Effect of ohmic blanching on drying kinetics, physicochemical and functional properties of garlic powder

Abstract

This study aims to perform an optimization of process parameter for ohmic blanching of garlic to focus on the drying characteristics of the garlic powder at different temperatures. Comparative analyses on physicochemical and functional properties of differentially blanched garlic powder are carried out. The browning intensity was found to be lesser in garlic with ohmically blanched at 26.66 V/cm for 30 s. Process optimization was carried using different thin layer models, out of which Midilli–Kucuck was found to best fit model (R² = 0.9954). Rate of drying was significantly higher in ohmically blanched garlic compared to conventional blanching. Obtained garlic powder by differential blanching methods was analyzed for physicochemical and functional attributes specifically; diallyl disulphide content was retained up to 945.8 mg/kg, 928.7 mg/kg and 667.6 mg/kg, respectively.

Introduction

India develops a substantial quantity of fruits and vegetables throughout the world, with post-harvest losses of more than 25%. For human consumption, fruits and vegetables are the main sources of vitamins and minerals. High post-harvest losses demand to preserve the agricultural produce, thereby reducing scarcity during unavailability. Garlic is a spiced vegetable that is available throughout the year. It is known for its health benefits mainly due to various bioactive compounds such as organosulphur compounds, phenolic compounds, and polysaccharides. Garlic consists of a bulb containing pungent cloves. It can be processed into various forms including dried garlic powder, garlic flakes, raw garlic, garlic pulp and oil. Garlic is likely susceptible to degradation due to respiration and microbial spoilage that occurs during transportation and storage (Dawn and Shreenarayanan 1997). Removal of moisture content from the garlic cloves is a significant part of food processing. Drying is a substitutive method to minimize the losses up to a particular extent (Gupta et al. 2013). It works on the principle of lowering water activity and reduction of moisture level, thereby increasing the shelf-life of the product. Water vaporization heat is large during the process of drying and so it is considered as an energy-intensive process (Compaore et al. 2019). Tray drying is a common process used for garlic. Drying of fruit and vegetable products is an important form of preventing degradation, as it decreases water activity. Garlic, as agricultural produce, is rich in phosphorus, sulphur, potassium, selenium, vitamin and zinc. The garlic skin protects the clove from losing water to the surrounding environment. Drying causes some quality changes like loss of flavour, colour and rehydration behaviour during drying of garlic. This can be controlled by blanching garlic before drying. Blanching not only prevents the enzymatic reaction but also influences the heat and mass transfer by increasing the cell permeability. Blanching by the conventional method is done by immersing the produce in hot water (70–100 °C) for a particular time. Due to the low heat penetration rate, the texture of the product and rehydration property of the product gets affected. Ohmic heating can be used as a substitute method for blanching familiarly known as ohmic blanching (OB). The conversion of alternating electric current that passes through the food which is placed in between electrodes, leading to electrical resistance thereby generating heat in the food. Blanching by the ohmic heating method has many advantages including reduced leaching of solids, maintain product quality and nutritional value as compared to conventional blanching (CB) (Castro et al. 2004; Wang and Sastry 2002).

For an intended drying system, it is significant to study the drying kinetics of the sample of interest. It determines the drying time which has a direct effect on the dryer unit size. Drying kinetics is the transport properties of the food material and is dependent on the biological, chemical and physical properties of the food sample. The transport properties are important to determine the operating conditions such as velocity, air temperature, and humidity (Compaore et al. 2019). During the drying process, the mathematical models are used to analyze the heat and mass transfer. They predict thin layer drying at different conditions. Based on the internal and external resistance, thin-layer drying is classified into three categories including theoretical, semi theoretical and empirical. Under semi theoretical modelling falls Henderson and Pabis model, Wang and Singh model, Lewis model, Two-term exponential model and Page model. These models are based on the external resistance to moisture transfer between product and air.

Organosulphur compounds, saponins, phenolic compounds and polysaccharides are the bioactive compounds present in garlic. Organosulphur compounds such as diallyl disulphide (DADS), diallyl thiosulfonate, etc. are the major active constituents. It is reported that diallyl disulphide could prevent non-alcoholic fatty liver diseases, protects the digestive system (Lai et al. 2014). DADS also have the anticancer activity of preventing tumorigenesis (Saud et al. 2016). It has protective effects against chemically induced toxicity and carcinogenesis, cardiovascular, and age-related diseases. DADS is responsible for the odour and flavour of garlic. The activity of this compound occurs when a cell is broken, alliin break to produce allicin with the activation of the allinase enzyme. Allicin is attributed to the strong aroma of garlic and by a reduction reaction, it is converted to diallyl disulfide (Chunli et al. 2015).

This manuscript discusses the optimization of the process parameter for blanching garlic ohmically (different voltage gradients) and conventionally at different treatment period. Also, the drying kinetics study of garlic at various drying temperatures is done. The presented work investigates the effect of blanching and drying temperature on drying time, rehydration ratio, enzymatic browning, other physicochemical and functional properties of garlic.

Materials and methods

Sample preparation

Garlic of Ooty 1 variety was collected from a wholesale market, Guduvanchery. The initial moisture content of the garlic was obtained by oven dry method. The average moisture content was found to be 65.6%.

Ohmic heating apparatus

The ohmic heating system consists of a voltmeter, ammeter, power supply, titanium electrode and step-up transformer. The area of an ohmic heating cell used is 67.2 cm². The distance between the titanium electrodes is 6 cm. A Teflon coated thermometer is used to measure the temperature at different locations of the cell and the temperature was uniform when measured at different locations (Poojitha and Athmaselvi 2016).

Blanching of garlic

Garlic was peeled manually and cut into sizes of equal thickness. 250 g of the sample was used for each trial. In the ohmic heating chamber with a capacity of 500 ml, 350 ml water was added and ohmically heated at 13.33, 20 and 26.66 V/cm (for optimization). When it reached the boiling temperature, 250 g of garlic was charged and blanched for 10, 30 and 50 s. It was then subjected to cold water and pat dried to remove excess water. Similar way conventional blanching was also done to compare the efficiency. In a vessel with a capacity of 500 ml and dimension similar to the ohmic heating chamber, 350 ml water was added and conventional heating was done at low, medium and high heat (to have the same heating rate as that of ohmic heating). Once the water starts to boil, 250 g of garlic was charged and held for 10, 30 and 50 s. The process parameters such as voltage gradients, heating rate and time were optimized for further drying studies.

Optimization of blanching

Blanching parameters were optimized based on browning intensity and total colour difference concerning voltage gradient, conventional heating rate and treatment time.

Browning intensity

The blanched and unblanched garlic were analysed for browning intensity according to the method of Phisut and Jiraporn (2013) with slight modifications. After blanching (ohmic and conventional method). 10 g of blanched garlic slices were crushed with mortar and pestle. It was then diluted to 100 ml with distilled water and the solution was centrifuged at 3000 rpm for 15 min. The browning intensity was measured at 420 nm using UV spectrophotometer.

Colour analysis

Hunter colour lab (Colour Quest) was used to measure the colour. The measurement is based on three variables, namely L, a and b. The L value represents the lightness, a* value indicates the redness (+ve) and greenness (−ve), while the b* value signifies the yellowness (+ve) and blueness (−ve). A Standard white reference tile was used to calibrate the instrument (Trirattanapikul and Phoungchandang 2014).

The total colour difference was calculated using the following formula:

$$\text{Total}\text{Colour}\text{Difference,}\text{TCD}=\sqrt{{\left({\text{L}}_{\text{o}}-\text{L}\right)}^2+{\left({\text{a}}_{\text{o}}-\text{a}\right)}^2+{\left({\text{b}}_{\text{o}}-\text{b}\right)}^2}$$ 1

L₀ Lightness of garlic before treatment, L Lightness of garlic after treatment, a₀ Redness/greenness of garlic before treatment, a Redness/greenness of garlic after treatment, b₀ Blueness/yellowness of garlic before treatment, b Blueness/yellowness of garlic after treatment.

Drying of garlic

Unblanched and blanched garlic samples were dried in a lab-scale tray drier at 50 °C. Weight of the garlic was taken for every 30 min and moisture content was calculated until it reached the equilibrium. Three replicates of each experiment were done to assure the effect of process parameter on drying behaviour and quality of the garlic powder.

Moisture content

Moisture content was measured by hot air oven method (Association of Official Analytical Chemists 1990). The moisture content was calculated using the following formula

$$\text{Moisture}\text{content =}\frac{{\text{M}}_1-{\text{M}}_0}{{\text{M}}_1}\times 100$$ 2

where M₁ the initial weight of the sample, M₀ Final weight of the sample, Drying rate was calculated using the formula

$$\text{Drying, rate}=\frac{\mathrm{dm}}{\mathrm{dt}}=\frac{{\mathrm{M}}_{\mathrm{j}}-{\text{M}}_{\text{j + 1}}}{{\text{t}}_{\text{j}+1}+{\text{t}}_{\text{j}}}$$ 3

where dm/dt Drying rate, M_i Moisture content (% d.b) of sample at t_i, M_(i+1) Moisture content (% d.b) of sample at t_(i+1)

Physicochemical analyses of garlic powder

Physical properties including moisture content, density, colour, rehydration ratio, water absorption capacity and water solubility index of the blanched and unblanched garlic were analysed after tray drying. All tests were carried out in triplicates.

Density

5 gm of the garlic powder was taken in a measuring jar with a capacity of 25 ml. Bulk density was calculated in m/V (Kg/m³). Tapping the measuring jar will provide the tap density which is also expressed as m/V (kg/m³) (Caparino et al. 2012).

Water activity

Water activity ($a_w$) was determined using the method of Caparino et al. (2012). 2 g of the garlic powder was placed in a cup and placed inside the water activity machine till it gives a beep sound after which the water activity value was noted.

Water solubility index (WSI) and water absorption capacity (WAC)

0.5 gm of powder added to 6 ml distilled water was centrifuged at 3000 rpm for 10 min. The supernatant was dried at 105 °C to obtain a dry solid weight and the weight of residue was also measured (Anderson and Griffin 1969).

$$\text{Water}\text{solubility}\text{index},(\text{WSI})=\frac{\left(\text{Weight}\text{of}\text{dry}\text{solid}\text{in}\text{supernatant}\right)}{\left(\text{Weight}\text{of}\text{dry}\text{sample}\right)}\times 100$$ 4

$$\text{Water}\text{absorption}\text{capacity},(\text{WAC})=\frac{(\text{Weight}\text{of}\text{wet}\text{sediment})}{(\text{Weight}\text{of}\text{dry}\text{sample})-(\text{Weight}\text{of}\text{dry}\text{solids})}$$ 5

Rehydration ratio

5 grams of dried garlic slices were immersed in distilled water (50 mL) at room temperature and left to rehydrate. The garlic slices were taken out and weighed at an interval of 5 min until the weight reached constantly. The procedure was done several times to get an optimum result. Rehydration ratio was calculated by dividing the weight of rehydrated garlic slices with dry garlic slices (Trirattanapikul and Phoungchandang 2014)

$$\text{Rehydration}\text{ratio =}\frac{{\text{W}}_{\text{r}}}{{\text{W}}_{\text{d}}}$$ 6

W_r the weight of the rehydrated product, W_d the weight of the dried product.

Mathematical models of drying

To fit the experimental data of garlic, six mathematical models were used viz., Lewis model, Henderson and Pabis model, Newton model, Wang and Singh model, Page model, and Midilli-Kucuk. The formulae used were as follows

$$\text{Wang}\text{and}\text{Singh}\text{model}\left(\text{Mohapatra}\text{and}\text{Srinivasa}2005\right),\text{MR}=1+\text{at}+{\text{bt}}^2$$ 7

$$\text{Henderson}\text{and}\text{Pabis}\text{model}\left(\text{Henderson}1974\right),\text{MR}={\text{ae}}^{-\text{kt}}$$ 8

$$\text{Page}\text{model}\left(\text{Page}1949\right),\text{MR}=exp\left(-{\text{kt}}^{\text{n}}\right)$$ 9

$$\text{Lewis}\text{model}\left(\text{Bruce}1985\right),\text{MR}=exp\left(-\text{kt}\right)$$ 10

$$\text{Newton}\text{model}\left(\text{Aregbesola}\text{et}\text{al}.2015\right),\text{MR}=exp\left(\text{k}\ast \text{t}\right)$$ 11

$$\text{Midilli}-\text{Kucuk}\text{model}\left(\text{Midilli}\text{et}\text{al}.2002\right),\text{MR}=\text{a}exp(-{\text{kt}}^{\text{n}})+bt$$ 12

Drying kinetics models were analysed by plotting Moisture ratio (MR) against drying time.

$$\text{Moisture}\text{ratio}(\text{MR})=\frac{{\text{M}}_{\text{i}}-{\text{M}}_{\text{e}}}{{\text{M}}_{\text{o}}-{\text{M}}_{\text{e}}}$$ 13

where M_i is the moisture content at a particular time, M_e is the equilibrium moisture content and M_o is the initial moisture content.

Analysis of diallyl disulphide content by HPLC method

Garlic powder was analysed for diallyl disulphide. The quantification of diallyl disulphide content was done by High-Performance Liquid Chromatography (HPLC).

Solvent preparation

The solvent used for extraction of diallyl disulphide content from garlic powder was 90% methanol containing 0.01 N dilute Hydrochloric acid.

Standard preparation

HPLC (Reference) Standard of Diallyl disulphide was obtained from Sigma Aldrich. Working standard concentration ranging from 25 to 175 ppm was prepared with 90% methanol containing 0.01 N dilute hydrochloric acid. 1 ml from each concentration was pipetted out into vials for HPLC analysis.

Sample preparation

1 g of garlic powder was added to 30 ml of 90% methanol containing 0.01 N dil HCl. The mixture was shaken for 30 min and made up to 50 ml with the solvent mixture. It was then centrifuged at 1000 rpm for 15 min using benchtop REMI R-8C microcentrifuge (India). 1 ml from the supernatant was pipetted out into vials for HPLC analysis.

HPLC condition

A Shimadzu UFLC (LC-20AD, Japan) system coupled with a C18 column was used. HPLC conditions were as follows column; column temperature, 25 °C; flow rate 1 ml/min; mobile phase, methanol/water (86:14 v/v); wavelength, UV 210 nm; injection volume 20 µL (Ichikawa et al. 2006).

Statistical analysis

Three replications of colour, rehydration ratio, moisture content, water activity, bulk density, tap density, water absorption capacity and water solubility index were used to determine each parameter. MINITAB Version 17 for Windows was used to compute analysis of variance (ANOVA). Posthoc multiple pairwise Tukey test was performed to determine the significant temperature at a 95% confidence interval (Trirattanapikul and Phoungchandang 2014).

Results and discussion

Effect of blanching on browning intensity and total colour difference of garlic

Peeled garlic generally suffers undesirable changes in quality, such as rapid browning, due to polyphenol oxidase and peroxidase enzymes, which can be inactivated using thermal blanching. Optimization of blanching treatment is done based on browning intensity and total colour difference of garlic and is reported in Fig. 1. Browning intensity increased concerning the applied voltage gradient and treatment time. The absorbance value ranged from 1.63 to 2.49 for ohmic blanched garlic and 2.23 to 3.16 for conventionally blanched garlic. Another factor responsible for the browning intensity is the increase in pH which occurs due to application of heat. Maillard reaction or non-enzymatic browning reaction strongly depends on the temperature and pH. When the garlic was ohmic blanched at 26.66 V/cm for 30 s, the residence time was less and so the browning intensity was significantly lower when compared to garlic blanched for 50 s. Similar way garlic conventionally blanched at high heat for 30 s showed less browning than other treatments. The results were in agreement with Wong et al. (2008), in which Maillard reaction between amino acid and glucose was examined under acidic condition. Similar results are also reported in Karseno et al. (2017), where the effect of pH and temperature on browning intensity is studied.

Fig. 1.

Effect of differential blanching on a browning intensity and b total colour difference of garlic

Colour is one of the most important appearance attributes. Undesirable changes in colour may lead to a decrease in consumer’s acceptability and market value. Colour has a good correlation with the antioxidant abilities, oxidation and Maillard reactions. The total colour difference is considered as an indicator to evaluate the severity of the heat treatment and to predict the corresponding quality degradation caused by the blanching process. The total colour difference is also calculated to optimize the blanching parameter. It is done by using L, a, b value obtained before and after blanching. It indicates that due to less residence time, garlic ohmic blanched at 26.66 V/cm for 30 s showed less colour difference. The results proved that blanching could control the polyphenol oxidase enzyme. Similarly, the total colour difference was less in garlic conventionally blanched at high flame for 30 s. Since there was no significant difference in the browning intensity and total colour difference between 10 and 30-s treatment in both ohmic blanched (26.66 V/cm) and conventionally blanched (High heat) garlic, blanching for 30 s is considered as the optimized blanching parameter.

Change in moisture content during drying

The effect of blanching and unblanching on the difference in moisture content during the drying period is studied and plotted in the Fig. 2. With an increase in drying temperature, the moisture content decreased. It shows a non-linear decrease of moisture with drying time. The rate of removal of moisture content was high in ohmically blanched garlic compared to unblanched garlic and conventionally blanched garlic, as blanching influences the mass and heat transfer. Blanching generally increases the diffusivity of cell wall leading to faster removal of water. Initially, the material surface was saturated with water, and so with increasing air temperature, rapid drying took place between 60 and 240 min, when dried at 50 °C and 60 to 180 min, when dried at 70 °C. In the falling rate period, there is no water in the surface of material and drying was operated by moisture diffusion from the inside of the material to the surface. The equilibrium moisture content was observed as 4.9 ± 0.2%. The time taken for unblanched, conventionally blanched and ohmically blanched garlic to reach the equilibrium moisture content is 780, 720, and 660 min when dried at 50 °C. On the other hand, unblanched, conventionally blanched and ohmically blanched garlic took 540, 480, and 420 min, when dried at 70 °C. Ohmic blanching treats the garlic uniformly causing faster removal of moisture content, which is lacked in conventional blanching. In addition to the enzyme, inactivation blanching resulted in tissue softening, subsequently resulting in faster moisture removal by enhancing moisture transfer from inside the samples to the surface. Similar results were also reported in cauliflower where drying process parameter is optimized (Gupta et al. 2013).

Fig. 2.

Effect of differential processing followed by drying at 50 °C on moisture content of garlic

Effect of treatment on drying rate

The process of drying took place in the falling rate period as in Fig. 3. The drying rate ranged from 0.22 to 0.017 (Kg H₂O/h m²) when unblanched garlic is dried at 50 C, whereas it ranged from 0.37 to 0.021 (Kg H₂O/h m²) when ohmic blanched garlic is dried at 50 °C. With the drying time of 11 h, ohmic blanched garlic is found to have a higher rate of drying at 50 °C. Similarly, when dried at 70 °C, unblanched garlic had the drying rate ranging from 0.28 to 0.02 (Kg H₂O/h m²). Whereas, the drying rate of ohmic blanched garlic was in the range of 0.35 to 0.03 (Kg H₂O/h m²). The drying rate decreased with a decrease in moisture content indicating that the drying of garlic took place in the falling rate period. Ohmic blanched garlic is observed to have a faster drying rate. Similar trends were observed in garlic (Krokida et al. 2003; Sharma and Prasad 2001) tomato and pepper (Kaymak-Ertekin 2006), bitter-gourd (Mudgal and Pandey 2009) and probiotic impregnated murta (Zura-Bravo et al. 2018). The falling drying rate period ensued from the predominance of internal diffusion mechanism because of surface-bound water and product shrinkage. Due to the high temperature used for drying and due to the hydrophilic nature of garlic, the constant rate period was absent. Water molecules are tightly held by carbohydrates, proteins and sugar, which makes water movement difficult and therefore drying take place in the falling rate period (Aware and Thorat 2011). Blanching has proved to increase the drying rates in basils and carrots (Mazza 1983; Rocha et al. 1993). Generally blanching is done to improve the water permeability to the surface and to reduce the resistance of the moisture diffusion internally (Ando et al. 2016). These factors interact in a complicated manner to enhance the drying rate.

Fig. 3.

Studies on drying kinetics of garlic subjected to differential processing method and dried at 50 °C

Modelling of drying kinetics

Temperature, airflow velocity, the diffusion rate of water, sample dimensions are some of the factors that influence the drying process. Semi-permeable cell membranes get damaged due to high drying temperature, thereby leads to shrinkage and case hardening. These parameters can be minimized by doing the drying kinetics study, where a design is applied to obtain a high-quality product (Zura-Bravo et al. 2018). Thin layer modelling consists of three categories theoretical, semi-theoretical and empirical. Theoretical describes both internal and external resistance to moisture transfer. Whereas, semi- theoretical and empirical accounts for external resistance to moisture transfer between the product and the air. Page model, Henderson and Pabis model, Lewis model and Two-term model comes under semi-theoretical thin layer modelling. Wang and Singh model, and Midilli Kucuk, come under the empirical model. Among these models studied, drying method was the best fit with Wang and Singh model and Midilli Kucuk model with higher R² value of 0.9913 and 9959 for garlic ohmically treated and dried at 50 °C as in Table 1. Higher R² value results in the lower standard error value showing good fit. Wang and Singh model is the best fit for rough rice and grains.

Table 1.

Drying kinetics model of garlic powder (R² value)

Drying models	UB garlic dried at 50 °C	CB garlic dried at 50 °C	OB garlic dried at 50 °C
Lewis model	0.8657	0.9065	0.9176
Henderson and Pabis model	0.9459	0.9495	0.9633
Newton model	0.8657	0.9065	0.9176
Wang and Singh model	0.985	0.9861	0.9913
Page model	0.9808	0.9671	0.9913
Midilli Kucuck	0.9936	0.9881	0.9954

UB unblanched, OB ohmic blanched, CB conventionally blanched

Change in physicochemical properties

Garlic powder was analyzed for bulk density, true density, water activity, water solubility index, water absorption capacity, colour and rehydration ratio by AOAC methods as in Table 2. The density of food is one of the essential traits for processing, packaging, shipping and storage. During the process of drying, moisture removal tends to increase in solid content. There is no significant difference between treatment in case of bulk density and true density. Both blanched and unblanched garlic were dried at 50 °C to a moisture level of 3.2%. Water activity determines the shelf life of the product. Microbial growth is directly proportional to water content and water activity of the material. According to FSSAI regulation water activity for dried products should be below 0.6. The water activity of garlic powder was 0.4 for all treatments. Water absorption capacity (WAC) is the ability of the powder to absorb moisture from the atmosphere. Water absorption capacity is less in ohmic blanched garlic powder. There was a significant difference between the treatments. Water solubility index (WSI) is the ability to dissolve in water. With the significant difference between the treatments, solubility was less in unblanched garlic when compared to blanched garlic. Colour is an important factor for the overall acceptability of the product. And so it is expected to remain the same even after drying. The level of non-enzymatic browning and amount of colour indicators is based on the process parameters including temperature and time. Drying generally causes degradation of colour due to application of heat, affecting heat-sensitive bioactive compounds, leading to browning (Ozgur et al. 2011). On the other hand, blanching reduces colour degradation by preventing the polyphenol oxidase enzyme. The L value denotes the lightness of the product. Unblanched garlic dried at 50 °C showed a L value of 73.82 which showed an increase of 15.7% and 5.02% after Ohmic and conventional treatment respectively. Rehydration ratio of ohmic blanched garlic dried at 50 °C was high, providing the best result of ableness to gain freshness, than any other treatment and drying temperature. At high temperature, coagulation of protoplasmic protein and destruction of osmotic property of cell membrane occur thereby causing less swelling during rehydration (Gupta et al. 2013). The results are in line with ginger which was dried at 40 °C, giving the highest rehydration ratio (Phoungchandang et al. 2009).

Table 2.

Physicochemical analyses of garlic powder

Properties/treatment	UB garlic dried at 50 °C	CB garlic dried at 50 °C	OB garlic dried at 50 °C
Bulk density (g/ml)	0.83 ± 0.001a	0.83 ± 0.001a	0.83 ± 0.015b
True density (g/ml)	0.835 ± 0.005a	0.837 ± 0.005b	0.838 ± 0.01b
Moisture content (%)	3.2 ± 0.25a	3.2 ± 0.2a	3.2 ± 0.2a
Water activity (aw)	0.40 ± 0.03a	0.4 ± 0.02b	0.4 ± 0.01c
WSI	69.63 ± 0.28b	70.16 ± 0.35b	72.13 ± 0.32a
WAC	6.32 ± 0.02a	5.73 ± 0.047b	5.53 ± 0.306c
Color (L Value)	73.82 ± 0.12c	77.5 ± 1.26b	85.41 ± 0.05a
Rehydration ratio	0.81 ± 0.023a,b	0.83 ± 0.011b	0.87 ± 0.02a

UB unblanched, CB conventionally blanched, OB ohmic blanched, WSI water solubility index, WAC water absorption capacity

^a–eSignificant differences (P < 0.05) by post hoc multiple pairwise Tukey test

Effect on bioactive compounds

The functional properties of the garlic products are based on the bioactive compound present in it. In this study, diallyl disulphide content of the blanched and unblanched garlic powder is analysed. The reference standard of diallyl disulphide was prepared at different concentration using methanol and HCl. Using methanol and water as mobile phase, HPLC was performed and the chromatogram was detected at 210 nm. The retention time for diallyl disulphide is 2.9 min. A calibration curve was plotted with standard concentrations (25–175 ppm) and area. Y = 10.9X + 1905 is the linear regression equation obtained from the calibration curve with R² value of 0.992. The retention time depends upon the gas flow rate, length of the column and temperature difference between column and oven. To detect and quantify the low concentrations of a substance, the limit of detection (LOD) and limit of quantification (LOQ) were used. The LOD and LOQ for the standard diallyl disulphide are 3.5 and 10.72 mg/L. Garlic powder obtained by ohmic and conventional blanching technique was prepared for HPLC analysis and injected. The diallyl disulphide content was separated in C₁₈ column. The chromatogram thus obtained was shown in Fig. 4. Unblanched, conventionally blanched and ohmic blanched garlic dried to powder were found to have diallyl disulphide content of about 667.5, 928.7, and 945.8 mg/kg, respectively. During ohmic heating, the alternating electric current has a synergistic effect on enhancing the permeability of the plasma membrane thereby increasing the availability of bioactive compounds. Data on the effect of ohmic heating on availability of diallyl disulphide content is barely reported. Diallyl disulphide is responsible for the mechanism involved in controlling the occurrence of cancer. The degradation of the bioactive compound was influenced due to high temperature during drying.

Fig. 4.

Chromatogram of diallyl disulphide content in garlic dried at 50 °C

Conclusion

The influence of ohmic and conventional blanching on the drying behaviour, physiochemical, and functional properties of the garlic powder is investigated in this study. Blanching done ohmically at 26.66 V/cm for 30 s is found to have significantly lesser browning intensity and total colour difference. Shorter drying time and a faster drying rate were observed in blanched garlic than that of unblanched garlic. On all the treatments analysed, ohmically blanched garlic dried at 50 °C is found to have an acceptable quality with higher rehydration ratio. The drying kinetics was also observed to fit Wang and Singh model and Midilli Kucuk model with the highest R² value of 0.9913 and 0.9954. Hence it is concluded that unblanched garlic not only lowers the drying rate but also fail to maintain the quality of garlic powder. Similarly, amongst the treatments, ohmic blanched garlic dried at 50 °C is found to have good retention of diallyl disulphide content of about 945.8 mg/Kg. Conclusively, ohmic blanching helps not only in improving the drying rate of garlic, but also in maintaining the physiochemical, and functional properties.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.