Effect of increasing the rate of water temperature on the softening of root vegetables in boiling water

Abstract

In Japan, the empirical method for preventing root vegetable collapse is to place them in room temperature water and then heating them. This study aims to determine the effects of water temperature increase rate on the softening and optimum boiling duration of the Japanese radish and potato. We found that the lower the rate of increase in water temperature, the lower the softening rate constants (k) of both the radish and potato. With decreasing rates of water temperature increase, the optimum boiling duration of radish increased. However, because the potato softened easily before boiling, the difference in optimum boiling duration among all rates of water temperature increase was very small. The k and degree of methylation (DM) of pectin showed a linear relationship, which suggested that the decrease in DM is the main reason for the reduction in k at low rates of water temperature increase.

Introduction

Vegetable texture affects food palatability. Preventing excessive softening can prevent vegetable collapse, which is an important parameter for the quality control of vegetable cooking. Pectin, a cell wall polysaccharide, defines the texture of vegetables to a large extent. Pectin contains four major domains—homogalacturonan (HG), rhamnogalacturonan-I, rhamnogalacturonan-II, and xylogalacturonan. In vegetables, the functional properties of pectin are highly dependent on characteristics of the HG domain (Liu, et al., 2020). HG is a linear homopolymer composed of α-1,4 linked D-galacturonic acid (GalA) monomers, which is susceptible to β-elimination by heating (Albersheim et al., 1960). In HG, GalA is partly methylesterified on the C-6 carboxyl group. The degree of methylation (DM) is an important parameter for β-elimination and vegetable softening (Sila, et al., 2006; Fraeye, et al., 2007). Pectin methyl esterase (PME) activity is important for texture control because it catalyzes the demethylesterification of HG.

The optimum temperature for PME activity in root vegetables is 50–60 °C (Manabe, 1980; Puri, et al., 1982; Moens et al., 2021); thus, some pretreatments at approximately 60 °C have been tested to prevent excessive softening. Sila et al. (2006) reported that low-temperature blanching at 60 °C reduces the DM and thereby β-elimination, consequently reducing the thermal texture degradation of carrot. Low-temperature blanching or high-pressure pretreatments can prevent excessive softening (Kasai, et al., 1998; Sila et al., 2006; De Roeck, et al., 2010). However, these pretreatments require temperature control and high-pressure processing equipment, which may not be available in all food service facilities. Therefore, another method is needed to overcome the drawbacks of such pretreatments.

In Japan, root vegetables are empirically to place them in room temperature (approximately 25 °C) water and then heating to prevent vegetable collapse. Endo et al. (2012) simulated the changes in potato hardness during cooking and found that the potatoes cooked with a slow rate of water temperature increase had uniform internal hardness and avoided shape collapse. Cooking potatoes from room temperature equalized the internal temperature and increased hardness before boiling. This method is simple to implement in many food service facilities because the rate of water temperature increase can be controlled by adjusting the water weight or heating power. As previous study focused only on simulations, the experimental effects of the water temperature increase rate on the softening of vegetables during boiling remained largely unexplored. Moreover, the effects of the rate of water temperature increase on the softening rate constant (k) have not been studied.

When cooking root vegetables in water, if the rate of temperature increase from room temperature to boiling point is slow, it takes longer to reach the optimum temperature of PME, which is around 60 °C. It is expected that the DM will decrease owing to the long duration of PME activation. Therefore, the effect of the water temperature increase rate on the DM should also be evaluated. This study aims to evaluate the effects of the water temperature increase rate on the k and optimum boiling duration of root vegetables. Experiments were conducted with Japanese radish, a non-starchy vegetable, and potato, a starchy vegetable. Additionally, the relationship between the DM and k was investigated.

Materials and Methods

Root vegetable samples  Japanese radishes and potatoes were purchased from a local supermarket in Bunkyo-ku, Tokyo, Japan. Japanese radishes were used on the day of purchase, while potatoes were stored at 4 °C and used within a month. Japanese radishes were peeled and trimmed by 10 cm at both ends and cut into 1 cm cubes. Potatoes were peeled and cut into 1 cm cubes. All experiments were repeated three times.

Heating treatments  Eight 1 cm cubed samples were placed in a net-shaped bag, heated in a pot with room temperature water, and boiled for 120 min to measure the changes in hardness. The water temperature increase rate was maintained at 1–10 °C/min by adjusting the heating power and water weight. For rates of 1 and 2 °C/min, a volume of 8 or 7 L of reverse osmosis (RO) water was heated in a 10 L aluminum pot, respectively. For rates of 5 and 10 °C/min, a volume of 4 or 2 L of RO water was heated in a 5 L aluminum pot, respectively. As a control, raw vegetable samples boiled for 120 min.

For the preparation of alcohol insoluble solids (AIS), 10 g of samples were placed in a net-shaped bag (n = 10 bags), heated at 1–10 °C/min rates, and removed from the water once it started boiling. The removed samples were cooled immediately in water (approximately 20 °C).

Temperature measurement  Three 1 cm cubed samples were heated at the same time. Temperature of water and center of sample were measured using a K-type thermocouple. The samples were heated until the temperature in the center reached 99 °C.

Evaluation of potato collapse  As some potatoes collapsed during boiling, the ratio of the number of collapsed potatoes to the total number of potatoes was calculated. Collapsed potatoes were characterized by the loss of the cubic shape or the crumbling of the surface; the samples that collapsed when a small force was applied were also considered.

Hardness measurement  Hardness was measured using the TA.XT.Plus texture analyzer (Stable Micro System Ltd., Surrey, UK) with a load cell of 30 kg, stainless cylinder (ϕ3 mm) probe, and test speed of 1 mm/s. The maximum peak was expressed as the hardness. The hardness of 8 samples was measured per heating time in one experiment. If the potatoes collapsed, they were removed; hence, no more than 8 were used to measure hardness.

Kinetics analysis for softening  We use dimensionless hardness values. The softening ratio (x) was obtained using Eq. 1 (Kasai and Shimada, 1985) as shown below:

  

where y is the hardness (N) at time t, and y _e is the equilibrium hardness during boiling. For control samples, y₀ is the hardness of the raw sample. For samples heated from room temperature water, y₀ is defined as the hardness at the boiling point.

The changes in hardness were determined based on the following first-order rate law:

  

where k is the softening rate constant (min⁻¹), and t is the boiling time (min). The softening analysis was conducted to determine the changes in hardness during boiling. At t = 0, x = 0, Eq. 2 can be modified to yield

  

Calculation of optimum boiling duration  The changes in hardness were simulated using the k values obtained in this study. The optimum boiling duration was calculated as the time it took for the predicted hardness to reach the optimum value. The optimum hardness was determined according to Matsuura et al. (1989); Japanese radish, 2.9 N and potatoes, 2.4 N.

Preparation of AIS  Raw and heated Japanese radishes were homogenized for 5 min at 12 000 g with 99.5 % ethanol. The homogenates were suction filtered through a Buchner-type glass filter. Subsequently, the residue was washed with 70 % ethanol until the filtrate showed a negative reaction in the phenol-sulfuric acid test (Dubois et al., 1956), and once with 99.5 % ethanol and acetone. The residue was then dried to obtain the Japanese radish AIS.

The AIS of potatoes was prepared following the method described by Marle et al. (1997). Raw and heated potatoes were homogenized for 5 min at 10 000 rpm in 99.5 % ethanol or RO water, respectively. Subsequently, 3 000 units of α-amylase (Sigma-Aldrich, USA) were added per 100 g of heated samples, and the samples were incubated at 37 °C for 6 h. The heated and raw samples were filtered through a sieve (100 µm) and washed with 70 % ethanol until the filtrate showed a negative reaction in the phenol-sulfuric acid test. The residue was washed once with 99.5 % ethanol and acetone and dried to obtain the potato AIS.

Pectin extraction  Hexametaphosphate solution (2 %, pH 4.0) was added to AIS and incubated at 80 °C for 2 h. Samples were then suction filtered through a Buchner-type glass filter. This procedure was repeated four times. The obtained filtrate was used to measure the pectin DM.

DM measurement  Pectin DM was calculated as the ratio of the moles of methanol to the moles of GalA. The amount of GalA in the pectin extract was measured using the colorimetric method described by Scott (1979). Methanol concentration was measured following the method of Anthon and Barrett (2006). The pectin extract was treated with 0.5 M NaOH and allowed to stand at room temperature for 60 min. Subsequently, 0.5 M HCl was added to neutralize the sample, and 0.01-unit oxidase per each microliter of alcohol and 200 mM phosphate buffer (pH 7.2) were added; the sample were then incubated at 30 °C for 10 min. Purpald solution (5 mg/ml) was then added and the sample was further incubated at 30 °C for 30 min. Finally, the absorbance was measured at 550 nm wavelength.

Statistical analysis  Data was analyzed using Tukey's test (significance level, p < 0.05) implemented in SPSS ver. 21.0.

Results and Discussion

Effect of the rate of water temperature increase on the shape collapse ratio and change in hardness during boiling  Temperature of water generally and at the center of the sample was compared in Fig. 1. Because changes in the temperature at the center of Japanese radish and potato were nearly identical, only the result of Japanese radish was shown in Fig. 1. When the sample is only boiled, sample temperature rose rapidly and reached 99 °C within 2 min. When heated starting from room temperature, the sample temperature increase rate was almost the same as the water temperature increase rate. Therefore, we considered that the rate of internal temperature increase is to be the same as the rate of water temperature increase in this study.

The ratio of shape collapse decreased as water temperature rate decreased (Fig. 2). At the 1 °C/min rate, no collapse was observed in samples boiled for 45 min. Low water temperature increase rates maintained the samples around 60 °C for a longer time than high rates did, thereby promoting hardening and preventing shape collapse. Because hardness measurements should use the same sample shape, we excluded collapsed samples from this study.

Fig. 2.

Changes in the ratio of shape collapse of potato during boiling.

Control indicates only boiling treatment.

No significant differences in ratio of collapse among control, and all rates of water temperature increase were detected. Mean ± S.D. (n = 3).

The changes in the hardness of Japanese radishes and potatoes were measured during heating (Fig. 3 and 4). The slower the water temperature increased, the slower the changes in the hardness of both the Japanese radish and potato. Japanese radish y₀ values (white circles) were almost identical regardless of the rate of water temperature increase, while potato y₀ values decreased as water temperature rates decreased. The y _e values (white triangle) of Japanese radish at low rates of water temperature increase were slightly higher than those at high rates. In contrast, the y _e values of potatoes were almost the same regardless of the water temperature increase rate. These findings agreed with our previous study, which demonstrated that after a brief preheating at 60 °C, the Japanese radish y _e value increased when standing at 20 °C, whereas the potato y _e value was almost the same in raw and stood samples after preheating (Sato et al., 2016). These results indicated that the effect of the water temperature increase rate on hardness is different for each kind of vegetable. Japanese radish differed in the y _e values by a high degree of hardening. In contrast, potatoes showed a difference in the y₀ values owing to a high degree of softening.

Fig. 3.

Changes in the hardness of Japanese radish during heating.

◯, y₀ (the hardness at the boiling point); △. y _e (the equilibrium hardness). Mean ± S.D. (n = 3).

Control indicates only boiling treatment.

y₀ of control sample indicates the hardness of the raw sample.

Fig. 4.

Changes in the hardness of potato during heating.

◯, y₀ (the hardness at the boiling point); △. y _e (the equilibrium hardness). Mean ± S.D. (n = 3).

Control indicates only boiling treatment.

y₀ of control sample indicates the hardness of the raw sample.

Effect of the rate of water temperature increase on the k and optimum boiling duration  The hardness measurements (Fig. 3 and 4) were used to calculate x. The first-order plots of x during boiling (Fig. 5) were used to obtain k values (slopes) (Table 1). The lower the rate of increase in water temperature, the lower the k of both Japanese radish and potato. The k values of Japanese radish were smaller than those of the potato at all rates of increase in water temperature. It has been reported that the higher the DM, the faster β-elimination (Fraeye. et al., 2007). However, DM of Japanese radish tended to larger than that of potato at all rates, as described below in Fig. 7. Previous studies have also reported that k of Japanese radish was smaller than that of potato (Matsuura, et al., 1989; Sato, et al., 2016). Matsuura et al., (1989) reported starchy vegetables tended to have higher k than non-starchy vegetables. Therefore, the difference in k among different types of vegetables is thought to be more influenced by non-pectin, such as starch, than by pectin.

Table 1 Effect of the rate of increase in water temperature on the k and optimum boiling durations of Japanese radish and potatoes.

Rate of water temperature increase	Japanese radish			Potato
	k (min−1)	R2	Boiling duration (min)	k (min−1)	R2	Boiling duration (min)
Control	0.187	0.984	7.2	0.351	0.998	6.8
10 °C/min	0.175	0.984	8.1	0.283	0.982	6.6
5 °C/min	0.128	0.990	12.0	0.197	0.961	8.7
2 °C/min	0.079	0.996	22.9	0.141	0.989	9.7
1 °C/min	0.065	0.997	23.6	0.110	0.988	8.9

k is the softening rate constant.

Control indicates only boiling treatment.

Fig. 5.

First-order plots of the softening of Japanese radish (a) and potato (b) ■, Control; ◆, 10°C/min; ●, 5°C/min; ×, 2°C/min; *, 1°C/min

Control indicates only boiling treatment.

Fig. 7.

Effect of the rate of water temperature increase on the DM of Japanese radish (a) and potato (b). DM means degree of methylation. Mean ± S.D. (n = 3).

Same letters do not differ significantly (p < 0.05). No significant differences in DM among raw and heated Japanese radish were detected.

To evaluate the effect of k on heating duration, we calculated the optimum boiling duration at which the predicted hardness reached the optimum value (Table 1). The lower the rate of increase in water temperature, the longer the optimum boiling duration for Japanese radish. Although the optimum boiling duration for potatoes was slightly longer for the slower rate of water temperature increase, the difference of it was only about 3 min and not as significant as for Japanese radish. To verify these results, the simulated changes in hardness during boiling are shown in Fig. 6. As previously described, the initial values (y₀) of Japanese radish in Fig. 6(a) were almost identical regardless of the rate of increase in water temperature. The lower the rate of increase water temperature, the slower the changes in hardness during boiling. This indicates that low rates of increase in water temperature prolonged the time in which the hardness reached the optimum value. In the case of potatoes, the lower the rate of increase in water temperature, the softer the initial value (y₀) (Fig. 6(b)). This pattern indicated that the hardness of potatoes at the boiling point is closer to the optimum hardness at low rates of increase in water temperature than at high rates. As a result, regardless of the rate of water temperature increase, the difference in the optimum boiling duration of the potato was very small, even though the k value decreased as the low rate of water temperature increase. As the k values of the potatoes were greater than those of Japanese radishes, the potatoes softened more easily before boiling than Japanese radish. Vegetable softening occurs mainly at temperatures over 80 °C (Matsuura et al., 1989). The long time that the samples remained around 80 °C during the water temperature increase softened the potato before boiling. In Japanese radish, the extent of hardening is greater than that of the potato (Kasai, et al., 1994); moreover, the hardening effect was greater than the softening effect. Based on these results, it is evident that when root vegetables are placed in them in room temperature water and then heating, both softening and hardening can occur before boiling. Additionally, the optimum boiling duration varies depending on the kind of vegetable. For Japanese radish, the slower the water temperature increase rate, the longer the optimum boiling duration, which is mainly due to the hardening effect. For potato, the difference of the optimum boiling duration was very small regardless of the water temperature increase, owing to the large effect of softening before boiling; however, the lower the rate of water temperature increase, the lower the k. In actual cooking, vegetables are cut into larger than 1 cm cube, which is used in this study. Therefore, boiling duration should be set according to the size of vegetables (Endo et al., 2012). Additionally, when some kinds of vegetables are cooked in the same pot, the size of each vegetable should be varied to achieve optimum hardness. Although it is necessary to change the boiling duration and shape in actual cooking, it is a useful to know that controlling the rate of increase in water temperature can prevent vegetables from vegetable collapse.

Effect of the rate of water temperature increase on the DM  To investigate the effect of DM on k, the DM values of Japanese radish and potato were measured at the boiling point (Fig. 7). The DM values of potatoes heated at 5 and 10 °C/min rates were higher than those of the raw sample. The reason for this may be due to the difference in the method of AIS preparation between the raw and heated samples. Therefore, the methodology for the DM measurement of raw potatoes needs to be considered. While there is no significant difference in Japanese radish, the lower the rate of increase in water temperature, the lower the DM of both Japanese radish and potato, except for raw potatoes. The optimum temperatures for PME activity in Japanese radish and potato are in the range of 50–60°C (Manabe, 1980; Puri et al., 1982; Moens, et al., 2021). Cooking vegetables from room temperature water leads to longer exposure of PME to optimum temperature. Long time passage of PME activation caused reduction of DM.

Finally, the relationship between the DM and k was evaluated (Fig. 8). As described above, the DM of raw potato is not shown in Fig. 8. The DM decreased as k decreased. It is reported that vegetables rapidly softened by cooking at pH values above 5 and below 3. When vegetables cooked at pH value above 5, softening is ascribed to the degradation of pectin by β-elimination, but at low pH it is caused by hydrolytic cleavage of pectin (Fuchigami, 1983). Since RO water was used in this study, main mechanism of softening in this study is considered to be β-elimination. It is known that the lower DM results in a reduction in β-elimination (Sila, et al., 2006; Fraeye, et al., 2007). As a result of the reduction of β-elimination, k value of Japanese radish and potato might be decreased. The correlation of Japanese radish (R = 0.990) was higher than that of the potato (R = 0.911). Additionally, vegetables soften by β-elimination (Fuchigami, 1983) and β-elimination depends on DM (Sila, et al., 2006; Fraeye, et al., 2007). However, the DM of Japanese radish was higher than that of potato at all rates, though k of Japanese radish was smaller than that of potato. The water and fiber contents of Japanese radish are 95% and 1%, respectively (Ministry of Education, Culture, Sports, Science, and Technology, Japan, 2020)ⁱ⁾; therefore, pectin greatly affects the hardness of Japanese radish. As potatoes contain a large amount of starch, the effect of polysaccharides on potato hardness might be more complex than that in Japanese radish. Therefore, DM alone cannot explain the differences in k among the different kinds of vegetables. The extent of the effect of DM on k may be slightly different from the kinds of vegetables. However, a high correlation between k and DM was revealed in this study. This pattern suggested that the reduction in DM was the main reason for the decrease in k at low rates of water temperature increase.

Fig. 8.

Relationship between k and DM.

■, Japanese radish; ●, Potato

k is the softening rate constant.

DM indicates degree of methylation.

Conclusion

Our study revealed the effect of the rate of water temperature increase on boiling duration and DM of potatoes and Japanese radish. Findings of this study may contribute to texture control of root vegetables. This method is easy and useful for preventing excessive softening and vegetable collapse in food service facilities, particularly when no temperature or pressure controllers are installed.

Conflict of interest  There are no conflicts of interest to declare.

Abbreviations

k

softening rate constant (min⁻¹)

t

boiling time (min)

x

softening ratio (−)

y

hardness at time t (N)

y₀

hardness at t=0 (N)

y _e

equilibrium hardness (N)

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• i)  https://www.mext.go.jp/content/20201225-mxt_kagsei-mext_01110_011.pdf (Dec. 28, 2022)