions (10 mL) containing an α-amylase solution (1 mL, Type II-A from Bacillus sp., Sigma-Aldrich Chemical Co., USA) were added, and the solution was incubated for 1 h at 20°C in a shaking water bath. After incubation, the enzymatic reaction was stopped by adding 20 mL of 4 N NaOH with 95% ethanol (10 mL), and the pH of the solution was adjusted to neutrality with 4 N HCl. The volume was made up to 1 L with water. To 15 mL of this digested solution, 2 mL of 1 N HCl and 2 mL of iodine solution (0.2% I₂, 2% KI) were added, and the volume was made up to 100 mL with water. The solution was allowed to stand for 20 min, and the absorbance at 620 nm (Uvikon 930, Kontron, Zurich, Switzerland) was measured. The degree of gelatinisation (DG, %) was calculated according to the following equation:

DG (%) = (a – b)/(a – c) × 100,

where a is the absorbance of the total starch fraction, b is the absorbance of the starch fraction to be tested, and c is the absorbance of the residual material that was obtained after the complete digestion of the starch.

Instrumental texture profile analysis (TPA) of Garaetteok

The texture profile analysis of uncooked Garaetteok consisted of using a texture analyser (TA express; Texture Technologies Co., Scarsdale, NY, USA) attached to a 35-mm aluminum cylinder probe at 1 g of trigger force to compress 75% strain through the Garaetteok after its surface was detected. The compression test, pretest, and posttest speeds were 0.5, 4, and 4 mm/s, respectively. The test distance was 4 mm, and the waiting period between each compression was 1 s. Based on the TPA, the hardness (height of the peak), adhesiveness (negative area of the first compression), cohesiveness (ratio of the area under the second peak to the area under the first peak), and springiness (ratio of the distance recorded during the second compression to that recorded during the first compression) were determined. At least 10 samples of each rice variety were analysed. Garaetteok was cut into square pieces (11 × 20 mm), and frozen Garaetteok was defrosted at ambient temperature (20 ~ 23°C) for 1 h. To prevent moisture loss, it was thawed in an airtight sealed plastic bag.

To investigate the texture properties of cooked Garaetteok, it was cooked in hot water for 20 min, as per the cooking process for Tteokbokki. Square pieces (11 × 20 mm) of Garaetteok (100 mg, dry basis) in 70 mL of distilled water were boiled at 100°C for 3 min and cooked at reduced heat (60°C) for 17 min. Garaetteok that was cooked for a total of 20 min was drained and immediately used for the measurement of its textural properties, which were analysed using the same procedure as that applied in the analysis of uncooked Garaetteok.

Statistical analysis

All statistical analyses were performed using SPSS (Chicago, IL, USA). The values corresponding to moisture content and retrogradation rate are presented as the mean ± standard deviation of five replicates, and those corresponding to TPA are presented as the mean ± standard deviation of 10 replicates. Data were analysed using ANOVA and Duncan’s multiple-range test. Differences among samples were considered significant at P < 0.05.

Results and discussion

Moisture content of Garaetteok

For unfrozen Garaetteok, the moisture content ranged from 38.55% to 41.81%, as shown in Table 2. The moisture content of GC was significantly (P < 0.05) lower than those of the other samples. After Garaetteok was frozen and stored, the moisture content of G significantly decreased (P < 0.001), whereas those of GA and GAC did not. The addition of agar to GA and GAC resulted in no significant decrease, and the addition of casein to GC resulted in less of a decrease than that seen in G during the frozen storage period. These results indicate that the addition of agar or casein protected Garaetteok from moisture loss during frozen storage. Agar and casein hold water because of their internal hydrogen bonding.^[36] The branching of agar is believed to be responsible for its better hydration properties as well as its higher hydrogen-bonding activity.^[18] As a result, the enrichment of Garaetteok with casein may improve the processing properties of the frozen dough similarly as a dough strengthener in wheat bread.^[27] Kenny et al.^[27] explained that casein improves the gluten network by increasing water absorption and renders the dough more expandable, thus accounting for the positive effect on frozen dough quality.

Table 2. Moisture content of uncooked Garaetteok.

	Unfrozen	10-day frozen	20-day frozen	P
Control (G)	41.81 ± 0.18 b	37.99 ± 0.20b	37.85 ± 0.50b	< 0.001
+ Agar (GA)	40.91 ± 1.16 b	40.48 ± 1.03c	39.46 ± 0.37c	0.216
+ Casein (GC)	38.55 ± 0.74 a	36.17 ± 1.01a	35.67 ± 1.41a	0.036
+ Agar + Casein (GAC)	40.67 ± 1.27 b	39.20 ± 0.33bc	38.46 ± 0.49bc	0.189

Data are expressed as mean ± standard deviation.

P value: ANOVA by frozen storage.

Within columns means with the same subscript are not significantly different (P<0.05).

Retrogradation of Garaetteok

The retrogradation rate of unfrozen Garaetteok decreased significantly (P < 0.05) with the addition of agar (GA) and agar/casein (GAC), as shown in Table 3. As the frozen storage time increased, the retrogradation rate increased significantly (P < 0.001) in G and GAC. The smallest increase was observed in GC (P = 0.083). The retardation effect on the retrogradation may be related to the emulsifier action of casein on the starch. It has been reported that casein plays a role as an emulsifier and retards the retrogradation of starch in frozen wheat flour bread.^[27] The addition of milk proteins and gums were also reported to be effective in retarding bread staling via a slowing of the retrogradation of starch by contributing to the moisture-content-retention abilities of bread during storage.^[28]

Table 3. Retrogradation rate (%) of uncooked Garaetteok.

	Unfrozen	10-day frozen	20-day frozen	P
Control (G)	16.04 ± 0.13c	16.54 ± 0.11b	19.38 ± 0.51c	< 0.001
+ Agar (GA)	14.30 ± 0.23b	14.73 ± 0.43a	16.06 ± 0.33a	0.002
+ Casein (GC)	15.47 ± 0.29c	15.87 ± 1.56ab	17.45 ± 0.23b	0.083
+ Agar + Casein (GAC)	10.69 ± 0.50a	15.32 ± 0.10ab	17.69 ± 1.22b	< 0.001

Data are expressed as mean ± standard deviation.

P value: ANOVA by frozen storage.

Within columns means with the same subscript are not significantly different (P<0.05).

Textural characteristics of uncooked Garaetteok

The textural characteristics of Garaetteok were investigated regarding the supplementation with casein and agar and frozen storage time. As shown in Table 4, for uncooked Garaetteok, the addition of agar and casein significantly increased the hardness of unfrozen Garaetteok. As the frozen storage time increased, a significant increase was observed in all samples. In particular, after Garaetteok was frozen and stored up to 20 d, the hardness (9.49) of GC with casein was relatively lower than that of the other samples. The hardness (6.05) of unfrozen GC was significantly higher than that (4.77) of G. However, the hardness (9.49) of frozen GC was slightly but not significantly (p < 0.05) lower than that (10.23) of frozen G. Casein acted as an emulsifier and retarded the retrogradation of starch for frozen wheat flour bread.^[28,29] Shon et al.^[28] reported that casein has emulsifying properties and leads to a reduction in the hardness of bread crumbs via improved emulsion stability.

Table 4. Texture profile analysis (TPA) of uncooked Garaetteok.

		Unfrozen	10-day frozen	20-day frozen
Hardness (kg)	G	4.77 ± 0.61aA	7.75 ± 1.11aB	10.23 ± 1.56aC
	GA	7.10 ± 1.15bA	9.82 ± 1.31bB	10.24 ± 0.78aB
	GC	6.05 ± 0.50bA	7.56 ± 0.52aAB	9.49 ± 0.68aB
	GAC	7.52 ± 0.46bA	8.84 ± 0.69abB	11.66 ± 0.67bC
Adhesiveness (kg·s)	G	1.85 ± 0.67bC	0.63 ± 0.41bB	0.15 ± 0.08aA
	GA	0.33 ± 0.09aAB	0.27 ± 0.13aA	0.43 ± 0.19bB
	GC	0.58 ± 0.11aB	0.21 ± 0.10aA	0.18 ± 0.04aA
	GAC	0.60 ± 0.05aB	0.38 ± 0.10abA	0.32 ± 0.14abA
Cohesiveness	G	0.79 ± 0.02bB	0.73 ± 0.01bA	0.78 ± 0.02bB
	GA	0.65 ± 0.02a	0.70 ± 0.01ab	0.70 ± 0.01a
	GC	0.74 ± 0.01bA	0.80 ± 0.01bB	0.78 ± 0.01bB
	GAC	0.67 ± 0.01a	0.67 ± 0.01a	0.67 ± 0.01a
Springiness	G	0.79 ± 0.03aB	0.72 ± 0.02aA	0.75 ± 0.03aA
	GA	0.70 ± 0.12a	0.70 ± 0.01a	0.76 ± 0.03a
	GC	0.75 ± 0.10a	0.77 ± 0.03b	0.75 ± 0.05a
	GAC	0.70 ± 0.03a	0.69 ± 0.04a	0.74 ± 0.03a

G: control; GA: + agar; GC: + casein; GAC: + agar + casein.

Data are expressed as mean ± standard deviation.

^a Within columns means with the same lower case subscript are not significantly different (P<0.05).

^b Within rows means with the same capital subscript are not significantly different (P<0.05).

The addition of agar and casein significantly decreased the adhesiveness of un-cooked unfrozen Garaetteok. The adhesiveness significantly decreased from 1.85 for G to 0.33 for GA. The effect of agar and casein on the adhesiveness of Garaetteok can be explained by the formation of hydrogen bonds.^[18,22,37] Agar is compatible with starch for forming intermolecular hydrogen bonds,^[36] and thus, the adhesiveness due to gelatinised starch may significantly decrease with the addition of agar. Casein may aggregate and form a complex with starch by forming hydrogen bonds.^[33,37] As the frozen storage time increased, adhesiveness decreased, except for GA. This result may be due to the surface dryness of Garaetteok during the storage period. Surface water complexed with rice flour and, as a result, decreased the stickiness of the products that stuck to the probe.

The cohesiveness of unfrozen Garaetteok decreased from 0.79 for G to 0.65 for GA with the addition of agar and casein. During the frozen storage period, the cohesiveness of Garaetteoks with added agar (GA and GAC) showed no significant change, but GC with added casein showed a significant increase. The springiness (0.79) of unfrozen G decreased to 0.70 for GA and GAC and 0.75 for GC. During the frozen storage period, the springiness of G significantly decreased, but Garaetteoks with added agar or casein (GA, GC, and GAC) showed no significant change.

Textural characteristics of cooked Garaetteok

In Korea, Garaetteok is distributed in a dry or frozen state in markets. Therefore, the boiling of Garaetteok in water is an essential step to gelatinise the starch in dishes. The textural properties of Garaetteok after 20 min of cooking are shown in Table 5. The hardness of unfrozen Garaetteok was higher in GA (1.95) and GAC (2.29) and lower in GC (1.44) compared to that in G (1.64). As the frozen storage time increased, the hardness of all Garaetteoks significantly increased. The hardness before and after cooking (Tables 4 and 5) of 20-d frozen stored GC was lowest among the 20-d frozen Garaetteoks but not significantly different with G. These results indicate that casein slightly but not significantly reduce the hardness of cooked frozen Garaetteoks. At least, casein does not increase the hardness of cooked frozen Garaetteoks.

Table 5. Texture profile analysis (TPA) of cooked Garaetteok.

		Unfrozen	10-day frozen	20-day frozen
Hardness (kg)	G	1.64 ± 0.15aA	1.82 ± 0.23aAB	2.08 ± 0.17aB
	GA	1.95 ± 0.15bA	2.30 ± 0.30bB	2.79 ± 0.52bB
	GC	1.44 ± 0.21aA	1.56 ± 0.29aA	1.82 ± 0.17aB
	GAC	2.29 ± 0.35cA	2.61 ± 0.23cA	3.18 ± 0.54cB
Adhesiveness (kg·s)	G	25.52 ± 13.30a	39.06 ± 26.29ab	54.38 ± 49.32a
	GA	59.24 ± 24.83b	69.99 ± 56.09b	62.15 ± 10.14a
	GC	22.11 ± 13.18aA	11.63 ± 12.81aA	42.37 ± 34.42aB
	GAC	33.96 ± 11.98a	55.00 ± 28.83b	61.44 ± 30.80a
Cohesiveness	G	0.80 ± 0.02b	0.77 ± 0.01b	0.80 ± 0.01b
	GA	0.63 ± 0.01a	0.61 ± 0.01a	0.63 ± 0.02a
	GC	0.76 ± 0.01b	0.77 ± 0.03b	0.79 ± 0.01b
	GAC	0.64 ± 0.01a	0.61 ± 0.01a	0.60 ± 0.01a
Springiness	G	0.90 ± 0.02b	0.90 ± 0.02a	0.91 ± 0.02b
	GA	0.88 ± 0.03b	0.89 ± 0.06a	0.84 ± 0.03a
	GC	0.90 ± 0.04b	0.90 ± 0.06a	0.89 ± 0.03b
	GAC	0.82 ± 0.04a	0.84 ± 0.03a	0.84 ± 0.03a

G: control; GA: + agar; GC: + casein; GAC: + agar + casein.

Data are expressed as mean ± standard deviation.

^a Within columns means with the same lower case subscript are not significantly different (P<0.05).

^b Within rows means with the same capital subscript are not significantly different (P<0.05).

The addition of agar significantly increased the adhesiveness (59.24) of cooked unfrozen Garaetteok (GA). Although the addition of agar decreased the adhesiveness of uncooked Garaetteok (GA) as shown in Table 4, agar absorbed more water during cooking, complexed with rice flour, and increased the stickiness of the products. However, GC and GAC with added casein showed the no significant difference with G. The adhesiveness of 20-d frozen stored GC (42.37) was significantly (P < 0.05) higher compared to that of unfrozen GC (22.11).

Cohesiveness was significantly (P < 0.05) lower in GA and GAC with added agar and but not in GC compared to that in G, regardless of frozen storage time. Frozen storage time did not affect the cohesiveness of cooked Garaetteok. Springiness was significantly lower in GAC among unfrozen Garaetteoks. There was no significant effect of agar and casein after 10 d of frozen storage, but there was a significant effect in 20-d frozen cooked Garaetteok.

Conclusion

The effects of the addition of agar and casein on the textural properties of unfrozen and frozen Garaetteoks were investigated. A significant decrease in moisture content was observed in the control G during the frozen storage period. However, GA and GAC with added agar showed no significant decrease during the frozen storage period, whereas GC with added casein showed less of a decrease than that of G. Frozen storage significantly increased the retrogradation rate except for GC (P = 0.083). Regardless of frozen storage time, 20 min cooked GC had the lowest hardness value and a relatively low adhesiveness with no significant (P < 0.05) difference in cohesiveness and springiness. Above results indicate that the addition of casein effected no significant (P < 0.05) increase in hardness and relatively minimal differences in cohesiveness, adhesiveness, and springiness of frozen Garaetteoks compared with G. Although the addition of agar retained the moisture content of Garaetteok, it increased significantly hardness during the frozen storage period. Therefore, casein is an effective additive with relatively minimal impact on textural properties, of cooked Garaetteok regardless of frozen storage time.