Loss Assessment during Postharvest and Handling of Thai Garlic Used for Processing

Abstract

Garlic is one of the most economically important crops cultivated and consumed worldwide. The rising demand for garlic in the functional food market is driven by the growing interest in using processed products and supplements for benefits in health and wellbeing. Prior to processing, freshly harvested Thai garlic undergoes six distinct curing procedures; however, the losses and initial quality evaluation of the cured garlics have never been assessed. The research aims to evaluate losses and types of biomass during post-harvest processing using lab scale waste composition and mass–flow analyses, which align with the bio-circular green economic approach. Qualitative process flow diagrams (PFD) of each curing procedure were outlined, and the volume of post-harvest loss and types of biomasses were recorded. The study found that the overall losses during garlic curing were significantly higher than those associated with curing the bulb with root attached and the bulb alone. Moisture loss (>60%) was the greatest type of loss, followed by through biomass during initial and minimal processing. The aerial part accounted for >40% of total biomass loss, while root and skin were variable, depending on whether the initial process was conducted before or after curing. In terms of quality, the study found that the total phenolic and flavonoid content of garlic decreased after curing, and the level of total reducing sugar significantly decreased from the day of harvest. This result can be used as the criterion for handling Thai garlic after harvest. In addition, the biomass produced by postharvest processing can be utilised as a raw material for biorefinery extraction.

1. Introduction

Garlic is a highly traded international spice that has been consumed for thousands of years for its culinary and medicinal applications. According to the Food and Agriculture Organization (FAO) of the United Nations, global garlic production was estimated to reach more than 26 million tones in 2020, with a value of over US $9 billions [1]. In Thailand alone, the annual production is approximately 85,000 tons, and it continues to rise annually. Thai garlic is usually round or oblong and can range in size from small to large, depending on the variety and agronomic aspects [2]. The bulbs come with a circumference (8–15 cm) consisting of 10–20 individual cloves. The cloves are usually covered with a tunic of pinkish-white colour. The white fleshy part can be found when the tunic is removed. The root is connected to the base of the garlic bulb and is not usually utilized. After harvesting, fresh garlic passed through different postharvest and handling steps that involved initial processing and minimal processing, including cleaning, peeling, sorting, and curing. Garlic is a highly perishable crop, and postharvest losses can be substantial if adequate storage and handling procedures are not implemented. For this instant, curing is required to preserve the initial quality present at harvest and to satisfy consumer demand for extended accessibility of the bulbs [3,4]. Consequently, maintaining yield and improving bulb quality are the specified characteristics during the garlic value chain [4]. Although by-products, such as leaves, roots, and skin, are frequently discarded and cause environmental pollution adding mitigation costs to food producers, there are opportunities for value-adding component recovery [5,6,7]. Therefore, the raw material supply chain ought to be evaluated to improve economic and environmental sustainability in the development of this novel functional food product. During garlic production, various losses during the cultivation, harvesting, and post harvesting stages are considered (Supplementary Table S1) [4,8,9]. Postharvest handling of garlic is regarded as the most crucial step to ensure that the garlic remains fresh, safe, and nutritious and aims at waste and loss reduction.

In Thailand, the postharvest procedure was typically performed by the dealer (raw material producer), who performed postharvest and handling and prepared the product according to the need set out by the food industry. The initial processing includes, but it is not usually limited to, leaf trimming, root removal, and peeling of the outer skin. This dehydration method removes the excess moisture from the external surface of the bulb. The raw material specifications generally depend on the final product and when the product should be available [10]. The curing process took 7–14 days at the raw material processing site when only the bulb was cured. In contrast, curing the entire garlic plant may take 90–120 days. The report by Sunanta, et al. [11] mentioned that the moisture content of fresh garlic dramatically decreased from 72 to 50% after drying at 29 °C for 60 days. However, cured garlic with the aerial part and tunic can be stored for up to eight months, while the cured bulbs retained their shelf life of only two to three months (Anurak Jaijaruen, pers. comm., 15 March 2022). Garlic contained an elevated amount of antioxidant compounds, including polyphenol and flavonoid, which are common antioxidants in fruits and vegetables [12]. Furthermore, the sulfur-containing and volatile compounds of garlic are a response to its medicinal properties [13]. Since there are only two garlic crop cycles per year, garlic farmers must consider the garlic volume for each curing method. Shade drying is more preferable to sun drying to preserve the quality. However, the excessive moisture can be removed faster under full sun. The decision is made to fulfill the processing industry’s need and reduce transportation costs. The supply of fresh and processed garlic has significantly risen. Consequently, maintaining yield and improving quality of the bulbs are the specified characteristics of the garlic value chain [4]. During processing and handling, up to 30% of the husk, stem, and leaf biomass was produced [14,15]. This biomass is either wasted or used as animal feed, resulting in environmental problems and high management costs. Moreover, the majority of garlic biomass in Thailand is burned, resulting in severe environmental issues, such as the emission of air pollutants. Currently, the global campaign on sustainable food production has increased the awareness of biorefinery of agriculture biomass for value-added components [16]. It has been found that, apart from the garlic cloves, the skin and stalk of garlic, which pose environmental concern, also contain a significant amount of allicin and phenolic compounds [17]. In addition, the biomass also consists of carbohydrates, proteins, pectin, cellulose, lignin, and hemicellulose, with a low lipid content, among which cellulose, hemicellulose, and lignin account for up to 70% of the biomass’s total weight [6]. There are opportunities for value-adding component recovery despite the fact that the biomass, such as leaves, root, and skin, are frequently discarded and cause pollution of the environment, which increases food producers’ mitigation costs [5,6,7]. Moreover, when the bio-circular and green economy model (BCG) is present, optimising the processing to enhance the manufacturing capacity and decrease any possible processing losses is critical. In accordance with the industrial zero-waste campaign, the present work lays the foundation for future studies to improve production efficiency, by-product utilization, and environmental and economic sustainability of garlic postharvest processing. More importantly, the biological biomasses could serve as raw materials for the biorefinery’s sustainable processing in order to enhance the economic and environmental sustainability of this novel functional food product. The objectives of this study were to create qualitative process flow diagrams and to evaluate the losses using mass flow and lab scale waste composition analysis. The initial quality of the raw materials after curing were also compared. This work could provide valuable insights for garlic farmers, processors, and researchers in improving the efficiency and sustainability of the garlic supply chain.

2. Materials and Methods

2.1. Process Flow Diagram and Mass Flow Acquisition

A qualitative process flow diagram (PFDs) was hand-sketched to record postharvest and handling steps subsequentially, and mass streams flowed through each curing methods as conducted by a garlic producer in Mae Tang district, Chiang Mai, Thailand, who supplies garlic to major processing industries in Thailand (Anurak Jaijaruen, pers. Comm., March 2022). The recurring data were used to create representative postharvest PFDs. Fresh garlic (250 kg) (Figure 1) was harvested from Mea Tang district, Chiang Mai, at the commercial harvesting stage in 2021. The samples were transported to the laboratory immediately and subjected to different postharvest and handling procedures.

Prior to processing, fresh garlic was dehydrated using the following different curing techniques:

• Whole semi-sun drying: garlic plants were sun-dried in the field for seven days before being combined and hung in the shade for 90 days.
• Whole shade drying: garlic plants were bundled and hung in the shade for 90 days.
• Root bulb sun drying: bulbs without an aerial part, but with root, remained sun-dried on a grate for 14 days.
• Root bulb shade drying: bulbs without an aerial part, but with root remaining, were dried on a grate in the shade for 14 days.
• Bulb sun drying: bulbs without an aerial part and root were sun-dried on a grate for 14 days.
• Bulb shade drying: bulbs without aerial parts and roots were dried on a grate in the shade for 14 days.

For curing treatments C–F, an initial processing and/or minimal processing, including leaf trimming, peeling of the outer skin, and root removal (E and F), were performed before subjection to the curing process.

2.2. Loss Assessment and Calculation

The assessments of losses and by-products from the six curing methods were conducted using the combined adapted methods both by lab-scale waste composition analysis and mass balance, as described by Lebersorger and Schneider [18], Amicarelli, et al. [19], and Nath, et al. [20]. Throughout the process, the floor was covered with canvas, and the garlic plants were cleansed, trimmed, peeled, and bundled. After initial and minimal processing, the residuals were weighed and recorded. Each replicate contained a 2 kg sample of garlic plant material, and each postharvest procedure was performed in 20 replicates. Garlic samples were weighed for the calculation of water loss [20] as below:

$$\mathrm{Moisture} \mathrm{conversion} \mathrm{factor} (\mathrm{MCF})=\frac{100-M_1}{65}$$

(1)

where M₁ = initial moisture (considering the moisture content of garlic after curing is 65%)

$$\mathrm{Garlic} \mathrm{yield} \left({\mathrm{Y}}_2\right)={\mathrm{Y}}_1\times \mathrm{MCF}$$

(2)

where Y₁ = weight of the garlic sample at field moisture content (kg)

Y₂ = weight of garlic sample area at 35% moisture content (kg)

$$\mathrm{Loss} (\%)=\frac{\mathrm{Weight} \mathrm{of} \mathrm{biomass}}{{\mathrm{Y}}_2}\times \frac{1}{10}$$

(3)

2.3. Quality Assessments

At the completion of each post-harvest procedure, five replicates were randomly sampled from each postharvest procedure for chemical analysis. After cutting and peeling, the flesh of garlic was lyophilized using a freeze dryer (Beta 2–8 LSCbasic, Martin Christ, Osterode Am Harz, Germany), ground to a fine powder, and stored at −20 °C until further analysis (not over three months). For the analyses, the 100 mg of sample powder was placed into a test tube, and 5 mL of 80% methanol was added, and it was heated at 70 °C in a water bath for 30 min. The supernatant was collected after 12 min of centrifugation at 6800× g. The extraction step was performed in the same manner five times. All supernatants were mixed, and the volume was adjusted to 25 mL using 80% methanol. The extract (1.5 mL) was added to a 2 mL microcentrifuge tube before evaporating under a vacuum. Before conducting chemical analyses, the extract was redissolved in 1.5 mL of deionised water and thoroughly mixed by vortexing and sonication.

2.3.1. Total Phenolic Content

The total polyphenol content in the garlic sample was determined using gallic acid as the standard, according to the method described by Sunanta, Chung, Kunasakdakul, Ruksiriwanich, Jantrawut, Hongsibsong and Sommano [2]. In brief, 30 µL of sample extract was combined with 150 µL of Folin-Ciocalteu reagent, followed by 120 µL of 7.5% w/v NaCO₃ solution. The reaction mixture was incubated for 60 min in the dark at room temperature. The absorbance at 765 nm was measured using a spectrophotometer (SPECTROstar Nano BMG lab-04 TECH, Ortenberg, Germany), and the total polyphenols content was expressed as mg of Gallic Acid Equivalents (GAE)/g of dry garlic sample.

2.3.2. Total Flavonoid Content

The total flavonoid content of garlic was analysed using catechin as the standard [2]. The extract (25 µL) was mixed with 125 µL of distilled water, 7.5 µL of a 5% NaNO₂ solution was added to it, and it was incubated at room temperature for 5 min to react. After that, 15 mL of a 10% AlCl₃6H₂O solution was added. After 6 min of incubation, 50 µL of a 1 M NaOH solution and 27.5 µL of distilled water were added. The absorbance was obtained using a spectrophotometer at 510 nm. The total flavonoid content of garlic samples was expressed as mg catechin equivalents (CE)/g of dry matter.

2.3.3. DPPH Scavenging Activity

The free radical scavenging activity, based on the scavenging activity of the stable DPPH (2,2-diphenyl-1-picrylhydrazyl) free radical, was determined [2]. The 25 µL of methanol extract sample was added to 250 µL of ethanol solution containing 0.2 mmol/L DPPH. After 30 min of incubation at room temperature in the dark, the absorbance at 550 nm was measured using a spectrophotometer. The free radical scavenging activity was determined as a proportion of DPPH decolourization.

2.3.4. ABTS Scavenging Activity

The ABTS [2,2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid)] assay was performed according to the method of Adedapo, et al. [21]. Briefly, the working solution was prepared by mixing the two stock solutions, including 7.0 mmol/L ABTS solution and 2.45 mmol/L potassium persulfate solution. The solutions were mixed in equal quantities. The solution was allowed to react in the darkness at room temperature for 12–16 h, and then it was diluted with 80% methanol to obtain an absorbance of 0.7 ± 0.02 units at 734 nm. Garlic extract (10 µL) was added to ABTS solution, it was mixed and left standing for 30 min, and the absorbance was measured at 734 nm. The antioxidant activity of garlic samples was expressed as mg Trolox equivalent (TE)/g of dry matter.

2.3.5. Total Reducing Sugar

The total reducing sugar was evaluated using glucose as a standard, following a significant modified method of Marsden, et al. [22] with significant modification. The 3,5 dinitro salicylic acid (1 g) was dissolved in 20 mL of 2 mol/L NaOH, and 30 g sodium potassium tartrate was gently added and diluted to a final volume of 100 mL using distilled water. Garlic extract (100 µL) was treated with 100 µL of DNS reagent at 80 °C for 30 min, and the absorbance at 575 nm was measured using a spectrophotometer.

2.4. Statistical Analysis

The postharvest losses experiments were conducted using a completely randomised design (CRD) with 20 replicates. All experiment data were expressed as mean ± standard deviation (SD). The qualities assessment parts were conducted with five replicates. One-way analysis of variance (ANOVA) and Duncan’s multiple range tests were used to test the difference between mean (p < 0.05) using SPSS 24.0 version (SPSS Institute, Armonk, NY, USA).

3. Results

3.1. Process Flow Diagram and Mass Flow Acquisition

The harvesting stage of Thai garlic is when half of the garlic leaves start to dry out and turn brown, as described by Dhingra and Paul [23]. Fresh garlic is manually pulled from the ground, and the excess soil is shaken off. Approximately 5–10 kg of the garlic plant is then bunched with bamboo strips for transportation. Figure 2 depicts qualitative process flow diagrams (PFDs) of raw material processing and losses detected from six distinct postharvest procedures. In procedure A, the postharvest handling was similar to the described by Opara [24], and freshly harvested garlic was sun-dried for seven days by spreading it on the ground under the full sun and covering it with the garlic leaves from the next row. After seven days, the water loss of garlic was 30.47 ± 0.72% (LA1), as shown in

Table 1. The mass flow of garlic loss during postharvest handling of Thai garlic.

Step/Procedure	A	B	C	D	E	F
Initial process (%)
aerial part	ND	ND	(Init LA) 48.60 ± 0.57	(Init LA) 48.60 ± 0.57	(Init LB) 49.00 ± 0.05	(Init LB) 49.00 ± 0.05
Root	ND	ND	ND	ND	6.00 ± 0.07	6.00 ± 0.07
Pre-drying process (%)
Water	(LA1) 30.47 ± 0.72	ND	ND	ND	ND	ND
Curing process (%)
Water	(LA2) 34.16 ± 0.38	(LB1) 66.06 ± 0.73	(LC1) 21.29 ± 0.23	(LD1) 20.35 ± 0.22	(LE1) 18.54 ± 0.05	(LF1) 17.90 ± 0.05
Minimal process (%)
aerial part	(LA3) 7.54 ± 0.11	(LB2) 7.23 ± 0.16	ND	ND	ND	ND
Skin	(LA3) 0.98 ± 0.01	(LB2) 0.94 ± 0.02	(LC2) 0.04 ± 0.00	(LD2) 0.04 ± 0.00	(LE2) 0.03 ± 0.00	(LF2) 0.04 ± 0.00
Root	(LA3) 1.77 ± 0.03	(LB2) 1.70 ± 0.04	(LC2) 1.05 ± 0.01	(LD2) 1.08 ± 0.01	ND	ND
Total losses	74.92 ± 0.37 A	75.27 ± 0.47 A	70.98 ± 0.32 D	70.07 ± 0.33 E	73.58 ± 0.07 B	72.94 ± 0.07 C

ND is not detected; Values are mean ± SD; values followed by a different alphabet in the ‘total losses’ row is significantly different (p < 0.05) using DMRT. Postharvest procedure, including: A: garlic plants were sun-dried for seven days in the field before being combined and hung in the shade for 90 days; B: garlic plants are bundled together and hung in the shade for 90 days; C: garlic sample leaves are removed and then sun-dried on a grate for 14 days; D: garlic sample leaves are removed and dried on a grate in the shade for 14 days; E: all leaves and roots are removed from the garlic sample before sun-drying on a grate for 14 days; F: all leaves and roots are removed from the garlic sample, which is then dried on a grate in the shade for 14 days; LA, LB, LC, LD, LE, LF are the losses detected of the postharvest procedures A, B, C, D, E, and F, respectively.

. The garlic was then bunched (~2 kg) and hung in the shade for further curing for 90 days at 30 ± 3 °C.

Water was the only compound lost during the shade-dry curing (LA2) process, and it was found to be 34.16 ± 0.38%. Before further processing, dried garlic underwent minimal processing, including cutting, peeling, and sorting. This step resulted in the processing losses of 7.54 ± 0.11% of aerial part, 0.98 ± 0.01% of skin, and 1.77 ± 0.03% of root (LA3). The total loss of this procedure was 74.92 ± 0.37% of the fresh weight.

In the second procedure (procedure B), garlic plants were tightened with strips of dried bamboo to a total weight of 2 kg each and immediately hung in the shade on a rack. The temperature of this curing process was 30 ± 3 °C. As shown in Table 1, the water loss of garlic samples after 90 days of curing was 66.06 ± 0.73% (LB1). The minimal processing of dried garlic from procedure B resulted in a loss of 7.23 ± 0.16% of aerial part, 0.94 ± 0.02% of skin, and 1.70 ± 0.04% of root (LB2). The total loss of this procedure was 75.27 ± 0.47% of the initial weight.

The initial processing of procedure C was performed by removing the leaf 2 cm above the bulb, which resulted in the loss of scape biomass accounted for 48.60 ± 0.57% (Int LA). After curing under the sun for 14 days, the water loss was 21.29 ± 0.23% (LC1). Then, the outer skin was peeled, and the root was cut before further processing. In this process, the residual garlic skin was 0.04 ± 0.00%, and the root was 1.05 ± 0.01% (LC2). The total postharvest loss from this method was 70.98 ± 0.32%.

The initial processing of procedure D was similar to procedure C, with 48.60 ± 0.57% loss in scape biomass, followed by curing of the garlic bulb for 14 days in the shade on a grate at 30 ± 3 °C. The water loss during the curing process was determined as 20.35 ± 0.22% (LD1). Before garlic bulbs were transformed into BG, the outer layers of skin were peeled off, and the root was cut. The weight of the dry skin and the root were 0.04 ± 0.00% and 1.08 ± 0.01% (LD2) of fresh weight, respectively. The total postharvest loss in this method was 70.07 ± 0.33%.

In procedure E, in addition to the aerial part of the fresh garlic plant that was cut at 2 cm above the bulb, the root was also separated, resulting in the loss of 49.00 ± 0.05% and 6.00 ± 0.07% (Int LB) of leaf and root biomass, respectively. The garlic bulbs were then sun-dried at 43 ± 7 °C for 14 days, and the moisture loss was 18.54 ± 0.05%(LE1) after curing. Further, minimal processing of peeling the outer layer of skin of the dried bulb resulted in only a 0.03 ± 0.00% loss of dried skin (LE2). This procedure accounted for a total loss of 73.58 ± 0.07% of fresh weight.

For procedure F, the initial processing was identical to procedure E, with identical loss percentage. However, the garlic bulbs further cured for 14 days in the shade at 30 ± 3 °C resulted in the water loss of 17.90 ± 0.05% of the initial weight (LE1). After the minimal processing, as for procedure E, dried skin residual was 0.04 ± 0.00% (LF2) with the total harvesting weight loss of 72.94 ± 0.07%.

Ultimately, the most significant loss in all processes was moisture loss during the drying process, followed by biomass during the cutting and trimming processes. The aerial part accounted for more than 45% of total loss during the drying process, followed by the outer skin of the garlic bulb. Procedures A and B, where the whole garlic plant was used for curing, resulted in the greatest total loss (75%) compared to other procedures. In addition, these two treatments have the longest curing time (90 days) in contrast to the others which required only 14 days. Moreover, there was no significant difference in overall losses between procedures A and B, despite procedure A being subjected to seven days of pre-drying in the sun. Nonetheless, the pre-drying phase could lower the total weight of the garlic plant by approximately 30% in seven days, thus reducing transportation costs. The remaining four procedures were curing the bulbs subjected to different initial processing and drying methods. Unlike the procedures A and B, other curing methods subjected for initial processing prior to curing. Drying garlic bulbs with intact roots (procedures C and D) resulted in significantly lower overall losses than bulb drying alone (procedures E and F). This might be due to the roots covering the entire basal plate, preventing moisture loss from the inside. In comparison to drying in the shade, the drying under the sun at high temperature rapidly evaporated the moisture. Nevertheless, direct sunlight-induced sunburn on the flesh, as demonstrated in Figure 3, caused the flesh of garlic to become yellow and to wither, while the outer appearance remained unchanged.

In general, postharvest changes, such as water loss, shrinkage, cell wall breakdown, softening, physiological instability, chemical alteration, and rotting resulted in the storage life of fresh products becoming shorter. Nonetheless, these alterations are highly noticeable in climacteric, as opposed to non-climacteric, fruits and vegetables [25]. Garlic is one of the non-climacteric plants that can sprout, be attacked by microorganisms, and experience an increase in metabolic rate after being harvested [26,27]. The purpose of postharvest processing is to preserve garlic’s quality and to extend its shelf life. After curing or drying, the neck and skin are dehydrated, providing a protective layer around the bulb that protects against fungal and bacterial infection. Nevertheless, the chemical qualities could be diminished due to temperature and exposure to environmental conditions. During storage, garlic continues breathing by taking oxygen from the air and generating carbon dioxide [28]. Nonetheless, Atashi, et al. [29] and Hong, et al. [30] reported that physiological and biochemical reactions considerably slowed down at low temperatures and that high CO₂ can slow sprout development and decay. However, the biomass from the processing consisted of scape, skin, and roots, which have the scope to be utilised as raw materials for biorefinery processing or the recovery of highly valuable components.

3.2. Quality Assessment

Table 2. Chemical properties of garlic material after treatment with various postharvest procedures.

Chemical Properties	Harvested	A	B	C	D	E	F
Moisture contents (%)	79.93 ± 0.10 A	49.33 ± 0.36 D	49.55 ± 0.41 D	58.37 ± 0.33 B	58.40 ± 0.42 B	56.45 ± 0.14 C	56.36 ± 0.21 C
Total phenolic content (mg/g GAE)	0.74 ± 0.02 A	0.69 ± 0.03 AB	0.61 ± 0.02 BC	0.66 ± 0.02 AB	0.55 ± 0.03 C	0.73 ± 0.02 A	0.73 ± 0.03 A
Total flavonoid content (mg/g CE)	0.07 ± 0.02 A	0.04 ± 0.01 BC	0.04 ± 0.01 BC	0.06 ± 0.01 AB	0.05 ± 0.00 ABC	0.04 ± 0.00 BC	0.03 ± 0.00 C
DPPH scavenging activity (mg/g TE)	0.75 ± 0.01 A	0.67 ± 0.02 B	0.57 ± 0.02 C	0.52 ± 0.03 CD	0.57 ± 0.04 C	0.41 ± 0.01 E	0.49 ± 0.01 D
ABTS scavenging activity (mg/g TE)	1.94 ± 0.03 A	1.87 ± 0.04 ABC	1.79 ± 0.01 BCD	1.66 ± 0.02 D	1.72 ± 0.05 CD	1.72 ± 0.10 D	1.93 ± 0.03 AB
Total reducing sugar (mg/g)	7.60 ± 0.82 A	3.61 ± 0.49 B	3.81 ± 0.71 B	3.41 ± 0.87 B	3.36 ± 0.21 B	3.92 ± 0.70 B	5.41 ± 0.80 B

Values are mean ± SD; values followed by a different letter in the same row are significantly different (p < 0.05) using DMRT. Postharvest procedure, including: A: garlic plants were sun-dried for seven days in the field before being combined and hung in the shade for 90 days; B: garlic plants are bundled together and hung in the shade for 90 days; C: garlic sample leaves are removed and then sun-dried on a grate for 14 days; D: garlic sample leaves are removed and dried on a grate in the shade for 14 days; E: all leaves and roots are removed from the garlic sample before sun-drying on a grate for 14 days; F: all leaves and roots are removed from the garlic sample, which is then dried on a grate in the shade for 14 days.

illustrates the chemical profile of the garlic material from each postharvest and handling procedure. Moisture contents of the garlic after curing from each procedure are different. In procedures A and B, to remove moisture from the freshly harvested garlic by sun drying, the moisture contents must be much lower (~49%), while those subjected to initial processing before curing could maintain the same level of moisture content. Additionally, by keeping the root intact, the raw materials could reserve their moisture content to be as high as 58%. The initial moisture content of garlic raw material after curing contributes largely to the final quality of the product. The study by Sunanta, Pankasemsuk, Jantanasakulwong, Chaiyaso, Leksawasdi, Phimolsiripol, Rachtanapun, Seesuriyachan and Sommano [11] revealed that, when high-moisture-content materials were used, the black garlic product had a soapy texture, a brown color, and a small amount of bioactive compounds. In addition, this investigation found that the optimal moisture content of fresh garlic for black garlic was approximately 50%. Due to the diverse curing procedures of garlic prior to BG processing, the chemical transformation of garlic between each postharvest method was not reported.

The results showed that the total phenolic content of sun-dried or high-temperature-cured garlic samples did not differ significantly from the freshly harvested garlic (0.74 mg/kg GAE). However, the total phenolic content of garlic dried in the shade appeared to be degraded to 0.65 mg/g GAE with significant differences compared to the sun-dried garlic. This result is similar to the report of Naheed, et al. [31] and Petropoulos, et al. [32], where the total phenolic content dramatically decreased during storage at low temperatures. On the other hand, the garlic drying experiment conducted by Wongsa, et al. [33] showed that the total phenolic content increased at temperatures between 50 and 60 °C due to releasing bound phenolics from the plant cell at high temperatures. It was also argued that the total phenolic content might have significantly decreased during the curing process under the sun. However, the bound-phenolic acid might have released during the curing. Consequently, the total phenolic content of the sun-dried procedure was maintained. Similarly, Kayacan, et al. [34] and Chen, et al. [35] discovered that heat processing reduced the flavonoid content. This was in line with our result that the total flavonoid content in garlic decreased significantly after curing from 0.07 to 0.5 mg/g CE. The amount of total flavonoid content was retained significantly more by the shade-dry curing method than by the other curing method. The drying procedure was estimated to account for more than 97% of flavonoid degradation during the drying process, according to a report by Mohd Zainol, et al. [36]. Davey, et al. [37] explained that heat treatment compromised the integrity of the cell structure, resulting in losses by leakage, and they also explained that the degradation of macromolecules was the result of numerous chemical reactions that included enzymes, light, and oxygen. There is a close relationship between garlic’s phenolic and flavonoid content and its antioxidant activity. Due to its higher total phenolic and flavonoid content, freshly harvested garlic possessed the highest antioxidant activity compared to the final product. In this study, we found that the antioxidant activity of garlic after curing was significantly reduced in all procedures. This was comparable to that of Feng, et al. [38], who reported the reduction of antioxidant activity after the curing process of garlic. It is generally accepted that the drying procedure of fruits and vegetables also reduced their antioxidant activity, as stated by Kayacan, Karasu, Akman, Goktas, Doymaz and Sagdic [34], and Samoticha, et al. [39].

4. Discussion

After the curing process, the total amount of reducing sugars significantly dropped, similar to the results of Fei, et al. [40]. This finding also agrees with the experiment conducted by Salama, et al. [41], in which they discovered that the total sugar in onions significantly declined after being stored at 30 °C for five weeks, and the fundamental reason for consuming glucose is metabolic activity at high temperatures.

5. Conclusions

The mass flow was generated from six distinct postharvest handling techniques of Thai garlic for processing. The assessment of losses revealed that bunch curing resulted in the greatest overall loss due to the longest curing duration. The water loss was the most significant loss, followed by the biomass loss. Considering the greatest volume from the biomass, the aerial part had potential as the raw material for bio-refinery processing. Compared with the freshly harvested garlic, all initial quality assessments of the cured garlics were significantly reduced. Due to the extensive curing time, the bunch-cured garlic had the lowest moisture content. In addition, garlic cured under the sun had lower moisture content than those of shade-drying. The garlic underwent sun-curing, providing the greatest total phenolic content, while curing garlic without leaves gave the raw material of the highest total flavonoid content when compared to other curing methods. In terms of antioxidant activity, the DPPH scavenging activity during brunch curing was greater than that of the remaining processes. However, there was no significant difference in total reducing sugar after curing. The effect of moisture content on chemical properties should be evaluated based on the findings. The findings of this study could have important implications for the garlic industry, including reducing losses during garlic curing, establishing quality standards, and developing sustainable and efficient ways to utilize garlic biomass. These findings can help to improve the efficiency, profitability, and sustainability of garlic production and processing.