Assessment of the Anti-Microbial Activity of Dried Garlic Powders Produced by Different Methods of Drying

Abstract

The anti-microbial activity of a range of garlic products including dried garlic powder produced by different methods, commercial garlic products, and garlic oil was determined against a range of selected bacteria. The bacteria included food borne pathogens, spoilage agents, and health-beneficial agents, namely Staphylococcus aureus, Escherichia coli, Salmonella typhimurium, Bacillus cereus, and a mixed lactic culture consisting of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. The dried powders were produced using air-drying at both 60 and 80°C, vacuum-drying at 50 and 60°C, and freeze-drying at −20°C. In addition, five commercial products were tested, two of which are used as spices, and three as food supplements. Before testing, the moisture content of the dried garlic powders was raised to that of fresh garlic before drying. Garlic oil was used without any addition of water. In general, the results showed that the lactic culture was the most sensitive to the growth inhibitory active compound of garlic used in this study, followed by Staphylococcus aureus. In contrast Salmonella typhimurium and Bacillus cereus demonstrated the greatest resistance to garlic. Generally, fresh garlic produced the greatest inhibition followed by freeze-dried powder. The anti-microbial activity decreased with decreasing dried garlic powder concentration. The results showed that both drying temperature and time have major effects on retaining the active components responsible for the inhibition of microbial growth. The anti-microbial substances in the moist fresh garlic were also affected by moist-heating temperatures and time. Higher heating temperatures caused faster loss of anti-microbial activity. The decrease in growth-inhibition zones for moist-heated fresh garlic followed zero-order kinetics.

Keywords:

• Garlic
• Functional food
• Anti-microbial activity
• Air drying
• Freeze drying
• Vacuum drying

INTRODUCTION

Worldwide, the demands for functional or medical foods are increasing. Functional foods have a positive impact on an individual's well-being and health, or can retard the aging process. Producing functional foods is a knowledge-based process, thus providing added value to new functional products with increased profit margins compared to conventional food products. Garlic (Allium sativum) is one of the major functional foods or food components used for human consumption. Consumers prefer natural safe preservatives in foods instead of chemical preservatives. Garlic may have the potential to be used in food products as a natural preservative, thus providing safety, as a flavoring agent and also as a medical or functional component. The medicinal use of garlic can be traced back thousands of years. The broad spectrum of activities of garlic include anti-microbial, anti-helminthic, anti-protozoal, anti-fungal, insecticidal, antitumor, anti-thrombotic, anti-cancer, antiarthritic, hypolipidemic, and hypoglycemic properties.[1,2,3,4,5]

The antimicrobial activity of garlic has been recognized for many years. Allicin, a diallyl thiosulfinate (2-propenyl-2-propenethiol sulfonate), is the active agent in this activity.[6] Allicin is formed catalytically when garlic cloves are crushed and the enzyme alliinase from the bundle sheath cells mixes with its substrate, alliin, which is released from mesophyll cells.[7] Ajoene, another active component in garlic, has been shown to be active against a variety of pathogens[8] and against rodent malaria.[9] The antibacterial activity of garlic has been studied by many scientists.[2,7,8,10] Garlic and its extracts have been shown to inhibit numerous microorganisms, including Bacillus cereus, Staphylococcus aureus, Lactobacillus plantarum, Clostridium botulinum type B (but not type E), E. coli, Salmonella typhi, Bacillus subtilis, Pseudomonas pyocyaneus, Candida albicans, and a variety of yeast like fungi.[10] Adler and Beuchat[2] investigated the viability of Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes in garlic butter (4:1 butter/garlic) as affected by the type of raw minced garlic added to the butter, storage temperature and time. All pathogens retained their viability at 4.4°C, regardless of the presence of garlic. However, the addition of garlic to butter enhanced the rates of inactivation of the three pathogens at 21 and 37°C. The most rapid decline in pathogenic populations was observed at 37°C. Lemar et al.[11] studied the effects of fresh and freeze-dried extracts of garlic on the physiology and morphology of Candida albicans, an opportunistic pathogen. A concentration range of 0–20 mg/ml was tested. Fresh garlic extract has a greater efficacy than garlic powder as indicated by its effects on morphology and inhibition of growth. Verluyten et al.[12] studied the effect of different spices on the Lactobacillus cultures used in fermented sausage. They found that garlic reduced the maximum specific growth rate but it was stimulatory for biomass production.

Generally, garlic is commercialized as a fresh product in the local market, but in recent years, the producers have looked for new commercial uses. The major processes used for garlic products are: drying, distillation, maceration in oil, hydroalcoholic short extraction, and hydroalcoholic long maceration.[13] The potential health benefits from garlic consumption are largely dependent on the process used to produce a garlic product.[13] Drying is one of the most widely used methods in developing garlic products for use as food ingredients, preservatives, and functional or medical foods. Three basic methods of drying are used today: sun drying, a traditional method in which foods dry naturally in the sun; hot air drying, in which foods are exposed to a blast of hot air; and freeze-drying, in which frozen food is placed in a vacuum (< 610 Pa) chamber to draw out the water. Drying methods and conditions significantly affect the quality of the products. In general, freeze-drying is one of the methods producing high quality products by maintaining active components. However, freeze-drying is very costly in terms of capital equipment and operating costs.[14]

Low temperature drying processes involve drying sliced fresh cloves at temperatures < 50°C for 3 to 4 days. Some allicin is formed due to the slicing process. Allicin is converted to allyl sulfides, which are largely responsible for the typical garlic odor. The final product has many of the components of the fresh garlic clove, which include γ-glutamyl cysteine, the precursor to alliin and α-allylcysteine.[13] Madamba[15] optimized the air drying process of garlic slices based on color, optical index, and rehydration ratio and found the optimum drying temperature to be 70°C for 2 mm garlic slices, while airflow rate and relative humidity showed no effect. Vazquez et al.[16] observed lower rehydration when garlic slices were dried at 40 or 50°C compared to 60°C. Jebson and He[17] developed a two-batch air-drying method to prepare high quality garlic powder at low cost. In this process, garlic was dried as large pieces at high temperatures (up to 90°C), but a smaller portion was dried as thin slices and at low temperature in order to preserve the enzyme allinase. The special garlic flavor is derived by enzymic action after the two powders are mixed and rehydrated before consumption. Madamba et al.[18] studied the effects of air-drying (70°C and 14.5% relative humidity) on the shrinkage, density, and porosity of garlic. Shrinkage was found to be fiber-oriented and different from the reported isotropic shrinkage of fruits and vegetables. Porosity decreased with the decrease of moisture content and reached a plateau and then increased with the decrease of moisture content. Sorption behavior of dried garlic slices was also reported.[19,20]

Although drying is one of the major methods used for garlic products, there is negligible information available on how different methods of drying could affect its functionality. Most of the high quality dried garlic products are developed using costly freeze-drying methods. Other low cost drying methods, such as air-drying could be used to develop dried garlic products, which may be comparable to freeze-dried products. In this case, optimum drying conditions and strategy should be used. This study was targeted to investigate the anti-microbial activities of dried garlic powder produced by different methods of drying. Four types of common food poisoning bacteria (Staphylococcus aureus, Escherichia coli, Bacillus cereus, and Salmonella typhimurium) and Lactic acid bacteria were considered to test the antimicrobial activity of garlic powders.

MATERIALS AND METHODS

Garlic Sample

Eight kilograms of fresh Chinese garlic (Allium sativum) were purchased from a local super market in the Muscat area and stored at room temperature (20°C) until used for the experiments. Average mass of 10 randomly selected garlic bulbs was 51.9 ± 2.9 g.

Drying Procedure

The bulb was separated into individual garlic gloves, peeled and any damaged ones were discarded. The average mass of 10 cloves was 4.5 ± 1.1 g. The sorted cloves were sliced into 2 mm thick strips along the longitudinal axis and spread on a stainless steel mesh tray. The air-drier (Gallenkamp, United Kingdom) was a forced convection, cabinet drier, capable of maintaining temperatures from room temperature up to 150°C. The drier was preheated to the desired temperature before placing the sample inside. Air was circulated parallel to the slices. The second sample batch was dried in a vacuum oven (Gallenkamp, United Kingdom) at less than 2 kPa pressure and the third batch was dried in a freeze-drier. For this technique the garlic slices were frozen in a freezer at −40°C for at least 48 hours, and then placed in an automatically controlled freeze drier (SP Industries Company, New York). The plate temperature was set at −20°C with a vacuum of 108 Pa in the chamber while the condensing plate temperature was set at −65°C. Table 1 shows the different drying methods with their processing temperatures and times.

Table 1 Drying techniques used.

Drying method	Temperature (°C)	Time (h)
Freeze-drying	−20	72
Air-drying	60	20
	80	20
Vacuum-drying	50	120
	60	72

Powder Preparation

The dried slices were ground with an electrical kitchen grinder to form a powder and stored in sealed containers at −18°C until used for the anti-microbial activity experiments. Two types of commercial garlic powders (as spices) referenced as A and B, two types of commercial garlic tablets marked as C and D, and commercial garlic oil capsules marked as E were purchased to compare with the dried garlic powders produced in our laboratory. The water content and total solids of each powder sample were measured gravimetrically by drying samples (3 replicates) in a convection air-oven at 105°C for at least 18 hours.[21] Moisture content was expressed on a wet basis (kg/100 kg sample). About 3 g of each sample were mixed with a predetermined mass of water in order to form a mixture having the same moisture content as fresh garlic, which was 68.2% (wet basis, kg/kg sample). No water was added to the commercial garlic oil capsules as it would be immiscible.

Antimicrobial Activity Analysis

Bacterial strains and inoculum preparation

Six types of bacteria namely Staphylococcus aureus, Escherichia coli, Salmonella typhimurium ATCC 14082, Bacillus cereus, and a mixed lactic culture consisting of Lactobacillus delbrueckii subsp. bulgaricus + Streptococcus thermophillus were selected to determine the anti-microbial activity of the different garlic products. The anti-microbial activity was determined by the diffusion agar method developed by Barry.[22] Mothershaw and Al-Ruzeiki[23] detailed this method to determine the anti-microbial activity of natural spices against Salmonella typhimurium. Bacterial cultures were prepared from stock cultures that are maintained in Sultan Qaboos College of Medicine. A loop full of each bacterial culture was put into 9.2 ml volumes of nutrient broth (Oxoid, Basingstoke, Hampshire, UK), and incubated static in air at 35°C for 24 hours except the lactic culture which was incubated at 42°C.

Preparation of bacterial media and plating

Bacterial lawns were prepared on plate count agar (PCA, Oxoid). A sterile cork borer was used to make wells (1.5 cm diameter) in the PCA. To prepare bacterial lawns, approximately 0.1 ml of each overnight culture was spread over the surface of an agar plate. Eighteen agar plates were used for each culture and three agar plates were used for each garlic powder sample. Fresh garlic was used to compare against the dried garlic samples. The fresh garlic sample was crushed using a sterile pestle and mortar to enhance the liberation of the antimicrobials[24] and also to facilitate the complete filling of the well. The wells were filled carefully with one of the garlic samples to ensure that they were completely filled (0.5 g) and so that the area of agar around the well was not contaminated with the inhibitor. The garlic oil was squeezed aseptically directly into the wells. The plates were incubated aerobically at 35°C for 24 hours after filling all wells. Following incubation, the plates were examined for zones of growth inhibition around the wells and the average diameter of the zone was measured. The same procedures were repeated for all test bacteria, except the lactic culture plates, which were incubated similarly at 42°C.

Effects of dilution on the antimicrobial activity of freeze dried powder

To determine the effect of dilution on the antimicrobial activity of garlic, the activities of different concentrations of freeze-dried garlic powder were measured using a culture of Staphylococcus aureus. The anti-microbial activities of diluted samples were tested using the same procedures described above.

Effects of moist heating on the antimicrobial activity of fresh garlic

The effect of moist heating time and temperature on the antimicrobial activity of garlic was also determined. Two different temperatures (50°C and 75°C) were used to heat the fresh garlic. The fresh garlic was peeled and sliced in order to increase the surface area and to facilitate the heating process. The sliced fresh garlic was heated in a closed, air tight, sterile container using a hot air oven. After preheating the oven the enclosed garlic samples were placed inside the oven and heated for predetermined times at the test temperatures. The heated garlic samples were stored in the freezer at −40°C until used for microbial analysis. The garlic samples were thawed and crushed prior to testing their inhibitory activity against Staphylococcus aureus. Control media for each culture without garlic samples were also included in the study to confirm culture viability.

Statistical Analysis

All data were statistically analyzed using SAS (GLM procedure, and Duncan's Multiple Range Test) to determine any significant effects of the drying and heating methods on the antimicrobial activity of the garlic.[25]

RESULTS AND DISCUSSION

Properties of Processed Garlic Powders

The moisture content of the garlic products varied (Table 2). On average fresh garlic has a moisture content of about 68% (wet basis), whereas the other products contained less than 10% moisture. Varying drying methods and drying times resulted in different levels of final moisture content in the product. The freeze-dried garlic produced in this study had the lowest moisture content of 2.3%. The standard deviations show that the variability of moisture in freeze-dried garlic samples was greater than in the other dried samples. The commercial garlic powders had moisture contents in a similar range to the garlic powders prepared in this study. The moisture content of garlic oil (commercial product E) capsules was considered to be zero.

Table 2 Moisture content of garlic samples.

Garlic sample	(n = 3) Average moisture content (%)
Fresh garlic	68.24 (1.28)
Air dried at 60°C for 20 h	7.60 (0.09)
Air dried at 80°C for 20 h	2.40 (0.06)
Vacuum dried at 50°C for 120 h	8.50 (0.24)
Vacuum dried at 60°C 72 h	5.90 (0.02)
Freeze dried at −20°C for 72 h	2.30 (1.26)
Commercial A (powder spices)	6.04 (0.05)
Commercial B (powder spices)	4.82 (0.26)
Commercial C (powder tablets)	7.12 (0.20)
Commercial D (powder tablets)	6.63 (0.18)
Commercial E (oil capsules)	0.00 (0.00)
Note: Values in parentheses are standard deviations.

The characteristics of the different powders were visually analyzed. Freeze-dried garlic powder was whiter, finer and easily free flowing compared to the vacuum dried powder followed by air-dried garlic powder. The higher temperature used in air-drying resulted in a more yellowish color, whereas the lower vacuum drying temperature caused a whiter color. This could also be due to the different drying time used in air and vacuum drying, since browning or color change depends on both the time and the temperature of drying. A clumping tendency was observed in vacuum dried powder after grinding. Commercial garlic powders were white in color, while tablets were yellowish. Commercial garlic oil was also yellowish in color.

Effect of Garlic and Garlic Products on Selected Microorganisms

Table 3 shows the inhibitory effect of different garlic products on selected microorganisms. As the moisture content of all products was raised to that of fresh garlic (68.24%) any effect of varying moisture content on the inhibitory activity was excluded. The lactic culture showed the greatest growth inhibition zone and was thus generally more sensitive to garlic products than the other types of bacteria considered in this study. Verluyten et al.[12] found that garlic extracts reduced the maximum growth rate of lactic culture (species were not reported) used in fermented sausage, but garlic addition was stimulatory for biomass production. The differences between the findings of the two studies may be associated with the difference in species of lactic acid bacteria used. For potentially pathogenic organisms, B. cereus and S. typhimurium appeared to be the most sensitive of the tested strains while S. aureus demonstrated the greatest resistance to garlic. Kumar and Berwal[26] found that E. coli was the most sensitive to garlic extracts when compared to S. aureus, S. typhi and Listeria monocytogenes. It is difficult to find any study regarding powders produced by different methods in order to compare our results. Most of the investigations in the literature used fresh garlic. In this study, generally fresh garlic showed highest inhibitory activity against all tested organisms followed by freeze-dried powder. Farbman et al.[27] found that extract of fresh garlic was active against a wide variety of Gram-positive and Gram-negative bacteria. In our study, in general, no specific trend was observed related Gram-positive versus Gram-negative.

Table 3 Growth inhibition of selected microorganisms by different garlic samples.

	Inhibition diameter (cm)
Method of drying	Staphylococcusaureus	Escherichia coli	Salmonellatyphimurium	Bacillus cereus	Lactic culture
Fresh garlic	5.50 (0.44)^ab	4.95 (0.68)^ab	4.70 (0.32)^a	4.70 (0.17)^a	8.00 (0.00)^a
Freeze-drying	5.70 (0.26)^a	5.43 (0.94)^a	4.32 (0.15)^b	4.20 (0.36)^bc	5.77 (0.40)^b
Air-drying (60°C)	4.63 (0.15)^de	4.95 (0.97)^ab	3.93 (0.20)^c	3.93 (0.64)^c	5.27 (0.31)^bcd
Vacuum-drying (60°C)	4.33 (0.50)^e	4.80 (0.35)^ab	3.80 (0.24)^c	4.57 (0.21)^ab	4.77 (0.40)^cd
Vacuum-drying (50°C)	0.00 (0.00)^g	0.00 (0.00)^f	0.00 (0.00)^g	0.00 (0.00)^e	0.00 (0.00)^f
Air-drying (80°C)	0.00 (0.00)^g	1.07 (1.20)^e	0.00 (0.00)^g	2.37 (0.06)^d	0.00 (0.00)^f
Commercial A	5.03 (0.23)^cd	4.13 (0.06)^bc	3.53 (0.06)^d	3.97 (0.12)^c	5.47 (0.15)^bc
Commercial B	5.17 (0.23)^bc	3.93 (0.06)^bc	3.97 (0.06)^c	4.00 (0.10)^c	4.67 (1.18)^d
Commercial C	3.30 (0.17)^f	3.23 (0.06)^cd	2.70 (0.10)^f	2.63 (0.12)^d	3.87 (0.15)^e
Commercial D	3.33 (0.21)^f	2.87 (0.06)^d	3.10 (0.26)^e	2.67 (0.06)^d	3.40 (0.10)^e
Commercial E	0.00 (0.00)^g	0.00 (0.00)^f	0.00 (0.00)^g	0.00 (0.00)^e	0.00 (0.00)^f
Note: Mean values within a column carrying different subscripts differ significantly (P < 0.05). Values in parentheses are standard deviations.

Garlic vacuum dried at 50°C produced no growth inhibition on any test microorganisms, probably due to the inactivation of active components during the long drying time (120 hr) (Table 3). In addition, air-drying at 80°C resulted in very low inhibition due to the higher drying temperature used. Air-drying and vacuum drying at 60°C produced powders that are active, but the potency was in general slightly lower than that produced by the freeze-dried powder. The results showed that both drying temperature and exposure time have a major role in retaining the active components responsible for the inhibition of microbial growth. The garlic oil (commercial E) showed no inhibition, which may be due to the low diffusion of active components from the oil phase into the agar medium.

Effect of Dilution on the Antimicrobial Activity of Freeze Dried Garlic Powder

Table 4 shows the effect of diluting the freeze-dried garlic powder on the inhibitory activity detected. There is a trend of decreasing inhibition with increasing dilution (Fig. 1). The inhibition decreased between 9.6 to 30.8% depending on the type of microorganism used in the experiments when the dilution factor increased from 40 to 90%. This indicated that even when the water content was increased as high as 90% the active components were still at concentrations exceeding the minimum required for bacterial growth inhibition.

Table 4 Effect of diluting the garlic sample on the growth inhibition of selected microorganisms.

	Inhibition diameter (cm)
Dilution (kg water/100 kg paste)	Staphylococcus aureus	Escherichia coli	Salmonella typhimurium	Bacillus cereus	Lactic culture
40	5.27 (0.15)^ab	4.57 (0.23)^bc	4.20 (0.26)^a	4.87 (0.32)^a	6.60 (0.10)^a
50	5.33 (0.15)^a	5.33 (0.31)^a	4.27 (0.15)^a	4.83 (0.15)^a	6.33 (0.15)^ab
60	5.37 (0.15)^a	5.20 (0.10)^a	4.13 (0.15)^ba	4.50 (0.20)^ab	6.17 (0.32)^ab
70	4.90 (0.00)^bc	4.83 (0.06)^ab	4.10 (0.10)^ab	4.30 (0.10)^b	5.80 (0.26)^bc
80	4.80 (0.10)^c	5.00 (0.26)^ab	3.73 (0.15)^bc	3.70 (0.10)^c	5.43 (0.15)^cd
90	4.33 (0.21)^d	4.13 (0.15)^c	3.40 (0.00)^c	3.37 (0.12)^c	5.17 (0.15)^d
Note: Mean values within a column carrying different subscripts differ significantly (P < 0.05).
Values in parentheses are standard deviations.
Sample size = 3.

Figure 1 Effect of dilution of freeze-dried garlic powder on growth inhibition of selected microorganisms.

Effect of Heating Time and Temperature on the Antimicrobial Activity of Fresh Garlic

The effect of heating time and temperature on the inhibition zones of fresh garlic paste for Staphylococcus aureus were tested. As the heating time increased the level of antimicrobial activity decreased (Table 5). Extended temperature exposure was much more effective at inactivation than dilution. This could be visualized from the Tables 4 and 5 based on change in temperature (50 to 75°C, i.e., 1.5 times increase) and dilution (40% to 60%, i.e., 1.5 times dilution). Fig. 2 shows decreasing trends of inhibition with the heating time at 50 and 75°C of moist garlic. The decreasing effect can be modeled with zero-order kinetics as shown in the following equation:

(1)

Table 5 Effect of heating time-temperature on the antibacterial activity of fresh garlic on the growth of Staphylococcus aure us.

	Inhibition diameters of Staphylococcus aureus induced by garlic heated at two temperatures (cm)
Time (minute)	50°C	75°C
0	5.93 (0.058)^a	5.47 (0.058)^a
5	4.07 (0.058)^bc	4.90 (0.10)^b
15	4.30 (0.10)^b	4.83 (0.058)^b
30	4.20 (0.20)^bc	4.57 (0.058)^c
60	3.97 (0.058)^cd	4.47 (0.115)^c
120	4.23 (0.252)^bc	3.93 (0.058)^d
360	3.70 (0.265)^e	2.03 (0.058)^e
720	3.77 (0.153)^de	0.00 (0.00)^f
1440	3.00 (0.00)^f	0.00 (0.00)^f
2880	1.93 (0.058)^g	0.00 (0.00)^f
Note: Mean values within a column carrying different subscripts differ significantly (P < 0.05). Values in parentheses are standard deviation.

Figure 2 Effects of heating time and temperature on the inhibitory activity of garlic on the growth of Staphylococcus aureus.

Where D is the inhibition diameter at any time (cm), D_o is the initial inhibition diameter (cm), and t is the heating time (min), respectively. The slope (k_o ) of the equation provides the inactivation rate constant (min⁻¹). The higher value of k_o indicates higher degradation of active components, thus higher heating temperature increased the destruction rate as shown in Table 6. The results in this study suggest that drying methods with different temperatures and times variably affect the antimicrobial activity of dried garlic powder product. Thus high quality dried powder could be developed using appropriate drying strategy. The high r² value (0.98) at 75°C suggested that the data fit zero order kinetics, this may be due to the sharp rate of decline in activity. The lower r² value obtained for 50°C indicated that biphasic (change in slope in the kinetics) inactivation kinetics may exist. However, more work needs to be done investigating a wider range of heating temperatures before suggesting a conclusive trend.

Table 6 Inactivation rate constant as a function of heating time.

Heating Temperature	Do (cm)	ko (min^−1)	r^2	MSE
50°C	4.95	9.07 × 10^−4	0.70	0.35
75°C	4.42	7.16 × 10^−3	0.98	0.08
Note: r^2: regression co-efficient; MSE: mean square error.

CONCLUSION

This study presents data on the anti-microbial activity of dried garlic powders produced by air-drying at 60 and 80°C, vacuum-drying at 50 and 60°C, and freeze-drying. In addition, five types of commercial products were used. The types of bacteria used were Staphylococcus aureus, Escherichia coli, Salmonella typhimurium, Bacillus cereus, and lactic culture bacteria. The results showed that growth of the lactic culture was more sensitive compared to the other types of bacteria considered in this study. In general, Bacillus cereus and Staphylococcus aureus appeared to be most sensitive while Salmonella typhimurium demonstrated the greatest resistance to garlic. Fresh garlic was the most-inhibitory followed by freeze-dried powder, overall. Commercial garlic oil showed no inhibition, which may be due to the low diffusion of active components from the oil phase into the agar medium. The anti-microbial activity decreased with decreasing dried garlic powder concentration. Higher heating temperature (75°C) caused a faster reduction of the inhibitory activity of fresh garlic during moist-heating conditions. Garlic powders produced using appropriate drying methods and strategy could be used as a natural preservative. In addition, garlic powder could be more effective in preservation when added to the food product before heating, for example as in marinades.

ACKNOWLEDGMENT

The authors would like to acknowledge the support of Sultan Qaboos University (IG/AGR/BIOR/05/01) and Norwegian Food Research Institute (MATFORSK) towards this project. They also express sincere appreciation to Dr. Shyam Sablani for assisting in preparation of freeze-dried garlic samples.