Introduction

The genus Allium (Amaryllidaceae) is among the largest monocot genera, involving more than 900 species distributed in the northern hemisphere, apart from A. synnotii G. Don in the Cape Region of South Africa. The primary evolution center of the aforementioned genus is in the Irano-Turanian biogeographical region and the Mediterranean basin. Türkiye has more than 200 Allium taxa in 15 sections. Almost one-third of these taxa are endemic, showing that Türkiye is an important part of the southwestern Asian Allium diversity center [1]. Therefore, Türkiye is one of the largest garlic-producing countries in the world, and a garlic variety called Kastamonu garlic makes up a significant part of this production. Kastamonu garlic is the most common garlic clone in Türkiye due to its high soluble solids content, strong taste, and ability to be preserved for a long time [2]. Garlic, which takes a significant place in the economic and social development of the rural population in Kastamonu, is described as white gold, is grown particularly in Taşköprü district, and received a “Geographical Indication” as “Taşköprü Garlic” in 2009 [3]. According to FAO data, 17.54 million tons of garlic were produced in 1.66 million hectares of land in the world in 2022 [4]. In 2023, 54 thousand tons of fresh and 143 thousand tons of dried garlic were produced in Türkiye’s approximately 13,900 hectares of land. Kastamonu, Gaziantep, and Kahramanmaraş provinces are top in Türkiye’s garlic production. Garlic production in Kastamonu province corresponds to approximately 25% of the total production of Turkiye [5]. While China ranks first in production, cultivation area and exports, Türkiye exported 4.9 thousand tons according to 2021 data [6]. The amount of exports remained low compared to production in Türkiye. Efforts are continuing to increase the export of this geographically indicated garlic.

Kastamonu has a temperate climate in the coastal parts of the Black Sea and a steppe climate in the inner parts and receives most of its precipitation in spring and winter. In Taşköprü district, where garlic production is concentrated, the soils are medium calcareous, mostly clay loam in structure, with a slightly alkaline pH. The high-quality characteristics of Kastamonu garlic are probably due to the interaction between its genotype and the environment in which it is grown [2]. Taşköprü garlic, whose cloves are arranged in regular rows, is a variety resistant to storage. Garlic cultivation in Taşköprü, which stands out with its quality and storage life, has a high yield depending on the climate and soil structure [7]. Harvest time is July. After harvesting, the garlic is kept in the field for 7–10 days to dry [8]. Produced garlic is divided into three classes according to the size of the heads and their quality. Among these, the best quality is large garlic. Garlics with a head size smaller than that are called medium garlic, and garlic with a head size smaller than medium garlic is called small garlic. After being classified, some of the garlic ready for sale is taken to the market and sold, while the other part is taken to warehouses, stacked and stored there. Storage takes place in normal warehouses. Garlic can also be stored on hangers, in cardboard boxes or in crates. Kastamonu garlic has a longer storage life than other garlic, approximately 10–11 months [7]. Taşköprü garlic has high soluble solids content, is rich in minerals, ascorbic acid, aroma components, and sulfur compounds and displays high antioxidant activity [9].

Garlic (Allium sativum L.) has numerous compounds that are beneficial to health, such as organic sulfites, saponins, polysaccharides, and phenolic compounds. Garlic is widely consumed in cuisine and has been utilized as a traditional medicine in China for a long time. It has been recently stated that garlic has extraordinary biological functions, such as antioxidant, anti-cardiovascular, anticancer, anti-inflammatory, immunomodulatory, antidiabetic, antiobesity, antibacterial, antifungal, anti-hyper thrombotic, antihypertensive, lipid and cholesterol-lowering, immune-enhancing, and prebiotic properties [10,11,12]. Garlic has been utilized in food and traditional medicine for years. In recent years, garlic has also started to be commonly used in medicine due to the bioactive compounds in its composition. This vegetable is used in human nutrition as fresh, dried, or cooked. Cooking is an important procedure to provide the basic properties the consumer desires. However, various positive or negative changes occur in the nutritional content of foods due to the temperature treatment and different methods applied during cooking. Cooking methods such as boiling, microwaving, pressure cooking, grilling, baking, and frying, cause general flavonol loss and can adversely impact vegetables’ nutritional value and texture. Cooking softens the cell walls and facilitates carotenoid extraction. It has been stated that vitamin loss in vegetables during cooking varies depending on the cooking method. Oils undergo physicochemical changes during deep-fat frying. Furthermore, the food dries out, and oil penetrates the food. Hence, foods fried in the oil contain considerably high thermo-oxidized and polymerized products, which are nutritionally undesirable [13].

When preparing dishes, vegetables such as onion, garlic, shallot, etc., are fried until their special flavors are obtained to improve the flavor profile of dishes. The oxidative degradation of fat and the Maillard reaction primarily determine the flavor during frying [14]. It is indicated that many reducing compounds are formed during the Maillard reaction, causing aroma and color changes, and some of them exhibit toxic, carcinogenic, or mutagenic properties. It is reported that acrylamide, hydroxymethyl furfural (HMF), and heterocyclic amines formed due to the Maillard reaction are toxic compounds [15].

Garlic is a vegetable usually cooked but can also be consumed raw. Studies have generally investigated the changes in total phenolic content, antioxidant capacity, phenolic profile, and volatile flavor compounds in different garlic varieties after stir frying, deep fat frying, oven cooking, sautéing, and boiling cooking treatments [12, 16,17,18,19]. Bi et al. [18] reported that roasted garlic has the highest volatile component content, while boiled and steamed garlic has lower levels. In another study, the highest antioxidant activity was detected in raw, fried, and boiled garlic, respectively [20]. Nevertheless, there is no study in the literature on garlic cooked using bleached parchment paper, unbleached parchment paper, oven bags, and different vegetable oils by employing the roasting method, a popular cooking technique nowadays as an alternative to deep-fat frying. At this point, it is essential to know what changes occur in cooked garlic’s antioxidant activities during common domestic procedures (boiling, frying, roasting, and microwaving). Therefore, the present study aimed to reveal the effects of cooking Taşköprü garlic, which is grown with a geographical indication in Türkiye, using bleached parchment paper, unbleached parchment paper, oven bags, and four vegetable oils (olive oil, sunflower oil, hazelnut oil, and corn oil) on the total phenolic content, antioxidant activity (DPPH and ABTS), and phenolic compound profile. Accordingly, it was aimed to identify the optimal cooking method and conditions that result in keeping the antioxidant capacity and radical scavenging activity of vegetables at the maximum level and enhancing their functional activities.

Materials and methods

Material

Taşköprü garlic from Kastamonu province was purchased from a local market in Erzurum province. Garlic was stored in a dark and cool environment until cooking and analysis.

Methods

Roasting procedures

It is a matter of curiosity what kind of effects the use of four different ways open, wrapped in bleached parchment paper, wrapped in unbleached parchment paper, and placed in an oven bag, which has become widespread recently instead of cooking in an open tray, has on the composition of cooked garlic. For these reasons, it is envisaged that these materials will be used in this study to determine the effect of these materials on garlic cooking.

Palm, corn, cotton, soy and sunflower oil are generally preferred for frying. Olive oil can be used in frying because it contains low amounts of linolenic fatty acid [21]. The oils used in this study were decided by considering some issues. In a previous study, hazelnut oil (HO), corn oil (CO), and olive oil (OO) were selected, which were reported to have low peroxide values calculated after 15 repeated frying processes [22]. Apart from these oils, sunflower oil (SO) was also preferred due to its widespread use.

A steel tray was used to roast the garlic. After the garlic skin was peeled, it was washed with water and placed in a sieve to drain the remaining water. Garlic portions of 30 g were immersed in 60 mL of oil (OO, SO, HO, and CO separately) to ensure that the entire surface of the garlic was covered with oil before cooking, and then the excess oil was removed by filtration. The oiled garlic was roasted on a steel tray in four different ways (open, wrapped in bleached parchment paper, wrapped in unbleached parchment paper, and placed in an oven bag). The oven cooking temperature was determined as 180 ºC, according to the vegetable cooking temperature specified in the oven instructions. In addition, Bubola et al. [23] roasted potato, carrot, and onion at 180 ℃ and Bi et al. [18] cooked garlic in the oven at 180 ºC and 20 min. In preliminary tests, the cooking time was determined when the garlic became soft enough to be eaten. Garlic was cooked at the oven temperature of 180 °C and a cooking duration of 20 min.

Extraction

The cooked garlic was dried in a lyophilizer at -86 ℃ (FDU-8612, OPERON, Korea). Then, the cooked dried garlic samples (15 g) were ground with a grinder, 150 mL of methanol was added, and they were mixed in an orbital shaker (Orbital Shaker SSL1) for 6 h. After this procedure, the filtrate was filtered using filter paper (Whatman No: 1), and the final filtrate was used as a stock extract. The extracts were transferred to an amber bottle and stored at -20 °C until analysis [24]. The extracts were used to determine total phenolic content, antioxidant activity (DPPH and ABTS), and phenolic compound profile.

Total phenolic content

To determine total phenolic content (TPC), 1 mL of the prepared extracts was taken, and 46 mL of distilled water and 1 mL of FCR (Folin-Ciocalteau reagent) were added. After the mixture was left for three minutes, 3 mL of 2% sodium carbonate (Na2CO3) solution was added and mixed in a magnetic stirrer for two hours. Afterwards, absorbance was measured on a spectrophotometer (PG Instruments T60V) at 760 nm. The samples’ total phenolic content was computed as gallic acid equivalent (mg GAE/kg) with the equation acquired with the help of the graph prepared using the gallic acid standard [25].

Antioxidant activity

DPPH assay

DPPH· (2,2-diphenyl-1-picrylhydrazyl) free radical scavenging activity was determined by diluting sample extracts prepared at various concentrations (10–30 µg/mL) to 2.0 mL with methanol. Afterwards, 0.5 mL of DPPH solution (1 mM) prepared with methanol was added to the samples; the samples were mixed thoroughly by vortexing and incubated in the dark at room temperature. Absorbance was measured against the blank at 517 nm in a spectrophotometer [26]. The samples’ antioxidant activity was computed as Trolox equivalent (mM Trolox/g extract) with the equation acquired with the help of the graph prepared using the Trolox standard [27].

ABTS assay

The extracts’ ABTS•+ radical scavenging activity was found following the method of Köksal, Gülçin [28] ABTS•+ radicals were produced by adding 2.45 mM potassium persulfate solution to ABTS (2,2’-Azino-bis(3-ethylbenzthiazoline-6-sulfonic acid)) solution prepared at a concentration of 7 mM with distilled water and stirring overnight at room temperature and in the dark. Stock extracts were also used as samples in the said analysis. After the stock extract solutions were transferred to test tubes in 3 parallels to form a concentration of 10–30 µg/mL, the total volume was completed to 1.5 mL with methanol, and 0.5 mL of ABTS•+ solution was added to each tube. The absorbance of the samples, which were vortexed and incubated at room temperature and in the dark for half an hour, was recorded against the blank at 734 nm. The control comprises 1.5 mL ethanol + 0.5 mL ABTS•+ solution. The samples’ antioxidant activity was computed as Trolox equivalent (mM Trolox/g extract) using the equation acquired with the help of the graph prepared with the Trolox standard [27].

Phenolic profile by liquid chromatography (LC-MS/MS)

The phenolic compounds of garlic samples were analyzed on an Agilent 6460 Triple Quadrupole LC-MS/MS device. A ZORBAXTB C18 4.6 × 100 mm column with a 3.5 μm particle size was used in the analysis. Deionized water (A) and acetonitrile (B) containing 0.1% formic acid were used as a mobile phase. The mobile phase program ran from 95:5 A: B to 5:95 A: B in 0–4 min, from 80:20 A: B to 20:80 A: B in 4–7 min, from 10:90 A: B to 90:10 A: B in 7–14 min, from 10:90 A: B to 90:10 A: B in 14–15 min, and from 5:95 A: B to 95:5 A: B in 15–15.10 min, with the mobile phase flow rate set at 0.4 mL/min. The column temperature was kept at 30 °C during the study. The injection volume was set at 5µL, nebulizer gas flow at 12 L/min, dryer gas flow at 5 L/min, detector temperature at 350 °C, and air block temperature at 250 °C. ESI (Electrospray ionization) and Agilent Jet Stream ionizers were utilized to ionize molecules. The MRM (Multiple reaction imaging) mode was employed for molecule identification. After MRM optimization was performed for each phenolic substance without column (quinic acid, fumaric acid, gallic acid, pyrogallol, keracyanin chloride, cyanidin-3-O-glucoside, chlorogenic acid, catechin, peonidin-3-O-glucoside, 4-OH-benzoic acid, epicatechin, epigallocatechin gallate, caffeic acid, vanillic acid, syringic acid, vitexin, naringin, ellagic acid, hesperidin, p-coumaric acid, sinapic acid, taxifolin, ferulic acid, rosmarinic acid, vanillin, myricetin, resveratrol, luteolin, quercetin, apigenin, naringenin, isorhamnetin, chrysin, galangin, and curcumin), the mixture that contained 1, 5, 10, 25, 50, 100, 200, and 500 ppb concentrations of the standards was analyzed. Calibration curves were drawn [29]. Then, limit of quantification (LOQ) and limit of detection (LOD) values were computed (Table 1).

Table 1 Limit of detection (LOD) and limit of quantification (LOQ) values of phenolic compounds (ng/mL)
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GC-MS analysis

The filtration of stock extracts through a PTFE 0.45 μm filter was performed; they were put in vials and utilized in GC-MS analysis. Chemical compounds (volatile and non-volatile) were identified with an Agilent 7820 A gas chromatography-mass spectrometry (GC-MS) device, a 7673 series autosampler, a 5977 mass spectroscopy detector, and ChemStation (Agilent Technologies, Palo Alto, CA, USA) software. An HP-5ms column with a 0.25 μm film thickness (30 m × 0.25 mm inner diameter) was utilized to separate the compounds.

GC-MS conditions were determined as follows:

  • Injection volume: 1 µl splitless injection mode.

  • Carrier gas: helium.

  • Flow rate: 1 mL/min.

  • Ionization energy: 70 eV.

The initial temperature was adjusted to 50 ℃. Following one minute, the temperature was increased to 100 ℃ at a rate of 20 ℃/min and maintained for 1 min. Afterwards, the temperature was increased to 180 ℃ at 10 ℃/min and maintained for 1 min, following which it was increased to 220 ℃ at 5 ℃/min and maintained for 5 min. Ultimately, the temperature was increased to 300 ℃ at 10 ℃/min and maintained for 5.5 min. The National Institute of Standards and Technology GC-MS library and reference standard substances were used to identify chemical compounds [30].

Statistical analysis

The data acquired in three replicates were analyzed in the SPSS 20.0 program. Analyses were carried out in three replicates, and the results were presented as mean values (± SD) with a standard deviation. The two-way analysis of variance (ANOVA) was performed to determine significant group differences (p < 0.05, p < 0.01) between means. Duncan’s multiple range test was carried out to compare mean values. Additionally, principal component analysis (PCA) was implemented on some data to identify similarities and differences between the samples (SIMCA-P + 14.1, UMETRICS).

Results and discussion

Effects of different oil/cooking material applications on the antioxidant properties of garlic

Due to the preparation method of garlic can potentially impact its medicinal properties [31,32,33], various studies have been undertaken to examine the effects of heating on garlic’s antioxidant properties [16, 34, 35]. This study aimed to determine the antioxidant capacity of garlic samples cooked using different oils and cooking materials. The antioxidant capacity was examined with ABTS and DPPH analyses because using a single antioxidant test does not entirely represent the situation, considering distinct reactive types and mechanisms are included in vivo oxidative stress. In addition, the total amount of phenolic compounds (TPC) was determined by Folin-Ciocalteu’s method.

Table 2 Duncan’s multiple comparison test results for antioxidant properties of garlic samples cooked with different cooking materials and oil types
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Table 2 shows the TPC and antioxidant capacity measured by scavenging DPPH and ABTS + radicals in terms of different oils (sunflower oil, hazelnut oil, corn oil, and olive oil) and cooking materials (no material, unbleached and bleached parchment paper, and oven bag) used for roasting. The TPC values of the cooked garlic samples varied between 339.85 and 370.24 mg GAE/kg in terms of the cooking material, while values ranged from 329.72 to 361.06 mg GAE/kg based on cooking oil (Table 2). The most effective cooking materials against DPPH free radical were unbleached and bleached parchment papers, while the most effective oils were corn and olive oils, similar to ABTS radical scavenging activity. Analysis of variance showed that the effect of cooking material and oil type on the TPC of cooked garlic was insignificant (p > 0.05). The bilateral interaction of cooking material x oil type was significantly (p < 0.01) effective on DPPH and ABTS radical scavenging activities and TPC. A significant and positive correlation was determined between DPPH and ABTS (r = 0.76, p < 0.01). However, no significant correlation was observed between TPC and free radical scavenging activities (DPPH and ABTS).

Fig. 1
figure 1

The effect of cooking material x oil type interaction on DPPH and ABTS radical scavenging activities and TPC of cooked Kastamonu Taşköprü garlic. *R only roasted; RUP Roasted with unbleached parchment paper; RBP Roasted with bleached parchment paper; ROB roasted with oven bag

Full size image

As seen in Fig. 1a, the TPC values of the cooked garlic samples ranged from 286.66 mg GAE/kg (corn oil + only roasted group) to 424.70 mg GAE/kg (corn oil + unbleached parchment paper group). The highest DPPH radical scavenging activity value was determined in the samples cooked with the sunflower oil + unbleached parchment paper and corn oil + bleached parchment paper combinations (0.88 mM Trolox/g extract), while the lowest was in the hazelnut oil + only roasted group (0.57 mM Trolox/g extract) (Fig. 1b). ABTS radical scavenging activity results showed that the olive oil + oven bag combination applied sample exhibited the highest antioxidant activity (0.48 mM Trolox/g extract). In contrast, the lowest activity (0.33 mM Trolox/g extract) was in sunflower oil + bleached parchment paper, sunflower oil + oven bag, and hazelnut oil + only roasted groups (Fig. 1c).

TPC, DPPH, and ABTS values detected in all samples cooked using different cooking materials and oil types were quite low compared to fresh garlic (634.92 ± 2.20 mg GAE/kg, 1.84 ± 0.00 mM Trolox/g extract, and 0.85 ± 0.00 mM Trolox/g extract, respectively) (data not shown in the table). The highest TPC loss was observed in the corn oil + only roasted group (approximately 55%). The highest decreases in DPPH and ABTS radical scavenging activities were found to be approximately 69% (in the hazelnut oil + only roasted group) and 61% (in sunflower oil + bleached parchment, sunflower oil + oven bag, and hazelnut oil + only roasted groups), respectively. When the samples were compared in terms of the oils used in cooking, it was observed that the samples with relatively best preserved total phenolic levels and antioxidant activity were those treated with olive oil. Olive oil, the dominant fat source of the Mediterranean diet, exhibits a singular fatty acid composition with a higher polyphenol content than other edible oils [36, 37]. The mechanism of action of oil on the antioxidant activity of cooked vegetables may be quite complex. However, this effect is likely due to the antioxidant compounds transferred from olive oil to vegetables. In addition, the fact that olive oil contains monounsaturated fatty acids (mainly oleic acid), which are more stable than polyunsaturated fats and, therefore, relatively more stable against oxidative damage, may be one of the reasons for the better preservation of the antioxidant activity of vegetables cooked with olive oil [38]. Indeed, in some studies conducted by Martínez-Huélamo and colleagues, it was reported that cooking vegetables with olive oil can increase the bioaccessibility and bioavailability of polyphenols, although the underlying mechanisms remain unclear [39, 40].

The significant decrease in antioxidant properties observed in all cooked samples compared to fresh garlic after different treatments may be related to the high cooking temperature (180 °C). Maillard products formed due to the high-heating process can cause changes in antioxidant properties [41]. According to Tomas et al. [42], cooking may cause polyphenols’ possible degradation or transformation, isomerization, and release from vegetable cells. Moreover, it should also be noted that the loss of phenolic compounds also depends on the processing time and the size of the food [43]. Similar results were reported for cooking different vegetables [44,45,46,47,48]. On the contrary, some studies have also reported increased [49,50,51,52] or unchanged [13, 53] antioxidant activity of various vegetables after cooking. Prior descriptions of the antioxidant activities of kinds of garlic have indicated substantial antioxidant activities in the fresh to cooked forms using DPPH and ABTS assays [16, 54, 55]. However, comparing the antioxidant properties of cooked garlic samples with other studies in the literature is challenging because the results obtained vary depending on various parameters such as the composition of garlic, cooking technique, processing time, temperature, and extraction technique, as well as units used for giving the results. In one study, Alide et al. [17] reported that the amount of total phenolic matter, which they determined to be 303.07 and 638.96 mg GAE/100 g in an aqueous extract of fresh garlic samples from Kenya, increased to 933.60-1273.30 mg GAE/100 g after boiling at 150 °C for 15–60 min. Ramirez et al. [35] determined that all domestic processing techniques (deep fried, sautéeing, boiling, boiling in a water/oil mixture) increased the total phenolic content and antioxidant capacity of garlic. However, Gorinstein et al. [55] cooked Polish, Ukrainian, and Israeli garlic in an oven at 100 °C for 20, 40, and 60 min and reported that the TPC and radical scavenging activity values of 47.3 mg/100 g fresh weight and 68.9% in fresh samples decreased to 31.7 mg/100 g fresh weight and 46.9% after cooking in proportion to time. Ali et al. [56] also reported that the TPC, DPPH, and ABTS values of the Iranian garlic decreased after 20 min of boiling. Mishra et al. [57] determined that the TPC and DPPH radical scavenging activity of fresh garlic, which was 78.45 mg/100 g and 98.92 mg Ascorbic acid equivalent/100 g, respectively, decreased to 74.39–76.67 mg/100 g and 88.28–94.56 mg AA eq/100 g, respectively, after roasting it at 190 °C for 5, 10, and 15 min. The researchers also stated that roasting exhibited the highest total phenolic content and radical scavenging activity compared to other heat treatment applications (frying, steaming, microwaving, and boiling).

Phenolic profile

At the molecular level, phenolic compounds and their functional derivates may be delineated as substances characterized by an aromatic ring containing one or more hydroxy substituents [58, 59]. It has been proven that phenolics found in cooked vegetables can still exhibit various bioactivities in the organism, either concerning their antioxidant activity or as modulators of anticarcinogenic processes [60,61,62].

In the present study, garlic samples roasted with the mentioned oils and cooking materials were analyzed for thirty-five phenolic compounds (quinic acid, fumaric acid, gallic acid, pyrogallol, keracyanin chloride, cyanidin-3-O-glucoside, chlorogenic acid, catechin, peonidin-3-O-glucoside, 4-OH-benzoic acid, epicatechin, epigallocatechin gallate, caffeic acid, vanillic acid, syringic acid, vitexin, naringin, ellagic acid, hesperidin, p-coumaric acid, sinapic acid, taxifolin, ferulic acid, rosmarinic acid, vanillin, myricetin, resveratrol, luteolin, quercetin, apigenin, naringenin, isorhamnetin, chrysin, galangin, and curcumin). However, the samples were found to contain only five of these compounds. The obtained phenolic data are presented in Table 3.

Quinic acid (1, 3, 4, 5-tetrahydroxy cyclohexane carboxylic acid) is a cyclohexane carboxylic acid with an astringent taste found naturally in various plant materials such as coffee beans, cinchona bark, potatoes, apples, pears, plums, peaches and tobacco leaves [63,64,65]. It can also be produced synthetically by hydrolysis of chlorogenic acid. Anti-diabetic, anti-inflammatory, antimutagenic, radioprotective and neuroprotective effects of quinic acid have been determined [66,67,68,69]. In analyzed garlic samples, quinic acid was the most dominant phenolic compound, and its amount in sunflower oil, hazelnut oil, corn oil, and olive oil groups varied between 22.98–29.43, 15.77–23.90, 17.64–22.84, and 16.92–26.65 µg/g, respectively. The only roasted group had the highest quinic acid content in all groups except those cooked using olive oil.

Fumaric acid ((2E)-but-2-enedioic acid) is an important organic acid with a fruit-like flavor. It is widely found in nature and has proven antioxidant and antimicrobial activities [70]. The fumaric acid contents of cooked garlic samples in terms of cooking oil were detected within the ranges of 5.07–6.63 µg/g (in sunflower oil group), 3.15–4.55 µg/g (in hazelnut oil group), 3.72–4.86 µg/g (in corn oil group), and 3.07–3.91 µg/g (in olive oil group).

Rosmarinic acid (α-O-caffeoyl-3,4-dihydroxy phenyl lactic acid) is a crucial phenolic compound for the food, cosmetic, and pharmaceutical industries [71, 72]. The substance was reported to be found in 39 plant species [72], but the primary source is rosemary plants [73]. Several remarkable biological properties of this phenolic acid have been demonstrated, such as hepatoprotective [74], anti-inflammatory [71, 75], anticancer [76] and neuroprotective activity [77]. It has found a place as a valuable antioxidant in the food industry thanks to its properties of free radical scavenging, chelating pro-oxidant ions, and inhibiting lipid peroxidation [78,79,80]. In roasted garlic samples, the highest rosmarinic acid content (0.55 µg/g) was found in the sunflower oil + only roasted group, whereas the lowest (0.20 µg/g) was found in the olive oil + bleached parchment paper group (Table 3).

Ferulic acid (4-hydroxy-3-methoxycinnamic acid) is a phenolic molecule commonly found in the plant kingdom (in particular in the umbrella family). It is usually cross-linked to lignin and polysaccharides and forms part of the plant cell wall [81]. It has been confirmed that it can eliminate free radicals and free radical-producing enzymes, lower blood lipid levels by inhibiting cholesterol synthesis, and protect against kidney and cardiovascular diseases [82,83,84,85]. The results obtained in the current study showed that only sunflower oil + unbleached parchment paper, hazelnut oil + bleached parchment paper, and olive oil + bleached parchment paper groups contained ferulic acid at 0.36, 0.16, and 0.02 µg/g, respectively.

Hesperidin (3′, 5, 7-trihydroxy-4′-methoxy flavanone) is a phenolic molecule found in tea, olive oil, and citrus fruits with proven antioxidant, anti-inflammatory, neuroprotective [86, 87] and antidepressive [88, 89] effects. Hesperidin was detected only in the roasted and unbleached parchment paper groups of garlic samples cooked using olive oil at 0.37 and 0.04 µg/g levels, respectively. When the samples were evaluated in terms of all phenolic compounds detected, it was observed that the individual phenolic contents of the samples cooked with sunflower oil were generally higher than the other groups.

Table 3 Phenolic profile of cooked garlic samples
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No previous study has evaluated the phenolic profile of garlic samples cooked with different oils (hazelnut, sunflower, corn and olive oils) and cooking materials (oven bag, bleached and unbleached parchment paper). However, a few studies investigate the effects of different cooking methods on the phenolic compound profile of garlic species. One of these studies was conducted by Lozano-Castellón et al. [90], who examined the phenolic and lipidic profiles of garlic samples cooked with new vacuum cooking techniques (slow cooking, low temperatures, and vacuum cooking) and conventional cooking methods (oven, pan-frying, and deep-frying) by using extra-virgin olive oil. In the mentioned study, 143 phenolic compounds screened with UHPLC-QTOF, mainly consisting of flavonoids, tyrosols, phenolic acids and lignans. After cooking, the most degraded phenolic group were detected as flavonoids. In addition, Lozano-Castellón et al. [90] identified the most distinctive phenolic compounds according to whether increased or degraded as sinensetin, luteolin-O-hexoside (flavones), kaempferol-O-trihexoside, quercetin-O-pentoside (flavonols), dimethylmatairesinol, lariciresinol-sesquilignan (lignans), 2-hydroxybenzoic acid, 4-hydroxybenzoic acid-O-hexoside, ferulic acid-O-hexoside, m-coumaric acid (phenolic acids), 4-vinylsyringol (alkylmethoxyphenols), isopimpinellin (furanocoumarins), carnosol (phenolic terpene), delphinidin-O-pentoside (anthocyanins), phloridzin (Dihydrochalcone), 8-prenylnaringenin (flavanone), glycitin (isoflavonoids), 3,4- dihydroxyphenylglycol, 5-heptadecylresorcinol, vanillin, and tyrosol acetate (other polyphenols), which were different from our present study. In another study, Ramirez et al. [35] evaluated garlic fried in sunflower oil (at 180 °C for 2 min), black garlic, garlic powder and garlic juice for some phenolic compounds. The researchers reported kaempferol, myricetin and quercetin flavanols at significant levels in cooked lyophilized garlic extracts obtained using a multi-phytochemical protocol (228.2, 291.77, and 108.75 µg g− 1 sample, respectively). De Queiroz et al. [91] reported in the HPLC analysis of raw, boiled and fried garlic samples that quercetin was detected at levels of 123.3, 112.4, and 93.9 mg kg− 1, respectively, whereas myricetin and apigenin could not be detected. These results show that the major phenolic compounds detected in the above mentioned studies using different cooking techniques are generally different from those in our current study and from each other. The variability of phenolic compounds among cooked garlic samples in different studies is directly related to the type of garlic and the climatic conditions of the growing region [92]. In addition, different biochemical mechanisms in synthesizing phenolic compounds during different heat treatment applications may also cause the difference. According to Volf et al. [93], phenolic compounds can maintain their stability at mild to moderate temperatures of 60–100 °C. Although De Greef et al. [94] reported that the bioactivity of various polar compounds in garlic, including flavonoids, phenolics, saponins, and sapogenins, did not change with cooking and storage, it is thought that the relatively high cooking temperature (180 °C) applied in this study may cause the degradation of phenolics. Because the reactions that occur depending on the type of cooking can vary according to cooking temperature, contact with air and other factors that can alter the oxidation and degradation processes [95, 96].

Discrimination of the cooked garlic samples with principal component analysis (PCA)

The PCA was carried out to evaluate the antioxidant properties (total phenolic content and DPPH and ABTS radical scavenging activities) and the profiles of the individual phenolic compounds of garlic samples cooked with different cooking materials and oils and to observe the differences. The outputs of the PCA, including the score scatter plot, the loading scatter plot and the biplot, are shown in Fig. 2a, b and c, respectively. Results showed that the first (PC1; 41%) and second (PC2; 33%) components accounted for 74% of the variation between garlic samples. The detected rate indicates that the study data are suitable for PCA. As ferulic acid and hesperidin were not detected in most samples, they were not included in the PCA analysis. The only roasted (SO-R), roasted with unbleached parchment paper (SO-RUP), roasted with bleached parchment paper (SO-RBP), roasted with oven bag (SO-ROB) groups of garlic cooked with sunflower oil, and the only roasted garlic group cooked with hazelnut oil (HO-R) were on the left side of the PC1, while the other groups were on the right side (Fig. 2a). The separation of SO-R samples from the others (Fig. 2a) indicates differences in the measured parameters. In the loading scatter plot (Fig. 2b), the close position of fumaric, quinic, and rosmarinic acids on the left side of the plot indicates that fumaric and quinic acids (r = 0.57, p < 0.01), fumaric and rosmarinic acids (r = 0.76, p < 0.01), and quinic and rosmarinic acids (r = 0.49, p > 0.05) are positively correlated each other. A similar situation was also observed for DPPH and ABTS (r = 0.76, p < 0.01), TPC and DPPH (r = 0.45, p > 0.05), and TPC and ABTS (r = 0.29, p > 0.05) on the right side of the plot (r = 0.76, p < 0.01). In the biplot plot shown in Fig. 2c, SO-R were located to the left of PC1, close to fumaric acid, quinic acid and rosmarinic acid. This location indicates that SO-R contain higher levels of these phenolic compounds than other cooked garlic samples. CO-RBP (corn oil + roasted with bleached parchment paper) and OO-ROB (olive oil + roasted with oven bag) samples, which are to the right of PC1, are also samples with high levels of ABTS, DPPH, and TPC. The SO-R sample also has high DPPH radical scavenging activity and total phenolic content. However, its low ABTS radical scavenging activity caused it to be positioned to the left of PC1.

Fig. 2
figure 2

Score scatter plot (a), loading scatter 3D plot (b), and biplot (c) of the PCA analysis (PC1 vs. PC2), including the antioxidant properties and individual polyphenols of cooked garlic samples. *R only roasted; RUP Roasted with unbleached parchment paper; RBP Roasted with bleached parchment paper; ROB roasted with oven bag; SO sunflower oil; HO hazelnut oil; CO corn oil; OO olive oil

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Chemical composition (volatile and non-volatile)

Garlic contains both volatile and non-volatile bioactive compounds. Sulfur compounds, aliphatic hydrocarbons, heterocyclic compounds, acids, alcohols, aldehydes, esters, ketones, ethers, and aromatic hydrocarbons are volatile compounds [97], while saponins, sapogenins, flavonoids, and phenols are non-volatile compounds [94]. Thiosulfinates, especially S-allyl-cysteine sulfoxide, dominate non-volatile aromatic compounds [98]. Important volatile compounds formed by the degradation of thiosulfinates are ajoenes, allicin, 1,2-vinyldithiin, allixin and S-allyl-cysteine, and sulfides (diallyl-, methyl allyl-, and dipropyl mono-, di-, tri- and tetra-sulfides, etc.) [99]. However, due to their relatively high volatility and chemical instability, most volatile components can be easily lost during the production, storage and preparation stages of foods (especially due to high-temperature applications) [100].

The percentage classification of the chemical compounds detected in garlic samples cooked with different oils and cooking materials is shown in Fig. 3. According to this classification, phenols and terpenoids were the constituent groups detected in all cooked garlic groups. Their contents ranged from 1.68 to 6.37% and 8.37 to 63.85%, respectively. Acids and alcohols were detected in the 1.62-24.69% and 1.09-10.66% range in all sunflower, hazelnut, and corn oil applications. However, no acids were detected in RUP, RBP, and ROB cooked with olive oil and no alcohols in RBP. Amides were found only in the hazelnut oil + unbleached parchment paper group (16.37%). Esters were found in sunflower oil + only roasted, sunflower oil + bleached parchment paper, hazelnut oil + unbleached parchment paper, hazelnut oil + bleached parchment paper, hazelnut oil + oven bag, olive oil + only roasted, and olive oil + unbleached parchment paper combinations at 8.57, 1.51, 12.11, 37.18, 31.86, 17.92, and 16.26% respectively. Sugars and sugar derivates were detected in sunflower oil in only roasted and unbleached parchment paper groups and hazelnut oil + unbleached parchment paper groups. Sulfur-containing compounds and vitamins and their derivatives were 0.97 to 13.22% and 3.84 to 24.14%, respectively, in all treatments except olive oil combinations. Steroids were detected only in hazelnut oil + oven bag and corn oil + only roasted groups (Table 4).

Fig. 3
figure 3

The percentage GC-MS classification of cooked garlic samples. *SO sunflower oil; HO hazelnut oil; CO corn oil; R only roasted; RUP Roasted with unbleached parchment paper; RBP Roasted with bleached parchment paper; ROB roasted with oven bag

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Table 4 Chemical composition of garlic samples treated with different oils and cooking materials
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The GC-MS analysis data of cooked garlic samples revealed twenty-nine compounds shown in Table 4. A total of 29 chemical substances were identified, which were 8 esters, 6 acids, 3 alcohols, 3 sugars and sugar derivatives, 2 sulfur-containing compounds, 2 terpenoids, 2 vitamins and vitamin derivates,1 amide, 1 phenol, 1 steroid compound (Table 4). Studies on the chemical composition of cooked garlic in the literature are limited. Bi et al. [18] detected the volatile flavor components of garlic after treatment with four cooking methods steaming, frying, boiling, and roasting by using gas chromatography ion mobility spectrometry (GC–IMS). In this study, they identified a higher number of compounds (A total of 73 volatile substances were identified, which were 27 thioethers, 26 aldehydes, 7 ketones, 4 esters, 4 alcohols, 2 furans, and 3 heterocyclic compounds) than in our study. The flavor components of cooked garlic samples are mainly consist of terpenoids. Among the terpenoids determined, β-sitosterol was detected in only roasted, roasted with oven bag, unbleached and bleached parchment paper groups of sunflower oil, hazelnut oil, and corn oil applied garlics. In addition, this compound could not be detected in all cooked garlic samples cooked with olive oil. Likewise, diallyl disulfide, diallyl trisulfide, α – tocopherol, and 24,25-dihydroxycholecalciferol could not be detected in all cooked garlic samples cooked with olive oil. The phenolic compound 2.4-di-tert-butylphenol was the only compound detected in various amounts in all cooked garlic samples. Squalene, the other terpenoid identified, occurs in varying concentrations in most vegetable oils and constitutes a significant part of olive oil’s hydrocarbon fraction [101]. Due to the six unconjugated double bonds in its structure, squalene strengthens immune function, resists skin aging and has hypolipidemic, antioxidant, antitumor, antibacterial and detoxification effects [102]. In support of this information, all garlic samples cooked with olive oil showed high levels of squalene, ranging from 52.11% (only roasted group) to 63.85% (roasted with unbleached parchment paper group). However, the lowest β-sitosterol (8.37%; 9.41%) was detected in the HO-RUP and HO-RBP samples compared to other samples. The β-sitosterol percentage values of roasted garlic vary according to the paper group (Table 4).

According to Table 4, trans-13-octadecenoic acid only in OO-R, cis-vaccenic acid, erucylamide, lauryl acrylate, ethyl palmitate in HO-RUP, ethyl trifluoroacetate in HO-ROB, propanoic acid. 3.3’-thiobis-. dihexadecyl ester, 9-hexadecenoic acid was detected only in HO-RBP, and D-melesitose in SO-R. Among these compounds, erucylamide is a fatty acid amide with antimicrobial activity [103]; It should be taken into consideration that trans-13-octadecenoic acid has pharmacological activity as being anti-inflammatory [104] and cis-vaccenic acid has anticancer properties [105]. Additionally, plasma and erythrocyte levels of cis-vaccenic and gondoic acids are inversely related to the likelihood of diabetes [106]. Furthermore, cis-vaccenic acid in plasma phospholipids has been associated with a lower risk of diabetes [107]. Trans-13-octadecenoic acid could only be detected in OO-R from cooked garlic samples. The use of other cooking processes in cooking with olive oil compared to traditional only roasted cooking showed a reducing effect for this compound. Cis-vaccenic acid was detected only in HO-RUP from cooked garlic samples. According to the results, we can say that the use of hazelnut oil and unbleached parchment paper in the roasting process showed a protective effect for the cis-vaccenic acid compound.

Conclusion

Garlic is a root vegetable rich in bioactive compounds with antioxidant activity. These compounds make it a functional ingredient used to treat various diseases. Recently, researchers have conducted many in vitro and in vivo studies investigating the bioactive components of various garlic species and their effects on various diseases. The present study was carried out to determine the antioxidant properties, phenolic compound profile and chemical composition of Taşköprü garlic grown in Kastamonu after cooking with different cooking oil types and different cooking materials. It has been observed that different cooking oils and different cooking materials lead to various changes in the measured properties. These results showed that using different oils and methods may be important factors for the antioxidant properties, phenolic compound profile, and chemical compound profile of oven-baked garlic, and consumers can choose the appropriate cooking method accordingly Based on the cumulative results, OO-ROB and CO-RBP were found to be the treatments that preserved the desired antioxidant properties of garlic. Therefore, it can be recommended to apply the mentioned methods for cooking garlic. Additionally, we can say that using hazelnut oil and unbleached parchment paper in the roasting process showed a protective effect for the cis-vaccenic acid compound. In terms of future studies, it is thought that it would be useful to analyse other biochemical compounds, such as the fatty acid profile, and to examine the effects of cooking materials and oils used for Taşköprü garlic on different garlic varieties.