Effect of Standing After Preheating on the Softening Rate of Root Vegetables

Abstract

Japanese radish and potato samples were preheated at 60 – 70°C for 10 min and maintained at 20°C for 0 – 7 h to control the changes in hardness. The slopes of the first order plots for softening and the Arrhenius plots of the softening rate constant were linear. The softening rate constants of Japanese radish and potato were reduced to 18 – 49% and 23 – 50%, respectively, compared to the fresh vegetables. The optimum cooking times of Japanese radish and potato samples with respect to hardness, calculated based on the softening rate constants, were 1.9 – 6.7 and 1.5 – 4.0 times longer, respectively, compared to fresh samples. To prevent browning and the elution of soluble components, samples were vacuum-packed, then pretreated, heated and subjected to sensory evaluation. All samples were evaluated for optimum hardness.

Introduction

Texture is an important factor affecting the palatability of foodstuffs. The hardness of vegetables is a main indicator of product quality. Pretreatments, such as preheating, involve hardening and are effective for the prevention of overcooking during high-temperature heating (sterilization, etc.). Vegetables cooked at high temperatures after pretreatment are characteristically harder than vegetables that are cooked without pretreatment (Kasai et al., 1998a). Hardening, which is caused by the de-esterification of pectin by pectin esterase and the formation of a bridge bond, occurs at 50 – 80°C, and especially from 60 – 70°C (Bartolome and Johan, 1972). Generally, cell membranes lose their semipermeability above 50°C. Hardening occurs after such loss of membrane functions.

Various types of vegetable treatments that are applied before cooking have been reported, for example, preheating at approximately 60°C, pressurization (Yamamoto et al., 1992; Fuchigami et al., 2002; De Roeck et al., 2010), and addition of metal ions (Domínguez et al., 2001; Fuchigami et al., 2002). In particular, preheating of various vegetables such as potato (Bartolome and Johan, 1972; Abu-Ghannam and Crowley, 2006), sweet potato (Truong et al, 1998; Fuchigami et al., 2002), and carrot (Fuchigami et al., 1995; Ng and Waldron, 1997; Smout et al., 2005) has been investigated. However, most examinations of vegetable preheating have been carried out for 30 min to 2 h in hot water, which might cause the elution of water-soluble components. The functions of Japanese radish cell membranes diminish after pressurization for 10 min, and the softening rate constant decreases by 50% during standing after pressurization (Kasai et al., 1998a). Therefore, standing is an effective measure to control hardness. However, the softening rate constants of root vegetables during standing after preheating have not been reported.

The softening rate constant differs among pretreatment methods (Kasai et al., 1998a), and depends on the cooking time after pretreatment. The cooking time that optimizes vegetable hardness can be calculated based on predicted changes in hardness using the softening rate constant and sensory evaluation (Kasai and Shimada, 1985). Knowledge of the softening rate constants of pretreated vegetables would allow the calculation of optimum cooking times.

In this study, short preheating of the vegetables followed by standing was used to control hardness. The softening rate constant of pretreated Japanese radish or potato was measured and the optimum cooking time was predicted.

Materials and Methods

Materials    Japanese radishes and potatoes were purchased from a local supermarket in Bunkyo-ku, Japan.

Sample preparation    Japanese radishes were peeled and cut 10 cm from both ends. Potatoes were peeled. Samples were cut into 1 cm × 1 cm × 0.5 cm sections to measure the softening rate constant, and into 1-cm and 2-cm cubes for the sensory evaluation.

Pretreatment    The samples were preheated with distilled water in a 3-L beaker using a water bath (ISOTEMP228; Fisher Scientific Co., Ltd., Hampton, NH, USA) at 60°C or 70°C for 10 min. After preheating, samples were placed in a vessel and covered to prevent drying of the samples. Then, samples inside the vessel were maintained at 20°C for 0 – 7h, as standing treatment.

Heating    Fresh or pretreated samples were heated at 85°C, 90°C, 95°C, and 99.5°C for 0.25 – 180 min. Vegetables were heated with distilled water in a 3-L beaker using a water bath; the temperature was set to 85°C or 90°C. Heating at 95°C or 99.5°C was carried out in an aluminum pan (diameter, 25 cm; height, 24 cm) on a gas stove, and the heat was adjusted to maintain a water temperature of 95°C or 99.5°C (±1°C). The water temperature was measured with a K-type thermocouple (Anritsu Co., Ltd., Kanagawa, Japan) with a data collector (AP-310; Anritsu Co., Ltd.) attached. After heating, samples were immediately cooled in running water and excess surface water was absorbed with a paper towel.

Vacuum packing    Samples were vacuum packed with a vacuum packing machine (V-280A; TOSEI Co., Ltd., Shizuoka, Japan). Vacuum packed samples were used for measurement of hardness and sensory evaluation.

Measurement of hardness    The hardness of samples was measured using a texturometer (GTX-2; Zenken Co., Ltd., Tokyo, Japan) under the following conditions: a V-type plunger, clearance of 1 mm, and bite speed of 6 times/min. The maximum peak was defined as the hardness.

Kinetics of softening and optimum cooking time prediction    The hardness value was made dimensionless. The softening ratio x (-) was obtained using Eq. 1 (Kubota et al., 1978).

  

where y₀ is the initial hardness (N), y_e is the equilibrium hardness at 99.5°C (N), and y is the hardness (N) at time t.

Changes in hardness were determined based on the first-order rate law as follows:   

where k is the softening rate constant (min⁻¹). Eq. 3 is obtained from Eq. 2.

  

Changes in the hardness of vegetables were predicted by using Eq. 1 and 3.

The optimum cooking time, at which the center of the sample reaches the optimum hardness, was calculated. The optimum hardness of Japanese radish was 7.1 N and that of potato was 7.8 N, as reported by Matsuura et al. (1989).

Analysis of heat conduction    The softening rate constant depends on the temperature. The internal temperature of vegetables was determined using a three-dimensional heat conduction equation. The finite difference equation based on the heat conduction equation was applied to predict the temperature at a given point j (Kasai et al., 1998b) as follows:   

where T is the absolute temperature (K), α is the thermal diffusivity (cm²/min), m is the x-coordinate, n is the y-coordinate, and p is the z-coordinate. The calculations were performed using Δt = 1 s and Δx = Δy = Δz = 1 mm. The thermal diffusivity previously determined by Kubota et al. (1981) was used. The surface temperature of the samples was assumed to be represented by the water temperature, and the structural components of samples were assumed to remain constant (Kasai et al., 1998b).

Sensory evaluation    The degree of cooking was evaluated using a 5-point rating scale from −2 (overcooked) to +2 (undercooked). A sample hardness in the range of −0.5 to 0.5 was regarded as optimal. The ten panelists who participated in the test were members of the cookery science laboratory of Ochanomizu University.

Statistical analysis    Data was analyzed using t-tests or Tukey's test (significance level, p < 0.05) implemented in SPSS ver. 21.0.

Results and Discussion

Temperature of preheating    The effects of preheating temperature on Japanese radish and potato were investigated because the two vegetables exhibit differences in the degree of hardening (Kasai et al., 1994).

In this study, Japanese radish was preheated at 60°C following the report of Manabe (1980), which showed the optimum temperature of pectin esterase involved in hardening was approximately 60°C.

Potatoes were preheated at 60°C or 70°C for 10 min, allowed to stand in a vessel at 20°C from 0 to 7 h after treatment, and boiled for 10 min. The hardness of each sample was then measured. The hardness ratio of pretreated to untreated potatoes is shown in Fig. 1. A standing time of 0 h means no standing time after preheating. As the ratios were greater than 1, hardening occurred at both temperatures.

Fig. 1.

Hardness ratio of potato samples boiled for 10 min following preheating at 60°C or 70°C and standing at 20°C to boiled non-pretreated samples

□, 60°C; ▪, 70°C

The potato was cut into 1-cm cubes. * p < 0.05 : significantly different between 60 and 70°C at 2 and 7 h.

In particular, the hardness ratio of potatoes preheated at 70°C was significantly greater than that at 60°C. Furthermore, the hardness ratio of potatoes preheated at 70°C tended to increase as the standing time increased, although the increase was not significant. Based on these results, we set the preheating temperature of potatoes as 70°C (the temperature at which the samples become harder).

Effect of pretreatment on hardness during heating    Pretreated samples were heated at 85 – 99.5°C, and changes in hardness were measured to calculate the softening rate constant (Figs. 2, 3). Changes in hardness of pretreated Japanese radish (Fig. 2) decreased compared to that of the untreated sample. In particular, that of the sample left to stand for 7 h was noticeably delayed and the difference in changes of hardness among temperatures became smaller. Based on this result, the degree of hardening increased in a standing time-dependent manner at room temperature. Changes in hardness of potato tended to be delayed after pretreatment; however, the effect on potato left to stand for 7 h was not as large as that observed for Japanese radish.

Fig. 2.

Changes in the hardness of pretreated Japanese radish samples during cooking. Japanese radish samples were preheated at 60°C for 10 min followed by standing at 20°C

▵, 85°C; ×, 90°C; ●, 95°C; ♦, 99.5°C

Fig. 3.

Changes in the hardness of pretreated potato samples during cooking. Potato samples were preheated at 70°C for 10 min followed by standing at 20°C

▴, 85°C; ×, 90°C; ●, 95°C; ♦, 99.5°C

Kinetic analysis    The softening ratios (x) of Japanese radish and potato were calculated based on the hardness value shown in Figs. 2 and 3, and first-order plots were obtained. Only the first-order plots of vegetables after standing for 7 h are shown in Fig. 4. The plots of all other samples were linear, as in Fig. 4, and the softening rate constant at each temperature was calculated from the slope of these lines and used to generate Arrhenius plots. As an example, the Arrhenius plots of samples after standing for 7 h are shown in Fig. 5. Since the slopes of all other samples were linear, as shown in Fig. 5, the softening rate constant of pretreated samples followed the Arrhenius equation (Eq. 5), like untreated samples, as reported by Kasai and Shimada (1985).

Fig. 4.

First order plots of softening for Japanese radish and potato samples after standing for 7 h x represents the softening ratio.

▴, 85°C; ×, 90°C; ●, 95°C; ♦, 99.5°C

Fig. 5.

Arrhenius plots of the softening rate constant (k) of Japanese radish and potato samples after standing for 7 h

○, Japanese radish; ●, Potato

  

where A is the frequency factor (min⁻¹), E is the apparent activation energy (J/mol), and R is the gas constant [8.314 kJ/(mol·K)]. A and E, calculated from the Arrhenius plots, are shown in Table 1. Moreover, we calculated the softening rate constants using Eq. 5 (e.g., the softening rate constant at 99.5°C, k_99.5), and these are also shown in Table 1.

Table 1. Apparent activation energy (E), frequency factor (A), softening rate constant at 99.5°C (k_99.5), and the ratio of k_99.5 for treated to untreated samples

		Standing time (h)	E (kJ/mol)	A (min−1)	k99.5	Ratio of k99.5 for treated to untreated samples	y0 (N)	ye (N)
Japanese radish	Untreated		181	1.97 × 1024	0.094	1.00	28	2
	Preheated	0	173	8.51 × 1022	0.046	0.49	32	3
		2	223	8.70 × 1029	0.041	0.44	34	5
		7	148	1.10 × 1019	0.017	0.18	38	5
Potato	Untreated		173	3.95 × 1023	0.229	1.00	30	1
	Preheated	0	185	1.12 × 1025	0.115	0.50	30	1
		2	223	2.27 × 1030	0.122	0.53	34	1
		7	143	5.95 × 1018	0.053	0.23	36	1

Japanese radish was preheated at 60°C and potato at 70°C.

k_99.5 was calculated using the Arrhenius equation.

y₀ is the initial hardness.

y_e is the equilibrium hardness at 99.5°C.

In comparison with the k_99.5 of the untreated samples, that of the treated Japanese radish samples and potato samples was reduced to 18 – 49% and 23 – 50%, respectively. That is, the rate constant of each sample was reduced to about 50% after preheating for a short time and was reduced to about 20% during standing. These results indicate that the softening rate constant is reduced to half by preheating for only a short time. Based on the k_99.5 of Japanese radish and potato, the softening rate constant of Japanese radish was smaller than that of potato, and the degree of hardening in Japanese radish was greater than that of the potato samples.

Therefore, standing at room temperature after preheating for a short time is a useful method to reduce the softening rate from the viewpoint of hardness control.

Optimum cooking time predictions for pretreated samples    The samples were heated for the optimum cooking time that was calculated using the softening rate constant and sensory evaluation was carried out. The optimum cooking time for 1-cm and 2-cm cubes of Japanese radish and potato, which is reached when the center of the sample reaches the optimum hardness, was calculated based on the softening rate constant (Table 2). For Japanese radish samples that were preheated and subjected to standing, the optimum cooking time was between 1.9 and 6.7 times longer than for fresh radishes, and that of potato was between 1.4 and 4.0 times longer. Specifically, it takes 2.5 hours to reach the optimum hardness for Japanese radish after standing for 7 h. The effect of standing after preheating on the hardness of Japanese radish samples was greater than for potato samples, suggesting that the optimum cooking time for Japanese radish was much longer than that for potato, as the former showed a greater effect of standing. This result agreed with the previous paper that the degree of hardening for Japanese radish was more marked than for potato, carrot and burdock (Kasai et al., 1994). This might be due to the high pectin esterase activity of Japanese radish. In addition, some interactions might have occurred among tissue components (Kasai et al., 1997). Although the cooking time is prolonged by pretreatment, this pretreatment can prevent overcooking, avoiding the rapid softening following the collapse of vegetable tissues.

Table 2. Optimum cooking time (θ) of Japanese radish and potato samples and sensory evaluation scores

		Standing time (h)	θ (min)		Sensory evaluation score
			1 cm	2 cm	1 cm	2 cm
Japanese radish	Untreated	Fresh	21.3	25.5	0.20 ± 0.41	0.47 ± 0.44
	Preheated	0	43.2	47.3	0.12 ± 0.76	−0.10 ± 0.49
		2	57.9	62.2	0.38 ± 0.82	−0.03 ± 0.52
		7	143.6	147.5	−0.10 ± 0.25	−0.15 ± 0.36
Potato	Untreated	Fresh	8.0	12.2	0.45 ± 0.44	0.14 ± 0.50
	Preheated	0	14.3	18.5	0.13 ± 0.39	−0.27 ± 0.60
		2	14.8	19.2	0.02 ± 0.19	−0.40 ± 0.63
		7	32.4	36.4	−0.19 ± 0.55	−0.30 ± 0.33

θ is the calculated optimum cooking time, defined as the point at which the center of the sample reaches the optimum hardness

A 5-point rating scale from −2 (overcooked) to +2 (undercooked), (n = 10)

There were two points in the pretreatment process that needed to be considered. Since preheating did not inactivate enzymes, the potato turned brown during standing. In addition, prolonged cooking time due to the pretreatment, especially for Japanese radish, caused the elution of water-soluble components. Therefore, we examined the effect of standing after briefly preheating, followed by boiling of vacuum-packed samples to prevent browning and the elution of soluble contents. To investigate the effect of vacuum-packing on hardness, packed and non-packed Japanese radish and potato samples were heated at 99.5°C, for 5 – 30 min. The results are shown in Fig. 6. There were no significant differences in hardness between packed and non-packed samples for both fresh samples and those subjected to preheating. Vacuum-packed samples were preheated, allowed to stand, and boiled. The samples boiled for the optimum cooking time were removed from the packaging and subjected to sensory evaluation. All vacuum-packed samples boiled for the optimum cooking time after short preheating had a sensory evaluation score of between −0.5 and 0.5, indicating optimum hardness (Table 2). Therefore, the predicted optimum cooking time using the softening rate constant of the samples after preheating and standing was recognized as appropriate. In general, vacuum-packed cooking can prevent the elution of water-soluble content of foodstuff. It was especially effective for long-term cooking of the pretreated vegetables in this study.

Fig. 6.

Comparison of hardness between vacuum-packed and non-vacuum-packed samples

□, Non-vacuum-packed; ▪, Vacuum-packed

Japanese radish and potato samples were cut into 1×1×0.5 cm sections. No significant differences in hardness between non-vacuum-packed samples and vacuum-packed samples were detected.

In this study, the hardness of Japanese radish and potato samples was investigated. Our observations revealed that the softening rate constant decreased with a short duration of preheating and subsequent standing, and the optimum cooking time was prolonged. Based on these results, the softening rate constant can be controlled using a combination of preheating and standing after preheating. This pretreatment can effectively control hardness and prevent overcooking of foods at high temperatures.

Conclusion

Moderate hardening that occurred during standing at room temperature after pretreatment was quantified, indicating the effectiveness of this method to prevent the overcooking and associated loss in quality of foods. The softening rate of root vegetables could be controlled by adjusting the standing time after preheating. We believe that these observations will be useful in maintaining food quality during the food processing of vegetables.

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