Impact of Dried Garlic on the Kinetics of Bacterial Growth in Connection with Thiosulfinate and Total Phenolic Content

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The research provides valuable insights for designing garlic-based products with enhanced antimicrobial effectiveness for both food preservation and health applications.

Abstract

The health properties of garlic (Allium sativum L.) are attributed to thiosulfinates, flavonoids, phenols, and bioactive polysaccharides. These compounds, however, can degrade during processing methods. As hot air-drying is a commonly used preservation method due to its relatively simple operation, this study investigated the effects of garlic slices dried at various temperatures (50, 70, and 90 °C) on the growth kinetic parameters of model strain Escherichia coli ATCC 25922, the total thiosulfinate content (TTC), and the total phenolic content (TPC). Observations showed that the concentration of garlic extracts was a significant factor influencing the kinetics of bacterial growth, while the garlic drying temperature appeared to have no effect on E. coli activity. Analysis of TTS in fresh and dried garlic did not reveal statistically significant differences in their levels. However, hot air drying at 50 °C significantly reduced the TPC by nearly 25%, whereas drying garlic at higher temperatures (70 °C and 90 °C) did not lead to a significant loss in TPC compared to the raw samples. The determined growth kinetic parameters of the tested E. coli strain could serve as a basis for selecting optimal drying process conditions and extract concentrations when designing garlic products with enhanced antimicrobial properties.

1. Introduction

The discovery of antibiotics—chemical compounds from groups such as tetracyclines, cephalosporins, aminoglycosides, and macrolides, characterized by their therapeutic effectiveness—revolutionized the field of medicine [1]. They offered unprecedented methods of combating bacterial infections and saving countless human lives. However, these extraordinary compounds are now at risk of losing their effectiveness due to the increase in microbial resistance. This phenomenon is accelerated by the excessive and inappropriate use of antibiotics in both human medicine and agriculture. The problem of reduced effectiveness associated with the emergence of drug-resistant bacteria may have global consequences for maintaining public health. The adaptation of bacteria to grow and evolve in response to antibiotics necessitates a continuous search for methods and substances to combat them. As a result, pharmaceutical companies face significant scientific, regulatory, and financial challenges in bringing new antibiotics to the market. As plants can provide many complex and structurally diverse compounds characterized by antimicrobial properties, the search for new drug molecules is often carried out in the natural environment [2]. Much research has been devoted to investigating plant extracts or their essential oils for their potential use as antimicrobial agents [3,4]. In recent years, there has also been an increasing interest in the use of plant preparations in natural therapies.

The research on the evaluation of the antimicrobial activity of plant components uses different types of bacteria as model microorganisms. Escherichia coli is predominantly found in the digestive systems of humans and other warm-blooded animals and in a wide variety of environments [5]. In food, the presence of this bacteria is undesirable, as it can lead to economic losses by causing product quality deterioration and severe foodborne illnesses. This ubiquity makes E. coli a frequently used biological model organism in a wide variety of research focused on both basic and applied sciences, including food, environmental, medical, and pharmaceutical fields [6,7].

Garlic (Allium sativum L.), utilized by humans since ancient times, is a natural source of molecules with antibacterial activities. It is an important food crop, cultivated mainly for its underground bulbs, that is widely used both as a culinary commodity and a source of bioactive compounds that promote health by helping to combat various chronic conditions such as heart disease, gastric diseases (inhibiting Helicobacter pylori growth), infections, and atherosclerosis and exhibiting anticarcinogenic activity [8,9,10,11,12]. The health benefits of garlic are largely attributed to the antioxidant activity of the organosulfur compounds produced through the enzymatic conversion of alliin to allicin [13]. It is noteworthy that the vital importance of garlic as a potential antimicrobial agent lies in the mechanism of allicin action, which directly impacts thiol enzymes or specific proteins essential for cell division, thereby preventing the growth and development of resistance in most bacteria [7]. The properties of the bioactive compounds found in garlic, such as flavonoids, phenols, bioactive polysaccharides, various trace elements with antioxidant properties, and antimicrobial allicin activity, contribute to its popularity as an ingredient in the food industry and culinary recipes, where it is valued for enhancing taste and its ability to help preserve food products [11,14].

Given the seasonal nature of garlic harvests, long-term storage is necessary to ensure a consistent supply of high-quality bulbs throughout the year. The quality of stored garlic tends to deteriorate over time; nevertheless, storage practices primarily focus on preserving culinary rather than biochemical quality. It is worth emphasizing that the content of bioactive compounds in garlic, and thus its antimicrobial properties, depend on many factors, such as the plant variety, growing factors, as well as storage and processing after harvesting [13]. Due to the high moisture content, which affects its germination and the degradation of its nutritional and pharmacological values, garlic has a relatively short storage period. Storing garlic at lower temperatures can extend this period to 9 months at a temperature of −1.5 °C [15], but low-temperature storage significantly increases the costs incurred by producers; therefore, to enhance the shelf life of harvested garlic, a drying process is often employed. This method effectively protects garlic from the growth of microorganisms and inhibits many degradation reactions that occur with increased water availability while preserving its nutritional properties. Moreover, reduced water content facilitates the storage and distribution of this food crop.

Various drying techniques, including hot air drying, vacuum freeze drying, air impingement drying, pulsed vacuum drying, microwave–vacuum drying, and infrared radiation drying, each with distinct advantages and drawbacks, are utilized to produce dried garlic [16,17]. Understanding these methods enables the optimization of drying processes, ensuring the production of high-quality dried garlic for various applications. The choice of drying technique often depends on factors such as desired product features, cost, and available equipment. Hot air drying is commonly used due to its relatively simple and cost-effective operation [16]. However, there are disadvantages with this technology, such as high temperature, which can cause degradation of some of the bioactive compounds, affecting their quality and nutritional value as well as therapeutic activity. Hence, understanding the optimal drying conditions (a combination of temperature and drying time) that preserve or enhance garlic’s antimicrobial activity is crucial for ensuring the long-term effectiveness of the resulting products. This knowledge may lead to the production of more effective dietary supplements and medicinal preparations. Moreover, comprehending the impact of drying conditions on the bioactive compounds and antimicrobial properties of garlic could also expand the application of garlic in medicine, dietetics, and the food industry.

Considering the nutritional and medicinal value of garlic, as well as the need for effective processing to ensure high-quality products, this research focuses on studying the growth dynamics of a model strain of E. coli in response to the antimicrobial activity of garlic subjected to convective drying at various combinations of time and drying temperatures. Additionally, the total thiosulfinate content (TTC), including allicin, the predominant antimicrobial component of garlic, and total phenolics content (TPC) were evaluated in the examined garlic products.

2. Materials and Methods

2.1. Chemicals

The chemicals used in the study: L-cysteine, 5,5-Dithiobis (DTNB), Folin–Ciocalteu reagent, sodium hydrogen carbonate (Na₂CO₃), and gallic acid were purchased from Merck (Merck KGaA, Darmstadt, Germany). Nutrient broth was obtained from BTL (BTL, Poland). All chemicals used were of analytical reagent grade.

2.2. Plant Material

Fresh spring garlic of the Jarus variety was obtained from a local farmer from a region of Greater Poland (51°53′24.2″ N 18°12′37.0″ E). After harvesting, the garlic bulbs were cleaned, selected by removing those with visible damage, and then stored at 4 °C until use. Before the experiments, garlic bulbs were divided into cloves and peeled. The cloves were then cut into pieces (approximately 2 mm thick) using a mechanical slicercut and left for 30 min to form allicin by alliinase. The moisture content of raw garlic, determined by an electronic moisture analyzer (Sartorius MA30, Göttingen, Germany), was 52.49 ± 0.98%.

2.3. Strain

Escherichia coli ATCC 25922 was used as the model microorganism to evaluate the antimicrobial activity properties of raw and dried Jarus garlic gloves. The tested E. coli strain was cultured in nutrient broth (NB) supplemented with 2% glucose at 37 °C for 24 h. After incubation, 10 mL of the culture was transferred to the 90 mL of sterilized NB with 2% glucose and then incubated at 37 °C for 20 h to prepare inoculum for the main experiments. As E. coli is a facultative anaerobe and can grow in the absence of oxygen [18], the inoculation was cultured without shaking.

2.4. Drying Procedure

The garlic was dried in a convective dryer equipped with a drying air temperature controller, probes for measuring the air temperature (Cu-Constantan thermocouples), and relative humidity (the air RH with a capacitive sensor, type EE21-FT6B53/T24 of the E+E Elektronik Comp., Engerwitzdorf, Austria). To achieve steady-state conditions, the convective dryer was turned on about 1 h before drying experiments in each cycle. Once the dryer had reached the operating conditions, the garlic slices were placed evenly on the trays in a single layer and dried to a moisture content of approximately 10%. Drying of garlic slices was carried out at temperatures of 50, 70, and 90 °C. Drying temperatures were chosen to cover the typical range used for high-temperature drying and to capture the effect of these temperatures on the total loss of thiosulfinate (of which allicin is the main component), which may affect antibacterial properties. The airflow velocity was 1.0 m/s. All experiments were performed in triplicate.

2.5. Antibacterial Activity Test

Raw, peeled, and chopped garlic cloves (5 g) were blended with 20 mL cold ultrapure water for 60 s. After blending, another 5 mL of cold, ultrapure water was added. Then, the solution was mixed, centrifuged at 4500× g for 10 min at 4 °C, and filtered through a 0.22 µm sterile syringe filter (Millex—MP, Millipore, Burlington, MA, USA). This process allowed us to obtain 20% (w/v) garlic extract. Then, the extract was diluted with cold ultrapure sterile water (1:4 and 1:8 dilutions) to obtain 5% (w/v) and 2.5% (w/v) sterile extracts. Extracts from dried garlic were prepared in the same manner, but the appropriate mass of dried garlic and extractant volume were used. The aqueous extracts were prepared in duplicate and used immediately for the experiments.

The volume of 0.5 mL of the appropriate garlic extract was added to the 4 mL of sterilized NB supplemented with 2% glucose (0.5 mL of water instead of garlic extract was used as a control sample). Then, 0.5 mL of the 20 h inoculum was added. After mixing, 300 µL of each culture was transferred into a sterile 96-well tissue culture plate as test samples with extract concentrations of 2%, 0.5%, and 0.25% and control samples. Growth experiments were performed in triplicate for each prepared extract and control samples. The plate with the lid was incubated for 18 h at 37 °C in a microplate spectrophotometer (Multiskan Sky, ThermoFisher Scientific, USA). Cell growth was monitored at 20 min intervals, and cyclic shaking (4 s at 5 min intervals) was applied between readings. Then, optical density (OD) of the cell culture was measured at 600 nm.

2.6. Kinetics of Bacterial Growth

For each combination of garlic drying temperature and extract concentration, kinetic parameters of tested E. coli strain, reflecting biological component characterizing the dynamics of bacterial growth, i.e., lag phase time (τ_lag), the maximum specific growth rate (μ_max), and the final level of OD at the stationary phase (OD_s) were estimated by plotting the OD_(τ) versus time (τ) and fitting the modified Gompertz model [19] using the STATISTICA 13 software package (StatSoft, Inc., Tulsa, OK, USA). The model representing the three typical microbial growth phases (lag, exponential growth, and stationary) was expressed by the following equation:

$${OD}_{\left(\tau \right)}={OD}_0+{∆}_{max}OD\cdot \mathit{exp}\left[-exp \left({\displaystyle \frac{{\mu }_{max}}{{∆}_{max}OD}}·\left\{{\tau }_{lag}-\tau \right\}+1\right)\right]$$

(1)

where Δ_maxOD is the total increase in bacterial population, calculated as the difference between the initial OD₀ level recorded at the beginning of the experiments and the final OD_s level recorded in the stationary phase (asymptotic level); μ_max is the maximum specific growth rate in OD (defined as the slope of the growth curve at the point of inflexion) (1/h); and τ_lag is lag period (the intersection of the line defining the maximum specific growth rate with the line defining the initial OD₀ level) (h), τ, time, after which the bacterial population reaches the level of OD_(τ) (h). The parameters of the models were estimated by minimizing the sum of square residues.

2.7. Total Thiosulfinate Content

Total thiosulfinate content (TTC) was determined according to Han [20] and Zheng et al. [16] with some modifications. Raw garlic cloves (2 g) were peeled to remove the dry protective layers, chopped, and blended with 20 mL cold ultrapure water for 60 s. After a 15 min incubation, the sample was centrifugated (4000× g, 15 min) and filtered through a 0.45 µm sterile syringe filter (SF25RC45C, Alchem, Poland). Dried garlic extracts were prepared using the same procedure, but the appropriate mass of dried garlic and extractant volume were used (calculated to dry weight). An amount of 50 µL of each filtrate was added to the 500 µL of cysteine solution (1.4 mM) and incubated at 26 °C for 15 min. Meanwhile, 300 µL of DTNB (5,5-Dithiobis, 430 mg/L in 0.1 M Tris-HCl pH 7.5) was transferred into wells of a 96-well tissue culture plate. After incubation, 20 µL of each reaction mixture was added to the well containing DTNB solution, and the plate was incubated at 26 °C for 15 min. Finally, the absorbance was measured at 412 nm. The TTC was determined based on the approach that a single thiosulfinate molecule interacts with two molecules of cysteine, which were used in excess [20,21]. The degree of cysteine consumption determined using Ellman’s Reagent (5,5′-dithiobis-(2-nitrobenzoic acid, DTNB)), allows for determining the TTC, which is equal to half the decrease in cysteine concentration. Each analysis was repeated six times.

2.8. Total Phenolic Content

The total phenolic compound (TPC) content was determined using the Folin–Ciocalteu procedure according to Ravindranath et al. [22] and Barkat [23], with slight modifications. A volume of 20 µL of each garlic extract (preparation described in Section 2.4) was pipetted into wells of a 96-well tissue culture plate, and 160 µL of ultrapure water was added. After adding 20 µL of Folin–Ciocalteu reagent, the plate was incubated in the dark for 10 min. Finally, 50 µL of Na₂CO₃ (20% w/v) was added, and the resulting mixtures were incubated for 20 min in the dark. Absorbance was measured at 760 nm. Gallic acid was used as a standard for the calibration curve, so the results are given as gallic acid equivalent. Each analysis was repeated six times.

2.9. Statistical Analysis

Data were presented as mean ± standard deviation (SD). Statistical analysis of data was conducted using the STATISTICA 13 software package (StatSoft, Inc., Tulsa, OK, USA). One-way analysis of variance (ANOVA) was performed on TPC and TTC to determine significant differences in compound levels between raw garlic and garlic samples dried at different temperatures, as well as between kinetic parameters of bacterial growth estimated for tested samples. The Tukey test (HSD) was used to compare differences among the mean values of examined bioactive components and growth kinetic parameters. All analyses were performed at a confidence level of p < 0.05.

3. Results and Discussion

3.1. Kinetics of Bacterial Growth

The antimicrobial activity of garlic, attributed to allicin, has been reported against a wide variety of microorganisms, including antibiotic-resistant Gram-positive and Gram-negative bacteria [24]. Some of these studies also included a quantitative evaluation of garlic antimicrobial properties by measuring the minimum inhibitory concentrations (MICs) and minimum microbicidal concentrations (MMCs) of various extracts against selected bacteria and fungi [25,26]. In turn, other deliberations were focused on different solvents for extracting dried bulbs garlic to evaluate their insecticidal, antimicrobial, and antioxidant activities [12,27].

Our study expands upon previous research by considering the effects of different concentrations of aqueous extracts obtained from raw garlic and garlic subjected to convective drying at various temperatures on the kinetic parameters describing the growth characteristics of E. coli. The selected bacterium is a model strain because it represents a widely distributed microorganism found in many environmental niches, including the human gastrointestinal tract, which, in some cases, may lead to foodborne illnesses. Moreover, considering findings of previous research indicated that allicin is thermally unstable and loses its antimicrobial activity within minutes when heated above 80 °C, while other thiosulfinates in garlic are more thermally stable, our experiments tested garlic subjected to drying at a range of temperatures 50–90 °C, including low, moderate, and high levels typically used in hot air drying [8,28,29].

The observations from our initial experiments carried out without garlic extracts demonstrated that growth curves of E. coli based on changes in optical density over time exhibited the typical three stages of microbial growth: the lag phase, the exponential growth phase, and the stationary phase (Figure 1). The addition of garlic extracts—obtained from raw and dried (at different temperatures) garlic samples—to the medium altered the activity of the tested strain and modified the patterns of growth; however, most of the E. coli curves still exhibited the three above-mentioned stages typical of microbial growth. The exception was the tests with the most concentrated extract (2%) obtained from both raw and dried garlic (regardless of the drying temperature), which exhibited complete growth inhibition for the examined bacterial strain during the analyzed period (18 h).

The main compound responsible for the antimicrobial properties of garlic, allicin, is formed from alliin through the action of the enzyme alliinase, which is activated by the disruption of cell walls in garlic tissue. It is worth mentioning that the activation of alliinase before garlic heat treatment is a critical factor in the development of the antimicrobial activity of the obtained products [25]. Heating whole garlic cloves without crushing them reduces the production of thiosulfinates from cysteine sulfoxides due to heat-induced inactivation of the enzyme before it converts alliin to allicin. In our study, the limitation of E. coli growth in samples containing 2% garlic extract indicates that the initial preparation of garlic cloves before the drying process (cutting them into 2 mm pieces and allowing a 30 min waiting period before placing them in the drying chamber) activated alliinase and enhanced the antimicrobial properties of garlic.

In further research, the modified Gompertz model was applied to the experimental data to assess the impact of garlic extracts on the growth kinetics of the tested bacterial strain. This model, widely used by researchers to describe microbial growth [3,19,30,31,32], was employed to predict biological parameters characterizing the dynamics of bacterial development in tested samples. The changes in the optical density (OD), describing the population level of the tested E. coli recorded in the experiments and fitted by models developed for different garlic treatment conditions, are depicted in Figure 2a–l. The growth patterns of the bacteria exposed to the garlic extracts were well-represented by the developed models. The values of model parameters, reflecting the lag time (τ_lag), the maximum rate of exponential growth (μ_max), and the total bacterial population increase (Δ_maxOD), offered valuable quantitative metrics for comparing the effectiveness of different extracts against the strain under investigation. The values of these parameters are presented in

Table 1. Growth kinetic parameters of Escherichia coli ATCC 25922 estimated for control samples and test samples containing garlic extracts based on raw garlic and garlic dried at different temperature conditions.

Growth Kinetic Parameter	Content of Garlic Extract (%)	Raw Garlic	Dried Garlic
			50 °C	70 °C	90 °C
lag time, τlag (h)	0	2.184	-	-	-
	2	>18	>18	>18	>18
	0.5	6.663	6.967	7.084	6.286
	0.25	4.258	4.010	4.017	3.906
maximum rate of exponential growth, μmax (1/h)	0	0.301	-	-	-
	2	0.000 *	0.000 *	0.000 *	0.000 *
	0.5	0.263	0.257	0.276	0.257
	0.25	0.285	0.270	0.275	0.292
total increase in bacterial population, ΔmaxOD	0	0.221	-	-	-
	2	0.000 *	0.000 *	0.000 *	0.000 *
	0.5	0.215	0.203	0.219	0.245
	0.25	0.220	0.217	0.185	0.186

* During the entire incubation, the OD oscillated slightly around the initial values.

.

Regarding the parameter estimates, the highest values of the maximum bacterial growth rate and the highest OD level in the stationary phase were observed in samples without the addition of garlic extracts, whereas E. coli activation in these tests occurred relatively quickly, after approximately 2 h.

The subsequent outcomes obtained in the study showed that the addition of garlic extracts to the growth medium significantly influenced the growth kinetics of the studied E. coli strain, but this influence was primarily observed as a delay in bacterial activity (Figure 3a–c). Each applied concentration of garlic extracts meaningfully (MS = 159.9, p < 0.0001) inhibited the activation of the tested bacteria by extending the lag time from 2 h for control samples without garlic extract to 4 h for samples containing 0.25% extract and to 6–7 h for samples containing 0.5% extract. The 2% extract concentration proved to be the most effective, as it entirely prevented bacterial development. Garlic extracts also significantly affected the growth rate of the tested strain (MS = 0.069, p < 0.0001), suggesting that garlic compounds may interfere with microbial metabolism and replication. Nevertheless, in samples with 0.25% and 0.5% extract content, the growth rate was only slightly slower compared to control samples, and only 2% of the extract content caused the bacterial growth rate to drop to zero. A similar statistically significant impact of garlic extracts (MS = 0.041, p < 0.0001) was observed in the case of the total increase in bacterial population density. The values for samples containing 0.5% and 0.25% extracts were comparable to samples without garlic extracts, while the addition of 2% extract decreased the total increase in bacterial population to almost zero.

The results presented in previous studies suggest that garlic thermal treatment can influence allicin content and, thus, its antimicrobial activity [25,33]. Nevertheless, in our study, statistical analysis revealed that garlic drying temperature had no impact on the estimated kinetic parameters of the tested bacterial strain, as all parameters recorded for samples with the same garlic extract concentration were similar (regardless of the thermal conditions of the drying process) to those computed for samples based on raw garlic. The lack of effect of drying temperature on the kinetics of bacterial growth may be explained by the shorter exposure time of garlic slices to thermal conditions during the drying process at higher temperatures. Specifically, to reduce the moisture content of garlic slices to the desired level of 10%, samples exposed to 90 °C were dried for only 80 min, whereas at lower temperatures, such as 50 °C, the drying process lasted 550 min. This shorter exposure time at higher temperatures maintained similar antimicrobial activity of garlic compared to samples dried for longer times at lower temperatures. Since temperature does not affect the growth kinetics of the tested E. coli strain, the only factor that can effectively inhibit or delay bacterial growth, which should be subject to optimization, is the concentrations of extracts derived from raw or dried garlic. This knowledge, gained from the study, helps in understanding the variability in kinetic parameters of bacterial growth and can be leveraged to design garlic products with enhanced antimicrobial properties.

3.2. Thiosulfinates Content

Thiosulfinates are molecules with free-SH groups found in garlic, primarily responsible for its health benefits, bioactive properties, and characteristic odor [34]. The biological and biochemical effects of thiosulfinates are attributed to their high reactivity and their antioxidant abilities. Among these bioactive compounds, allyl-2-propene thiosulfinate (allicin), constituting 50–90% of the TTC, exhibits dominant biological activity [20,34]. Previous findings showed that the antibacterial activity of aqueous allicin extracts depends on the incubation temperature and time [7]; hence, evaluating its level during garlic treatment is an essential matter. It is also important to emphasize that other thiosulfinates in garlic, although present in smaller quantities than allicin, may still possess significant biological activity—especially since studies considering this aspect have demonstrated that thiosulfinates exhibit antimicrobial properties comparable to that of allicin, and most of them are more heat-stable than allicin [35]. Since all garlic-derived thiosulfinates exhibit similar biological effects, it seems reasonable to monitor the antimicrobial quality of garlic based on the total amount of thiosulfinates, whilst the allicin content can be estimated by multiplying their entire quantity by the average value of their content range, which is 0.7 [20].

Earlier studies demonstrated that the concentration of thiosulfinate compounds in garlic bulbs varies depending on the garlic variety and conditions under which the plant was grown, processed, and stored [13,15,36,37]. One of them, examining the content of allicin in garlic slices dried at three temperatures (50, 60, and 70 °C), showed that the kinetics of allicin loss depends on drying temperature [8]. The authors emphasize that when producing garlic powder, it is crucial to optimize the combination of drying time and temperature to obtain a product with a high level of antimicrobial properties. In our research, we attempted to assess the impact of temperature and time conditions on the level of thiosulfinates in connection with the assessment of the antibacterial properties of extracts obtained from raw and convective-dried garlic. The results observed in experiments conducted in this study showed that the TTC in samples of dried garlic was slightly lower than in samples of raw garlic (Figure 4), but statistical analysis revealed that there were no significant differences in the TTC amount between all tested samples.

The content of the TTC in tested extracts obtained from raw and dried to 10% DW garlic slices ranged from 11.15 to 11.73 mg/g dry weight (DW), which was converted into allicin, according to the suggestion of Han et al. [20], gives a level of 7.81–8.21 mg/g DW. These results are slightly lower than those presented in recent findings, which reported an average allicin content of 13.12 mg/g DW in fresh garlic [38]. However, other studies that reported allicin content in crushed fresh garlic indicated that its content ranges widely from 0.16 to 13.0 mg/g DW [39].

Our results may seem surprising because exposition to high temperatures can cause alliinase inactivity and allicin degradation. There are reports indicating a detrimental effect of high temperatures on allicin content [29,38,40,41]. On the other hand, another study by Ratti et al. [28] found that garlic dried with hot air at 50 °C maintained a similar allicin content to raw garlic. In turn, Dogunturg et al. [8] observed the same allicin level in garlic drying at 50 °C for 240 min compared to that obtained at 70 °C for 120 min. A study by Ratti et al. [28] demonstrates even a higher loss of allicin content at 50 °C drying temperature compared to 60 °C drying, likely due to the longer drying time at 50 °C. Abano et al. observed an increase in allicin content during hot air drying with increased temperature [42]. Explaining the results, the authors suggested the possibility of the decomposition and rearrangement of allyl-S-cysteine sulfoxide to a larger number of dially thiosulfinates (allicin). A similar conclusion was reached in another study, which showed that heating alliin solutions at temperatures of 60 °C, 80 °C, and 89 °C resulted in the generation of significant numbers of organosulfur compounds [29]. The above-mentioned finding aligns with our observation of comparable TTC in garlic samples raw and dried at different temperatures. The lack of changes in TTC levels observed in the tested garlic extracts could also be attributed to the correlation between drying temperature and duration. It was noted that as the drying temperature increased, the time needed to reach the desired final moisture content of the tested samples of garlic significantly decreased (550 min at 50 °C, 170 min at 70 °C, and 80 min at 90 °C). These observations indicate that drying garlic at higher temperatures accelerates the drying process and yields a product of comparable quality to that achieved at lower temperatures. This was reflected in similar antimicrobial activity recorded for samples with the same level of extract based on raw and dried garlic.

3.3. Total Phenolic Content

The health benefits of garlic are attributed to its bioactive components, among which phenolics are particularly notable. These compounds possess antioxidant activities and are believed to play a role in preventing various degenerative illnesses such as cardiovascular diseases and cancer [43,44]. The numerous benefits of phenolic compounds highlight the importance of their preservation in natural food products to enhance consumer health and prevent chronic diseases. However, it has been demonstrated that processing techniques can influence the content of antioxidants in garlic. According to a literature review, raw garlic typically exhibits higher antioxidant activity compared to cooked garlic, while fermented varieties like black garlic demonstrate even greater antioxidant potential than its unprocessed form [37,45].

In this study, the impact of the drying process on TPC expressed as gallic acid equivalents (mg GAE per 100 g DW) was assessed using the Folin–Ciocalteu assay. The TPC in raw garlic was 148.7 ± 16.4 mg GAE/100 g DW (Figure 5).

Dehydration of garlic slices through hot air drying affected the level of phenolic compounds they contained (Figure 5). The significance of this effect was confirmed by the analysis of variance (MS = 1320.8, p = 0.0073). Similar findings on the heat sensitivity of phenolic compounds in various vegetables and fruits subjected to drying have been reported by other researchers [46,47,48]. Further analysis using the Tukey HSD test indicated that significant differences in TPC levels occurred only between garlic dried at 50 °C and the raw samples. Garlic dried at 70 and 90 °C exhibited slightly higher TPC than that preserved at 50 °C, though the levels remained lower than those in raw samples. Greater antioxidant retention in garlic dried at higher temperatures could be attributed to shorter drying times at higher temperatures, while longer exposure to thermal conditions at lower temperatures could lead to greater loss of phenolic compounds. These findings are in agreement with those observed in studies by Zhou (2016) and others [49,50,51,52]. It is also possible that higher processing temperatures facilitate the disruption of cell walls and the release of phenolic compounds from cellular constituents [49,53].

4. Conclusions

With increasing consumer health consciousness and a preference for natural foods free from artificial preservatives, there is significant potential for the use of natural garlic-based antimicrobials. The study provides valuable insights into the effect of different concentrations of garlic extracts derived from garlic, both raw and dried at various temperatures, on the kinetic parameters of E. coli ATCC 25922 growth. The thermal conditions of the garlic drying process did not have a significant impact on the pattern of E. coli development, and the only factor influencing the growth kinetics of the tested strain depended solely on the concentration of garlic extract. The addition of garlic extracts primarily affected the lag phase of bacterial growth, suggesting the potential for designing garlic-based products with enhanced antimicrobial properties. Additional investigation into the effect of drying processes at various temperatures on TTC, including allicin, revealed that, within the tested range of values, drying temperature had no significant effect on its levels. The analysis of the impact of the drying conditions on TPC showed the heat sensitivity of phenolic compounds. It was found that the longer processing time required to achieve the target moisture content resulted in greater loss of total phenols in samples dried at lower temperatures (50 °C) compared to those dried at high temperatures (70 °C and 90 °C).

Optimizing the garlic drying process to preserve its bioactive compounds and antimicrobial properties requires consideration of various criteria. The research has shown that the applied range of drying temperatures and corresponding drying time preserved the antibacterial properties of garlic. Considering these results, producers can focus on optimizing additional factors such as crew management, equipment usage, and minimizing operational costs. This approach could help ensure the maintenance of high-quality garlic products, retaining their health-promoting properties while ensuring production profitability.