Thermal kinetics of enzyme inactivation, color changes, and allicin degradation of garlic under blanching treatments

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Thermal kinetics of enzyme inactivation, color changes, and allicin degradation of garlic under blanching treatments

Article in Journal of Food Process Engineering · February 2019

DOI: 10.1111/jfpe.12991

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O R I G I N A L A R T I C L E

Thermal kinetics of enzyme inactivation, color changes, and allicin degradation of garlic under blanching treatments

Zhi Huang

¹

| Quan Zhou

¹

| Wei-Liang Wu

^2,3

| Jun Wan

⁴

| Ai-Min Jiang

¹

1College of Food Science, South China Agricultural University, Guangzhou, P. R. China

2Department of Nutrition and Food Safety, Guangdong Provincial Center for Disease Control and Prevention, Guangzhou, P. R. China

3School of Public Health, Southern Medical University, Guangzhou, P. R. China

4Institute of Tropical Crops, Guangdong AIB Polytechnic College, Guangzhou, P. R. China Correspondence

Ai-Min Jiang, College of Food Science, South China Agricultural University, Guangzhou 510642, P. R. China.

Email: [email protected] Funding information

Guangdong food quality and safety key laboratory construction project; poultry products precision machining and security national local joint engineering research center construction project; poultry products processing engineering research and development center construction project

Abstract

Blanching is a feasible and effective method widely applied to prevent greening in garlic puree.

However, this processing may cause the physical and chemical changes in garlic puree. There- fore, an investigation was carried out to systematically explore the effects of hot water blanching on enzyme inactivation (alliinase andγ-glutamyl transpeptidase, GGT), discoloration, allicin degradation, and antioxidant capacity of garlic puree. The results showed that the inactivation of alliinase and GGT was strongly dependent on blanching temperature and time. Garlic puree turned into white when blanched at 80C for 5 min or at 90C for 4 min, witha*value of

−3.24 ± 0.11, −2.80 ± 0.43, respectively. The inactivation kinetics of alliinase and GGT followed a first-order reaction, as well as the kinetic of green discoloration. However, allicin content was dropped by 71, 80, and 85% after blanching for 5 min at 70, 80, and 90C, respectively. A decline in the antioxidant capacity of garlic treated by blanching was also observed. The current findings indicated that blanching could contribute to inhabit garlic greening but had negative effects on the nutritional ingredients of garlic puree.

Practical applications

Garlic is prone to greening when processed into garlic puree, garlic juice and other products.

The occurrence of greening seriously hindered the further processing of garlic. Blanching is an effective and feasible method to prevent greening in garlic puree. In this study, the effects of blanching on enzyme inactivation, color changes, allicin degradation and antioxidant capacity of garlic puree were studied. This study provides a theoretical basis for the deep processing of garlic.

1 | I N T R O D U C T I O N

Garlic (Allium sativum L.), is one of the most favorite spices frequently used in cooking. It is well-known for high medical value including antimicrobial, anti-cancer, antioxidant and anti-diabetes (Block, 1992) because of multiple active ingredients. It is processed into various products, such as garlic puree, garlic juice and garlic powder. However, occurrence of greening in garlic during the processing seriously affected the exterior quality of products. As color is one of the most important appearance attributes, undesirable changes in color of garlic may lead to a decrease in consumer’s acceptance and market value. Therefore, how to effectively control garlic greening is an essential issue.

The previous literature reported that garlic greening is a multi- step process which is similar to the pink discoloration of onion, and it

is enzymatic reactions that caused garlic greening during the processing (Cho, Park, Choi, & Lee, 2012). It is proved that the formation of the precursors of green pigments is closely related toγ-glutamyl transpeptidase (GGT) and alliinase. The green discoloration of garlic was attributed to the increased of GGT activity (Li et al., 2008). In a study on the relationship between garlic greening and GGT gene expression, Cho et al. (2012) found that garlic greening is regulated by GGT. Also, alliinase is considered as another important enzyme contributed to the formation of the precursors of greening (Lee, Cho, Kim, & Lee, 2007). The study conducted by Kubec, Hrbácová, Musah, and Velísek (2004) found that the occurrence could be observed in a model sys- tem containing 1-Propenyl cysteine sulfoxide(1-PeCSO) and alliinase.

Blanching is a particularly important heat treatment used for food pretreatment. The main purpose of blanching is to inactivate quality- DOI: 10.1111/jfpe.12991

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changing enzymes responsible for deterioration reactions that contribute to undesirable color and off-flavors. Other objectives of blanching are to reduce the microbial load of products to improve its conserva- tion, to eliminate air in the intracellular space to prevent oxidation, to dissociate the wax on the tissues, to form superficial micro-crack on the product, and to soften tissues for special texture (Deng et al., 2017; Wang et al., 2017; Xiao et al., 2017; Xiao, Bai, Sun, & Gao, 2014; Xiao, Law, Sun, & Gao, 2014). Hot water blanching is the most popular and commercially adopted blanching method. Relative to other blanching methods, such as microwave blanching, infrared blanching, and high humidity hot air impingement blanching, hot water blanching is simpler to establish and easier to operate in term of equipment required. The studies on the blanching of garlic before cooking, freezing, fermentation and drying have been reported (Beato, Sánchez, de Castro, & Montaño, 2012; Chung & Kim, 2009; Fante &

Noreña, 2013; James, Seignemartin, Stephen, & James, 2009). How- ever, to the best of our knowledge, little information is available in the literature about the effect of blanching treatments on garlic greening.

In order to produce high quality garlic puree, immediate and complete inactivation of endogenous enzymes is a necessary prerequisite.

Thus, one of the objectives of this study was to investigate the effects of hot water blanching on the inactivation of alliinase and GGT in garlic, as well as the green discoloration. Moreover, the blanching treatments resulted in a series of undesirable changes, such as the loss of nutrients. So the allicin content and antioxidant capacity of garlic puree were also studied.

2 | M A T E R I A L S A N D M E T H O D S

2.1 | Chemical reagents

Freshly harvested garlic bulbs were purchased from Henan province and stored at 4 ± 1C for 3 months until used. Trichloroacetic acid, potassium ferricyanide, and β-mercaptoethanol were obtained from Tianji Fuchen chemical reagents Co., Ltd (Tianjin, China).

1,1-Diphenyl-2-picrylhydrazyl free radical (DPPH) was supplied by TCL chemicals Co., Ltd (Shanghai, China). Allicin was purchased from Shanghai Yuanye biological Co., Ltd. (Shanghai, China). Unless indicated otherwise, the chemicals and reagents were of analytical grade.

2.2 | Blanching treatment

The same size of the garlic cloves was peeled, washed and cut into 5 mm thick slices. The samples were then distributed into water bath with 100 ml of preheated water at selected temperatures (70, 80, and 90C) and blanched for varying times (1, 2, 3, 4, and 5 min), respectively. The sample of garlic slices was cooled in an ice-water bath immediately. Finally, garlic slices were grinded for 2 min to prepare garlic puree by a laboratory size grinder.

2.3 | Alliinase extraction and its activity

Crude extraction of alliinase was carried out according to the method described by Jr and Mazelis (1979) and Mazelis and Crews (1968) with some modifications. Ten grams of garlic puree was mixed with 10 ml of

0.05 mol/L sodium phosphate buffer (pH 7.0, containing 1.085 mol/L glycerol, 5 mmol/L EDTA, 0.086 mol/L NaCl, 0.006 mol/Lβ-mercaptoethanol) and centrifuged at 11,180×g for 20 min at 4C using a centrifugal machine (5804R, Eppendorf, Germany). The supernatant was collected and precipitated using ammonium sulfate. The above pro- cedures were carried out three times to obtain 30%, 45% saturation and final product-crude alliinase extracts, respectively.

The alliinase activity was assayed by the methods described by Frie- demann and Haugen (1943) and Yoo and Pike (2001). One milliliter of 0.005 mg/ml alliin aqueous solution was mixed with 1 ml crude enzyme solution. The mixture was incubated at 25C for 5 min and 2 ml of 0.6 mol/L trichloroacetic acid then was added to inhibit the enzymatic reaction. One milliliter of 0.005 mol/L 2,4-dinitrophenylhydrazine was added and the mixture was incubated at 25C for 5 min followed by adding 5 ml of 2.5 mol/L NaOH. The absorbance was measured using a UV–visible spectrophotometer (752 N, INESA, China) at 520 nm after 10 min. One unit of enzyme activity was defined as the production of 1μmol pyruvate per unit time.

2.4 | GGT extraction and its activity

The extraction of GGT was carried out using the methods reported by Lancaster and Shaw (1994) and Iwami, Yasumoto, Nakamura, and Mit- suda (1975) with some modifications. Ten grams of garlic puree was added into 10 ml of 0.05 mol/L Tris–HCl buffer (pH 8.0, containing 1 mmol/L EDTA, 5 mmol/L β-mercaptoethanol)and centrifuged at 11,180×g for 20 min at 4C. The supernatant was saturated with ammonium sulfate to 30% and then stood still for 1 h at 4C. This pro- cedure repeated twice to obtain 70% and saturation and crude GGT extracts. The determination was performed by adding 1 ml of 40 mmol/L L-methionine, and 1 ml of 0.4 mmol/L γ-glutamic-p- nitroaniline to 1 ml of crude enzyme solution. The mixture was incubated at 37C for 30 min and then 1 ml of 1.5 mol/L acetic acid was added to inhibit the enzymatic reaction. The absorbance was determined immediately by UV–visible spectrophotometer at 410 nm. One unit of GGT activity was defined as the production of 1 μmol p- nitroaniline per unit time.

2.5 | Color measurement

The garlic puree samples were placed in Petri dishes. The color was measured by direct reading in a colorimeter (SP62, X-rite, USA) on the basis of three variables, namelyL^*,a^*, andb^*, after the instrument was calibrated against a white plate and a black plate, respectively.

2.6 | Kinetic models

The quality changes of food during processing and storage can be described by different kinetic models. Generally, during the heat treatment, the quality properties of the food (such as nutrients, and enzyme) can be described by zero-order or first-order model. There are also many studies available on color change kinetics of food during storage and thermal processing, and the color change kinetics during a thermal process have been shown to follow the first-order, zero-order models. (Ahmed, Alsalman, & Almusallam, 2014; Ahmed, Shivhare, &

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Gsv, 2000; Demiray, Tulek, & Yilmaz, 2013; Wang et al., 2018; Xiao, Bai, et al., 2014; Xiao, Law, et al., 2014; Yang et al., 2018).

The inactivation kinetics of alliinase and GTT and the kinetic of discoloration and allicin degradation were analyzed and described by zero-order and first-order reaction, as represented by Equations (1) and (2). The dependence of the rate constant on temperature followed the Arrhenius law (Schwaab & Pinto, 2007), as represented by Equation (3).

At¼A0-k*t ð1Þ

InðA_t=A₀Þ ¼−k*t ð2Þ Inð Þ ¼k Inð Þk0-E_a=RT ð3Þ whereAtis residual enzyme activity or color value at blanching timet, A0is the enzyme activity or color value at zero time, andkis the inactivation or degradation rate constant (per min) at blanching temperature. k0 is the frequency factor (per min), T is the blanching temperature (K).Eais the activation energy (KJ/mol), andRis the uni- versal gas constant.

TheDvalue is decimal reduction time, which is the time needed for reducing the initial activity by 90%, as represented by Equation (4).

The following Equation (5) was used to expressed the half-life value of inactivation.Zvalue represents the temperature range that leaded to reducing the D value by one log-cycle, as represented by Equation (6).

D¼In 10ð Þ=k ð4Þ

t1=2¼In 2ð Þ=k ð5Þ

logð Þ ¼D −T=Z ð6Þ whereTis the blanching temperature (C).

2.7 | Allicin content

Allicin content was measured using the method reported by Friede- mann and Haugen (1943) and Yoo and Pike (2001). Four grams of garlic puree were washed into 100 ml volumetric flask with 0.5 mol/L trichloroacetic acid and remained stationary for 30 min. Ten milliliters homogenate was centrifuged at 1789×g for 10 min and the supernatant was collected. The assay was performed by adding 2 ml of 0.5 mol/L trichloroacetic acid and 1 ml of 0.005 mol/L 2,4- dinitrophenylhydrazine solution to 1 ml supernatant followed by adding 5 ml of 1.5 mol/L NaOH into the mixture. The absorbance of the mixture was determined at 520 nm using a UV–visible spectrophotometer.

2.8 | Antioxidant capacity

2.8.1 | DPPH scavenging capacity

The determination of DPPH scavenging capacity was measured as the methods described by Li, Miao, Wu, Chen, and Zhang (2014) with some modifications. Two milliliters of 0.06 mmol/L DPPH solution was added into 2 ml sample solution. Then, the mixture stood still in the dark for 15 min and the absorbance (Ai) was measured at 517 nm.

Two milliliters of 0.06 mmol/L DPPH solution was added into 2 ml of distilled water, and the blank absorbance (Ao) was measured at the

same wavelength. Finally, 2 ml of distilled water was added into 2 ml sample solution, and the background absorbance (Aj) was measured at 517 nm. The scavenging rate was calculated according to the following Equation (7):

Scavenging rateð Þ ¼% 1- A_i-A_j

=Ao

×100 ð7Þ

2.8.2 | Fe³⁺reduction capacity

Fe³⁺reduction capacity was measured as the methods described by Li et al., 2014with minor modifications. Two milliliters of 0.2 mol/L phosphate buffer (pH 6.6) and 2 ml of 1% potassium ferricyanide solution were added into 2 ml of sample solution, respectively, and then the mixture was incubated at 50C for 20 min. After 2 ml of 0.6 mol/L trichloroacetic acid was added, the mixture was centrifuged at 1006×g for 10 min at ambient temperature. Next, 2 ml of distilled water and 0.4 ml of 0.006 mol/L ferric chloride solution were added into 2 ml of the supernatant. Finally, the absorbance of sample solution was measured at 700 nm using a UV–visible spectrophotometer after 10 min of reaction. Results were expressed as absorbance values.

2.9 | Statistical analysis

Each experiment was performed in triplicate and the results were presented as mean ± standard deviation. The one-way analysis of vari- ance (ANOVA) was carried out using origin software (version 8.5, OriginLab, USA).

3 | R E S U L T S A N D D I S C U S S I O N

3.1 | Effect of blanching on alliinase and GTT

The activity of alliinase and GGT in fresh garlic was 0.23 ± 0.01, 1.30 ± 0.01 U, respectively. Figures 1 and 2 illustrated the changes of enzymes activity under different blanching treatments. It can be seen that the relative activities of alliinase and GGT significantly were decreased with the increase of blanching time (p< .05). For the

FIGURE 1 The relative activity of alliinase after different blanching treatments. (■) 70C; (●) 80C; (▲) 90C

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constant time, a significant decrease in relative activity with increase was also observed with blanching temperature increasing (p< .05).

The activities of alliinase decreased to 28.03 ± 0.14, 7.93 ± 0.42, and 1.60 ± 0.12% after blanching for 5 min at 70, 80, and 90C, respectively, while the activities of GGT decreased to 30.12 ± 0.45, 16.42 ± 0.83, and 5.97 ± 2.02%, respectively, which indicated that both alliinase and GGT were unstable in harsh environment especially at high temperature.

Alliinase is easily inactivated under high temperature (Lilial &

François, 2008). A blanching treatment at 90 or 100C for 15 min appeared to achieve complete inactivation of alliinase (Mochizuki et al., 1988; Rejano, Sánchez, Ade, & Montaño, 1997). Li and Xu (2006) reported that the activity of alliinase dropped sharply at 70C.

GGT was also poorly tolerated at high temperatures. Yin, Zhu, Fu, and Li (2009) studied the enzymatic properties of GGT in Lentinus edodes, and found it was completely inactivated when kept at 70C for 30 min.

3.2 | Kinetics of enzyme inactivation

The inactivation kinetics of alliinase and GGT were analyzed at 70–90C. Tables 1 and 2 summarize the kinetic parameters of zero- order and first-order reaction, which were calculated by Equations (1)– (1)–(6).

As shown in Table 1, it was observed that the first-order kinetics resulted in higher R²than the zero-order kinetics, so the first-order kinetic model could be used to describe the inactivation of alliinase and GGT under blanching. The dependence ofkvalues on temperature followed the Arrhenius law, with a determination coefficient of 0.996 and 0.992, respectively. With the increase in temperature, the kvalues increased. For alliinase, thekvalue at 90C was 3.2 times higher than that of 70C, whilekvalue of GGT increased twofold from 70C to 90C. It demonstrated that the inactivation of enzymes is accelerated at higher temperatures.

The half-life (t1/2) and the decimal reduction time D value are important parameters commonly used in the characterization of enzyme stability. In general, the highert1/2and D value were, the lower the sensitivity to temperature was. As shown in Table 2, the declines oft1/2andDvalues were observed with temperature increasing. Thet1/2value of alliinase varied from 2.756 to 0.876 min when blanching temperature increased from 70C to 90C, while t1/2 of GGT varied from 2.765 to 1.394 min. TheDvalues of alliinase and GGT at 70C were 9.155 min and 9.185 min, which were 3.1 and 2.0 times more than the corresponding ones of 90C, respectively. These results suggested that alliinase and GGT were more susceptible above 70C and higher temperature resulted in the inactivation of alliinase and GGT more easier and faster.

TheDvalue of the sensitivity to temperature can be expressed by theZvalue, which represents the temperature range that resulted in a 10-fold change in theDvalues (Huang et al., 2014). TheZvalue is obtained by plotting theD value on a log scale against the corresponding temperature, where the slope of the curve represents−1/Z.

The results showed that the Z values of alliinase and GGT were 40.161C and 67.110C, respectively.

The activation energies of alliinase and GGT were found to be 59.414 KJ/mol and 35.430 KJ/mol, which indicated that 59.414 KJ and 35.430 KJ energies were needed for inactivation of 1 mol alliinase and 1 mol GGT, respectively. The activation energies obtained from other researchers were significantly lower than our observations. Li and Xu (2006)reported that the activation energy of alliinase in garlic was 28.47 KJ/mol. Hanum, Sinha, and Cash (1995) reported that alliinase and GGT in onion had activation energy of 16.6 and 15.8 KJ/mol, respectively. Generally, an enzyme in an intact tissue or in a FIGURE 2 The relative activity of GTT after different blanching

treatments. (■) 70C; (●) 80C; (▲) 90C

TABLE 1 The coefficient of determination (R²) of enzyme inactivation kinetics

Temperature (C)

Alliinase GGT

Zero-order First-order Zero-order First-order

70 0.969 0.970 0.940 0.973

80 0.688 0.925 0.739 0.920

90 0.602 0.943 0.644 0.923

TABLE 2 Inactivation rate (K), decimal reduction time (D), half life (t1/2),Z-value, and inactivation energy (Ea) for thermal inactivation of alliinase and GGT

Temperature(C)

Alliinase GGT

K(min⁻¹) D(min) t1/2(min) K(min⁻¹) D(min) t1/2(min)

70 0.252 ± 0.022 9.155 2.756 0.251 ± 0.201 9.185 2.765

80 0.477 ± 0.068 4.828 1.453 0.338 ± 0.050 6.808 2.050

90 0.791 ± 0.098 2.911 0.876 0.497 ± 0.072 4.630 1.394

Ea(KJ/mol) 59.414 35.430

Z-value (C) 40.161 67.110

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homogenate is more stable than in its purified form (Cheng, Zhang, &

Adhikari, 2013). The enzymes in the study carried out by Li and Xu (2006) and Hanum et al. (1995) were purified, while the enzymes were extracted without purification in this study. Similar results were also obtained in the inactivation kinetic parameters of polyphenol oxidase (Cheng et al., 2013).

3.3 | Effect of blanching on the color of garlic puree

Color is often used to predict the corresponding quality degradation caused by blanching treatment. Thea*value in the colorimeter represents greenness. The largera*value is, the lighter the green is. In this paper, thea*value was used to describe the green degradation of garlic puree (Table 3). The a* values of fresh garlic puree was

−18.31 ± 0.10, indicating that the color of fresh garlic puree was grass green. As shown in Table 3, the increases ofa*values were found with blanching time or temperature increasing. This observation indicated that green discoloration can be enhanced by improving blanching temperature or extending blanching time. The white garlic puree was obtained after blanching at 80C for 5 min (Figure 3), witha* value of−3.24 ± 0.11. The garlic puree also turned into white after blanching at 90C for 4 min (Figure 3), witha*value of−2.80 ± 0.43.

Similar results that a moderate heat treatment (80–85C, 2–4 min) could effectively prevent garlic from green discoloration were reported by Li and Zhao (2009).

3.4 | Kinetic of green discoloration

The kinetic parameters of zero-order and first-order reactions were listed in Table 4. It is observed thatR²of the zero-order and first- order kinetic model are in the range of 0.859–0.944, 0.929–0.995, respectively. Obviously, the first-order model were suitable for pre- dicting green discoloration as compared to zero-order model. Thek1

value increased by 2.3 fold from 70C to 90C. The higher the temperature was, the greater and the faster the color changed. By plotting In(k) against 1/T, it was found that the dependence of the reaction ratekon temperature followed the Arrhenius law, with a determination coefficient of 0.975. According to the Arrhenius equation, the activation energy of green degradation was 42.618 KJ/mol. The results are different from those reported by other scholars. Ahmed et al. (2010) found that after the heat treatment of garlic, the color change (expressed byL*a*/b*) conformed to the first-order kinetic model and the discoloration activation energy of garlic puree is 13.78 KJ/mol. Ahmed, Shivhare, and Singh (2004) used a first-order kinetic model to describe the change in the color of coriander leaf (expressed by a*) and the activation energy for color degradation was 29.30 KJ/mol. Ahmed et al. (2000) treated green peppers with hot water and lye, respectively, and found that the color degradation (expressed byL*a*b*) of green chili puree also followed the first-order kinetic TABLE 3 Thea*value of garlic puree at 70–90C

Time (min) 70C 80C 90C

1 −17.76 ± 0.05âA −12.36 ± 0.09âB −9.22 ± 0.1âC 2 −16.38 ± 0.69^bA −9.81 ± 0.07^bB −6.45 ± 0.06^bC 3 −14.24 ± 0.24^cA −6.49 ± 0.12^cB −4.74 ± 0.35^cC 4 −11.34 ± 0.16^dA −4.33 ± 0.04^dB −2.80 ± 0.43^dC 5 −9.24 ± 0.11êA −3.24 ± 0.11êB −1.08 ± 0.05êC The results were presented as mean ± standard deviation (n= 3). The different small letters at the same temperature represent significant difference among different time. (p< .05). The different capital letters at the same time represent significant difference among different temperatures. (p< .05).

FIGURE 3 Garlic puree treated by different blanching conditions

TABLE 4 The coefficient of determination (R²) of green degradation

Temperature(C)

Zero-order First-order

k0 R² k1 R²

70 1.907 0.908 0.140 0.929

80 2.935 0.944 0.329 0.995

90 3.061 0.859 0.516 0.966

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model, and the activation energy was 11.4 and 16.0 KJ/mol, respectively. In short, there are many reasons for the difference in activation energy, including raw materials, color representations and processing methods.

3.5 | Correlation analysis

As illustrated in Table 5, there was a significant positive correlation between enzymes activity and green discoloration, which was in line with the previous results that GGT activity positively correlated with garlic green discoloration (Li et al., 2008). GGT is responsible for cata- lyzingγ-glutamyl 1-propenyl cysteine into 1-PeCSO, then 1-PeCSO was hydrolyzed into thiosulfinates by alliinase catalysis (Lawson, Wang, & Hughes, 1991). The 1-propenyl group in thiosulfinates was considered as color developer, which could react with amino acid to form a pyrrole compound, ultimately leading to greening (Li et al., 2008). Lukes (2010) found that green degradation positively correlated with the content of 1-PeCSO. Therefore, it can be confirmed that both of alliinase and GGT closely correlated with the green discoloration of garlic puree.

3.6 | Effect of blanching on allicin content

Hot water blanching can result in degradation of some substances such as allicin, and other thermal sensitive compounds. Allicin content of fresh garlic was 2.65 ± 0.07 mg/g. Changes in allicin of garlic puree underwent different hot water blanching were shown in Figure 4.

With the increase of blanching time, allicin content decreased significantly (p< .05) at 70 and 80C. At 90C, there was no significant difference in allicin content after 3 min (p> .05). At the same blanching time, a significant decrease of allicin content with temperature increasing was also observed. Allicin content was decreased by 71, 80, and 85% after blanching at 70, 80, 90C for 5 min, respectively. The loss of allicin in blanched garlic agreed with previous report by Kinalski and Noreña (2014) who found that blanching treatment resulted in significant reduction of thiosulfinates contents of garlic.

Yin and Cheng (1991) reported that the activity of organosulfur compounds in garlic was declined under high temperatures.

Several factors may contribute to the breakdown in allicin during blanching. As alliinase was responsible for catalyzing the conversion of alliin to allicin, the significant decrease in allicin was partly caused by the inactivation of alliinase during the blanching treatment (Miron, Rabinkov, Mirelman, Weiner, & Wilchek, 1998). Another reason for

the significant loss in allicin was the self-decomposition of allicin under high temperature. Allicin is an unstable and thermolabile compound that easily decomposed to more stable substance including sulfur-containing compounds (Jansen, Müller, & Knobloch, 1989;

Rybak, Calvey, & Harnly, 2004). Allicin even broke down into diallyl disulfides and sulfur dioxides during 20 hr at the room temperature (Rivlin, Budof, & Amagase, 2006).

3.7 | Kinetic of allicin degradation

As can be seen from Table 6, the coefficients of determination (R²) of the zero-order response kinetic model ranged from 0.789 to 0.970, and those of the first-order response kinetic model ranged from 0.952 to 0.995. At the same temperature, the R²of the zero-order model were all smaller than those of first-order model, indicating that alliicin degradation of garlic puree at 70~90C could be described by the first-order kinetic model.

The reaction rateķ1increased as temperature increased, which indicated that the degradation rate of allicin was accelerated by high temperatures. It is found that the dependence of the reaction rate k on temperature was in accordance with Arrhenius’law, with a determination coefficient of 0.999. The activation energy of allicin degradation was 15.39 KJ/mol. The results of this study was similar to that published by Ilic, Nikolic, Nikolic, et al. (2011) who reported that the activation energy for allicin degradation is 14.7 KJ/mol. Kinalski and Noreña (2014) performed a blanching treatment on garlic and used a modified Logistic model to fit the degradation process of thiosulfinate TABLE 5 Correlation analysis between enzymes activity and

color (a*)

Temperature(C) Correlation coefficient

Alliinase--a* 70 0.947*

80 0.952*

90 0.934*

GGT--a* 70 0.943*

80 0.980**

90 0.999**

*p< .05,**p< .01.

FIGURE 4 The content of allicin in garlic puree after different blanching treatments. (■) 70C; (●) 80C; (▲) 90C

TABLE 6 The coefficient of determination (R²) of allicin degradation

Temperature(C)

Zero-order First-order

k0 R² k1 R²

70 0.383 0.970 0.252 0.995

80 0.377 0.901 0.295 0.965

90 0.383 0.789 0.339 0.952

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in garlic. And the degradation activation energy of thiosulfinate was found to be 7.67 KJ/mol.

3.8 | Effect of blanching on antioxidant capacity

Antioxidant capacity of garlic puree subjected to different blanching conditions was showed in Figures 5 and 6. The scavenging rate of the fresh garlic to DPPH radical was 95.08 ± 0.36%, and the absorbance of the fresh garlic was 0.234 ± 0.004 in the experiment of Fe³⁺reduction. As to DPPH, an extensive loss of antioxidant capacity was observed over time (p< .05), except for blanching at 80C after 4 min.

Regarding Fe³⁺reduction, a significant decline in antioxidant capacity also occurred with increasing blanching time, except for blanching at 70C after 4 min. For constant blanching time, there was significant difference (p< .05) in antioxidant capacity among 70, 80, 90C.

Similar reports were published in the previous literature. Kinalski and Noreña (2014) found that a significant loss of antioxidant activity was observed when garlic were subjected to blanching in water at 80 to 90C and in 100C steam. Yin and Cheng (1998) reported that antioxidant capacity of allium family was decreased under heat treatment. In a study on the effect of heating on the antioxidant capacity of garlic, Prasad, Laxdal, Yu, and Raney (1996) observed that there was about 10% loss of antioxidant capacity when garlic was subjected to 100C.

The decreasing antioxidant capacity was not only due to the loss of allicin, but also the loss of other antioxidant nutrients in garlic, including ascorbic acid, which was not determined in this study. The loss of nutrients during hot water blanching is mainly due to leaching or diffusion. All water-soluble nutrients, such as vitamins, flavors, min- erals, carbohydrates, sugars, and proteins can leach out from plant tissues to the blanching water. A reduction in the antioxidant capacity of garlic was observed because of the loss of phenolic substances, which were destroyed by heat (Lawson, Wood, & Hughes, 1991; Willett, 1994). Similarly, Ornelas-Paz et al. (2013) mentioned that the decrease in antioxidant ability was associated with hydrophilic antioxidant which was easily leached into water, such as ascorbic acid and phenolic substances.

4 | C O N C L U S I O N S

Based on the results of current experiment, it was concluded that the first-order kinetics models provided an adequate description of the behavior of the enzyme inactivation, green discoloration and allicin degradation. The best blanching condition is 4 min of blanching at 90C, which not only can have an effective enzyme inactivation to inhibit greening in garlic puree but also can lead to a better retention of allicin. In summary, although the blanching treatments could inhibit garlic greening, it resulted in the loss of allicin and a decline in antioxidant capacity. The current findings provide a more comprehensive understanding of the method of hot water blanching.

A C K N O W L E D G M E N T S

This work was supported by the foundations of Guangdong food quality and safety key laboratory construction project, poultry products processing engineering research and development center construction project, and poultry products precision machining and security national local joint engineering research center construction project.

The authors were kindly thankful for these foundations.

O R C I D

Zhi Huang https://orcid.org/0000-0003-4276-9048

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How to cite this article: Huang Z, Zhou Q, Wu W-L, Wan J, Jiang A-M. Thermal kinetics of enzyme inactivation, color changes, and allicin degradation of garlic under blanching treatments.J Food Process Eng. 2019;e12991.https://doi.org/

10.1111/jfpe.12991

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