Aromatic Perspectives: An In-Depth review on Extracting, influencing Factors, and the origins of raisin aromas

Graphical abstract Factors affecting, extraction method and origin of raisin volatile organic compounds.


Introduction
Grapes are a non-climacteric berry fruit, generally growing in a cluster on deciduous woody vines.Grapes belong to the 'Vitaceae' family and are produced worldwide.The worldwide production of grapes increased per year by a million tons (MT), as shown in Supplementary Fig. 1.In the year 2021, China topped the list for grape production with 11.2 million metric tons, while Italy (8.15 MT) and Spain (6.07 MT) secured the 2nd and 3rd positions, respectively, according to the Food and Agriculture Organization of the United Nations (2023).Grapes can be consumed as fresh fruit or used in various products such as juice, jelly, vinegar, seed oil, wine, and raisins (Venkitasamy et al., 2019).
Raisins are dehydrated grapes produced in grape-growing regions and consumed throughout the world as a snack, as well as used in baking, brewing, and cooking.In terms of dried fruits production, raisins, and currants were in the top position with the production of 1,262 thousand metric tons (TMT) following dates (836 TMT), prunes (253 TMT), apricots (160 TMT), and figs (121 TMT) were reported by the global statistical organization (INC, 2016).According to Statista report, China is the second biggest raisin-producing (2.20 TMT) country after Turkey (Shahbandeh, 2022).The major production area for raisin is Turpan, Xinjiang, which accounts for nearly 75 % of the total production of China (Khiari et al., 2018).More than 20 different kinds of varieties were planted for making raisins.Among these, 'Thompson Seedless (TS) is a prominent variety because it is more suitable for processing and cultivated up to 85-90 % of the total grape area in the Turpan state of China (Inouye et al., 2016).A number of raisin varieties were produced based on consumer preferences, including small, medium, and large, as well as seeded and seedless grapes in green, black, brown, blue, purple, and yellow color.
Grapes, being seasonal fruit, are processed into raisins because they are not always available in the market.There are two primary classifications of raisin drying technologies: traditional and modern approaches.Traditionally, raisins are produced under the sun or in the air (Wang et al., 2015).In order to increase the rate of dehydration and ensure a higher quality of the raisins, various modern drying techniques have been used, including microwave, vacuum pulsed, oven, and infrared drying (Wang et al., 2016).The first raisins were thought to have been produced in the Near East by simply digging the grapes in the sand.Drying grapes significantly increased their shelf life and produced a high sugar content, making them ideal energy sources for laborers (Carughi et al., 2008).Furthermore, it inhibits the growth of mold, yeast, and bacteria, slows down enzymatic degradation, and inactivates basic physical and biochemical activities by eliminating the water until the appropriate level of moisture is obtained.(Khiari et al., 2018).Modern and traditional drying techniques significantly affect the changes in VOCs that contribute to raisin aroma (Torres et al., 2015;Wang et al., 2015Wang et al., , 2017)).
Raisins are comprised of sugar about 72 % by weight (Winkler et al., 1974), which mainly consist of fructose and glucose.They also contain higher contents of antioxidants, such as in apricots and prunes, but the concentration of vitamin C is lower than in fresh grapes.Raisins are found to be cholesterol-free dry fruit.The National Nutrient Database from the United States Department of Agriculture (USDA, 2018) reported the nutritional values of the raisin seedless (Vitis vinifera) per 100 g are mentioned in Table 1.
The aroma of raisins is an integral sensory quality that affects customer acceptance.It is the result of the accumulation of several VOCs, and the composition varies according to the variety and species (Schwab et al., 2008), geographical regions (Feng et al., 2022), vintage (Yuan & Qian, 2015), drying methods (Torres et al., 2015;Wang et al., 2015Wang et al., , 2017)), storage conditions and packaging materials (Kaack & Christensen, 2010).Approximately 120 VOCs are thought to make up raisins volatile profile, including free-form and glycosidically bound compounds.These VOCs belong to various chemical classes (aldehydes, esters, acids, alcohols, ketones, terpenes, and furans) and have a variety of flavor characteristics (fruity, green, floral, roasted, fatty, and chemical).The major sources of raisin VOCs include fresh grapes, Maillard reaction (MR), oxidation of unsaturated fatty acid, carotenoids, and lipids (Javed et al., 2018(Javed et al., , 2019;;Yuan & Qian, 2015).
Numerous reviews have been published on topics such as raisin drying methods, phytochemical attributes, and the associated health benefits (Khiari et al., 2018;Williamson & Carughi, 2010;Wong et al., 2013).Grapes drying has been extensively studied and there are many fascinating instances in the literature, we discovered that there is not a comprehensive review that addresses raisin aromas and the sources of VOCs such as UFAO, MR, terpenoid, and carotenoids pathways.In this review, we have explored factors that affect the raisin aromas including grape varieties, harvesting time, drying methods, climatic factors, processing methods (traditional and modern), and storage.Aroma is a key trait that has gained more attention these days.Aroma research has markedly advanced in several domains due to the rapid advancement of science and technology, particularly using Gas chromatography-Mass spectrometry (GC-MS) and other analytical tools.In this overview, the components of raisin aroma and the process by which distinct aroma compounds are produced were briefly examined, followed by a discussion of the variables influencing the volatile nature of aroma compounds and their biosynthetic pathways.

Extraction of raisin volatiles
The identified raisin VOCs were extracted via the distillation process and the solid phase micro-extraction (SPME) technique is depicted in Fig. 1.In earlier research, the volatile components of raisins were isolated and extracted using simultaneous distillation extraction and steam distillation (Buttery et al., 1981;Joulain & Fourniol, 1990;Ramshaw & Hardy, 1969).Ramshaw & Hardy (1969), utilized steam distillation (a technique that uses heat to vaporize the VOCs, which are then condensed and collected) to characterize 51 VOCs in sultana raisins, whereas only 38 VOCs through simultaneous distillation extraction (in this technique, volatile organic chemicals were separated from complex liquid matrices using a combination of solvent extraction and steam distillation) was discovered in Thompson seedless raisins (Buttery et al., 1981).Palma & Taylor (2001) used carbon dioxide, at high pressure and high temperature to extract 5-hydroxymethyl-2-furaldehyde from raisins and found a quantity of 0.128 mg/g.Overall 67 VOCs have been found in previous research, and numerous significant volatile components are still missing due to the limitations of analysis techniques (Guiné et al., 2010).Afterward, SPME (a technique that involves collecting the VOCs present in the headspace above a sample of raisins, and trapping them on a solid-phase microextraction fiber) was used for the first time and 77 VOCs were extracted from three raisin cultivars (Wang et al., 2015).SPME is an inexpensive, easy-to-operate, relatively solventfree, and sensitive technique as compared to others (Abdulra'uf & Tan, 2015;Silva et al., 2015).Furthermore, a number of studies were carried out, and the SPME technique was used to identify more than 100 raisin volatiles in different varieties during drying and storage (Javed et al., 2018(Javed et al., , 2019(Javed et al., , 2021;;Tesi & Lenzi, 1998;Thakur et al., 1996;Wang et al., 2017Wang et al., , 2020Wang et al., , 2021)).The SPME was found to be the most effective extraction method that was significant in the isolation of VOCs from raisins.Further work needs to be explored on a sophisticated technique that contributed a major role in extraction.

Factor affecting raisin aroma
The following factors such as grape variety, harvesting time, climatic, vintage, pre-treatment, processing method, and post-harvest handling and storage conditions (Fig. 2) have been identified to impact the quality and aroma profile of raisins, as detailed in Table 2 (Buttery, 2010;Wang et al., 2017).Raisins can be classified into different categories based on several factors, including grape cultivar attributes (such as size, color, and shape), the method of drying (such as green, golden bleached, sulfur bleached, and lexia), place of origin (like Patras, Malaga, Vostizza, Pyrgos, Symrne, and Valencia), size grading (including grade A, B, C, and substandard), as well as consumer preferences (like seeded, loose, and layer variations) (USDA, 2023).

Raisins varieties
Different grape varieties contain unique VOCs that contribute to their specific aromas.Raisins are generally produced from four grape varieties (TS, Black Corinth, Muscat, and Sultana).Overall, 90 % of Thompson Seedless Raisins (TSRs) are produced in the United States (Bhat et al., 2006).For the production of raisins, more than 20 different grape cultivars are grown in China, including Beauty Seedless, Centennial Seedless (CS), Crimson Seedless, Flame Seedless (FS), Horse Milk, Hamburg, Muscat, Thomson Seedless, Ruby Seedless, etc.Among these, the Flame Seedless raisin (red variety), TS (green variety), and Crimson Seedless raisin (red variety) are the major varieties for raisin production (Wang et al., 2015).
Different raisin varieties produced different kinds of aromas and were characterized into different classes like green, fruity, floral, fatty, and roasted.Wang et al. (2017) experimented on three different types of seedless raisins: TS, CS, and Zixiang seedless (ZS).They found that Centennial seedless raisins (CSRs) had higher levels of floral, fruity, green, and roasted aromas than TSRs because of the presence of 2-pentylfuran, benzeneacetaldehyde, (E)-2-nonenal and 3-ethyl-2,5-dimethyl pyrazine.The Zixiang seedless raisins (ZSRs) exhibited more pronounced floral and fruity aromas compared to TSRs and CSRs while displaying reduced intensity of green and roasted aromas.Furthermore, a comprehensive study was performed during the storage of raisins for 0, 3, 6, and 9 months.A total of 81, 94, and 98 VOCs were quantified in TSRs, CSRs, and ZSRs, respectively.The aroma profile revealed that floral notes were the dominant characteristic in ZSRs, while CSRs exhibited noticeable roasted and herbaceous flavors (Javed et al., 2018).However, different varieties can be distinguished by their distinctive fragrances due to the specific generation or intensity of volatiles.

Harvesting time
Harvesting of fresh grapes is the initial step for the production of raisins.The best harvesting time is on a warm and sunny day when they are high in sugar content and have no surface moisture.Subsequently, ripening is an important factor for harvesting, described as the development of a waxy layer on grape skin, softening, turnover of color (according to variety), and changing the taste from sour to sweet (Jin et al., 2017).The harvesting time has a significant impact on the quality of raisins, particularly due to variations in the levels of soluble solids.Harvesting time and total soluble solids (TSS) vary according to the raisin varieties and environmental conditions.The three cultivars most commonly used to make raisins such as TS, FS, and Crimson ceedless, were harvested with 20-22 • Brix of TSS (Wang et al., 2015).The grapes that were harvested before ripeness had a high level of acidity, which ultimately led to the undesired raisin product.The yields and quality of raisins significantly improved when grapes were harvested at their optimal ripeness (Sharma et al., 2013).Accordingly, harvesting the grapes at the appropriate physiological maturity improves the plumpness, juiciness, and appealing wrinkled texture of raisins.
times; in the first harvest the grapes were under the local harvest schedule (LHS), and then three further harvests were carried out with a ten-day gap.The results showed that the fourth harvest (30 days after LHS) had greater TSS (62.8 • Brix) and yield (2.14 kg) than the other harvests.Additionally, it takes less drying time to prepare raisins.In another study, the grape cultivars including TS, Manik Chaman, 2A Clone, Sonaka, and Merbein Seedless were harvested at five different intervals: 20 days before LHS, 10 days before LHS, LHS, 10 days after LHS, and 20 days after LHS.The findings demonstrated that the maximum rate of drying was achieved by raisins made from late-ripened grapes (20 days after LH) and a significant raisin recovery rate (26.20 %) was recorded in late-harvested TS grapes.Further, the sensory characteristics such as color, texture, flavor, and general acceptability with scores of 3.97, 3.89, 3.87, 4.09, and 3.96, respectively, as well as quality indicators like TSS/TA, ascorbic acid, and sugar content were increased in all raisin cultivars with each subsequent harvest date, reaching their greatest levels in the late harvest (Venkatram et al., 2015).Grapes that are harvested at different stages of ripeness will have different levels and types of volatile components.Therefore, to identify the optimal harvesting time, Gu et al. (2022) conducted a study on four grape varieties (Cabernet Sauvignon, Cabernet Gernischet, Cabernet Franc, and Merlot) harvested at seven different times from 3 September to 15 October, with 7-day intervals.They concluded that the peak sugars/acids ratio was achieved on day 21 for Merlot and on day 28 for the other three varieties.As ripening progresses, carbonyl compounds decrease while freeform alcohols, esters, and terpenes increase until the overripe stage.Additionally, bound-form aromas reach their peak on October 8th.This study suggests that optimal aroma maturity in tested grape varieties occurs later than the sugars/acids ratio in Ningxia, emphasizing the importance of aroma in grape ripening.Late harvesting enhances raisin quality and sensory attributes.Future research should explore the impact of harvesting time on raisin VOCs and aroma profiles.

Climatic factor
Grapevines are grown all over the world in a variety of climatic regions that offer a suitable environment for growing superior grapes and their products (raisins and wines).Climatic factors including temperature, humidity, light, and wind speed have a significant impact on the quality of raisins during the drying of grapes.The ideal temperature for drying grapes is between 40 and 45 • C, while low and high temperatures have an impact on raisins by delaying the drying process and changing their color, respectively (Sharma & Somkuwar, 2014).Higher humidity leads raisins to darken in color, and when these conditions persist for a while, raisins start to rot (Wang et al., 2016).In order to improve the color of the raisins, it is best to choose locations where the wind blows swiftly and with appropriate sunlight.
In earlier studies, two raisin varieties such as TS and FS were dried at 45-50 • C with airflow rates of 750 fpm responsible for better color and sweet taste as compared to sun-dried raisins (Gee, 1980).Langová and their Co.used five grape varieties including Bezsemenné, Perlette, Vrboska, Beauty seedless, and Jakubské that were dried under four different methods at various temperatures: drying at 40 • C, combination drying (70, 65, and 60 • C), drying at 70 • C, and lyophilizations.They found that those grapes dried at low temperatures (40 • C and lyophilized) had the best sensory quality in terms of smoothness, gloss, color, odor, chewability, juiciness, flavor, and overall impression as compared to the higher temperature (Langová et al., 2020).Chen and their   (Chen et al., 2022).Additionally, the interaction impact of temperature (20 • C and 30 • C) and relative humidity (70 % and 90 % RH) on the aroma profile of Torreya grandis nuts during drying was assessed, and both temperature and humidity were found to be effective.Particularly, terpenes were identified as the primary aromatic compounds, exhibiting a notable increase under conditions of elevated humidity.This augmentation can be attributed to the facilitation of D-limonene synthesis, a major constituent of terpene compounds, comprising a substantial proportion ranging from 63 % to 90.8 % (Hu et. al., 2022).The quality of the aroma is significantly influenced by temperature and humidity, which are important climatic factors.The majority of the research on raisins has focused on their quality parameter and sensory attributes.Further study is needed to identify the VOCs that are produced at various temperatures and to optimize the humidity for a richer aroma.

Processing method
The process of drying grapes is one of the most essential tasks for extending shelf life and minimizing financial losses.Furthermore, it improves the drying rate, as well as produces good quality products with distinctive aromas (Javed et al., 2018).The various methods that are being used to make raisins, such as traditional, modern, and chemical treatment, can also impact the VOCs present in the final product.

Traditional methods
Thousands of years ago, a sun-drying method is used all over the world, even nowadays.In this method, the sun is directly exposed to the grape bunches which were spread over the ground or on a plastic sheet or different sizes of tray.Sun-drying process is the most cost-effective and requires less time to make raisins as compared to other traditional methods due to the penetration of solar radiation into the grape berries, which produces heat on the surface and in the core of the berries.As a result, this mechanism accelerates heat transfer and quickly evaporates the moisture content (Mahmutoglu, 1996).Conversely, insect infection, dust, and other environmental risks lead to a significant decline in quality (Bhat et al., 2006).Furthermore, the raisins that were subjected to direct as well as intense sun radiation (due to temperature variations) showed differences in color, appearance, and aroma (Pangavhane & Sawhney, 2002).Air-drying or shade drying is also known as the traditional method of grape drying.This method is most commonly used in China, India, and Australia, in which natural air is used as a principal source for drying (Pangavhane & Sawhney, 2002).
In the early research (Buttery et al., 1981;Ramshaw & Hardy, 1969), the raisin was prepared from TS grapes by sun-drying method and identified 38 and 40 VOCs, respectively.They found that the key VOCs were octanoic acid, nonanoic acid, (E)-2-octenal, (E)-2-decenal, nonanal, furfural, methyl furfural, 2-hydroxybutan-3-one and diacetyl.In a subsequent experiment, 32 VOCs were found in the must made from sundried Pedro Ximenez grapes.Among these VOCs, hexanoic acid, isobutanol, isoamyl alcohol, benzyl alcohol, 2-phenylethanol, γ-hexalactone, γ-butyrolactone, and 5-methylfurfural were significantly higher in sun-dried must as compared to fresh grapes must (Franco et al., 2004).Around 25 years later, Wang et al. (2017) detected 91 free-and bound-form volatile chemicals in sun-dried raisins from the three cultivars which include TS, CS, and ZS using the sophisticated technique "SPME".They concluded that ZSRs performed exceptionally well because their floral OAV was much higher than the other two varieties, as well as, the fruity and green OAVs were also significantly differentiated.ZSRs' flowery and fruity aromas become prominent due to the presence of decanal, rose oxide, linalool, geraniol, and β-damascenone.In China, the dry and hot desert winds are used for raisins production in a specific ventilated room, which is made of clay bricks (Approx.3 x 4 x 6-8 m 3 ), along with several bricks-sized and cross-shaped openings in the walls (Fang et al., 2010).The air-dried raisins attained better color, appearance (Wang et al., 2016), and aroma quality (Wang et al., 2015) than sun-dried raisins, and also evade direct contact with sunlight.

Modern technologies
With the rapid advancement in agricultural technologies and mechanization; modern drying techniques have been commonly used in developing countries for rapid, controlled, and high-quality production of raisins by using less labor.Christensen (2000) reported that other modern technologies such as oven drying, microwave heating, freeze drying, and vacuum pulsed drying are used for raisins production.In recent years, microwave and oven drying have attained consideration owing to its less drying time, quick processing rate, instant, and accurate electronic control, as well as a clean heating process (Rattanadecho & Makul, 2016).In the microwave drying process, electromagnetic radiation is converted into thermal energy to vaporize the moisture from the grapes, whereas direct thermal energy is employed to dehydrate the food product in the oven drying process (Michailidis & Krokida, 2014).Furthermore, vacuum drying is a method in which fresh produce is dried under subatmospheric pressure and is magnificently used in raisins processing (Parikh, 2015).Recently, the infrared drying method has become more popular in the food industry due to its short drying time, producing the best final product, and the capacity to consume less energy.In addition, it is a more cost-effective method in comparison to microwave and vacuum drying techniques (Riadh et al., 2015).The main goal of these modern technologies is to obtain the best quality product by utilizing less energy and labor.The combination of these technologies can be helpful to achieve that goal.
The influence of several drying techniques (freeze-drying, ovendrying, microwave-dried, and shade-drying) on the volatile profile of strawberries was compared using the HS-SPME/GC-MS method, and 167 VOCs were characterized.The results revealed that shade air-dried, frozen, freeze-dried, and oven-dried (at 45 • C) strawberries retained more sweet and fruity, representing more than 68 % of the total aroma intensity.In contrast, the microwave-drying method showed a drastic loss of fruity esters (Abouelenein et al., 2021).Recently, a study was conducted on the changes in volatile organic chemicals, in which lemon flavedo was dried using integrated freeze drying (IFD), conventional freeze drying (CFD), and hot-air drying (AD).The findings indicated that IFD, followed by CFD and AD, preserved the most volatile aromas.Furthermore, CFD, IFD, and AD samples lost 56.07 mg/g, 47.92 mg/g, and 13.22 mg/g of total volatiles, respectively, compared with fresh samples (Hu et al., 2023).A series of research works have been done on the physical, physiological, and biochemical characteristics of raisins using modern drying techniques (Caglar et al., 2009;Kassem et al., 2010;Wang et al., 2017), but no one has looked at the impact of these techniques, particularly on aromatic components.
Accordingly, drying procedures have a significant impact on raisins' aromas.The high-temperature drying technique or sun-drying method did not retain the fresh aroma, but it produced a more roasted and fatty aroma.Contrarily, it was discovered that air-dried, shade-dried, and freeze-dried raisins preserved a fresh aroma and were also responsible for producing fruity, floral, and green aromas in raisins.

Storage
Raisins can be kept at room temperature for a few months without losing their taste, flavor, or color.The storage stability of raisins depends upon the room temperature, relative humidity (RH), and moisture level in raisins.The optimum range of RH for raisin storage is about 45-55 % (Bhat et al., 2006).However, to retain the quality for long-term storage, raisins can be kept in the refrigerator, as well as in a controlled atmosphere and modified atmosphere.
According to Kaack and Christensen (2010), storage conditions and packaging material significantly impacted the contents of VOCs present in tea.Javed et al. (2018) carried out a research study on the storage of air-and sun-dried raisins for 0, 3, 6, and 9 months in plastic and woven bags.They concluded that sun-dried raisins stored in plastic bags had greater total amounts of VOCs than those stored in woven bags.Furthermore, the fruity and floral aromas were noticeably affected by plastic bags during the storage periods (3, 6, and 9 months).Another study conducted on the storage of TSRs revealed that the fruity, floral, herbaceous, and roasted aromas improved during the storage period and became more compelling in 12 months (Javed et al., 2019).

Origin of raisin volatiles
Previous studies have reported that raisin VOCs are derived from different sources such as fresh grapes, unsaturated fatty acid autooxidation (UFAO), Maillard reactions, and carotenoids.A schematic diagram (Fig. 3) was proposed that illustrated the probable generation pathways of raisin VOCs based on previous research regarding metabolic aroma biosynthesis in grapes and wine.

Lipid oxidation
Lipid-derived VOCs are one of the major groups of food flavors.The consumer acceptability of foods was influenced as a result of the formation of VOCs by the process of lipid oxidation.Lipid oxidation is carried out in plant tissue by the enzyme at periods of maturity, growth, development, or tissue disturbance.In addition, it imparts a distinctive flavor to a variety of fruits and vegetables (Jelen & Wasowicz, 2011), including raisins (Javed et al., 2021).The fatty-derived compounds are also the source of fruity flavor, produced by enzymatic breakdown, α-oxidation, and β-oxidation in intact or disrupted fruits through the autoxidation of unsaturated fatty acid (Schwab & Schreier, 2002).
The oxidation process of unsaturated fatty acids is shown in Fig. 3A.The primary enzymes involved in this reaction included lipase, lipoxygenase (LOX), alcohol dehydrogenase (ADH), and hydroperoxide lyase (HPL).In terms of unsaturated fatty acids, TSRs and CSRs were found to have the highest concentrations of linoleic acid (C18:1) and linolenic acid (C18:2), which ranged from 1.6 to 2.2 ug/g (Wang et al., 2021).Both linoleic acid and linolenic acid are oxidized into 9-& 13-hydroperoxides by the action of LOX.The 13-hydroperoxides further oxidize to aldehyde compounds by HPL, as well as aldehyde compounds converted to alcohols by ADH (Stone et al., 1975).

Maillard reaction
The Maillard reaction (MR), particularly in heated/dried foods, has a significant effect on food quality.The nutritional value of food is impacted by browning due to MR, which may also result in the production of antioxidant molecules and a distinct flavor of food (Van Boekel, 2006).
MR is usually classified into three ( 3) stages (Fig. 3B).The first stage starts with a condensation process among reducing sugar and amino acids, leading to the formation of Amadori (if aldose is reducing sugar) or Heyns product (ketose reducing sugar).The second stage starts from the Heyns or Amadori product that converts into sugar fragmentation and discharge of the amino group.In the last stage, all kinds of dehydration, cyclization, fragmentation, and polymerization reactions are carried out, in which amino acid again takes part.In the case of flavor formation, Strecker degradation is of utmost importance.The several possible reaction pathways depended on temperature, pH, and kind of sugar, amino acid, and protein.

Terpenoids-derived volatile compounds
Terpenoids are the most prevalent and structurally diverse secondary metabolites in plants.They are vital to plant survival because they facilitate both direct and indirect defense mechanisms, attract pollinators, and boost various interactions between the plants and the environment.They are considered one of the most important classes that contribute to fruits' characteristically fruity and floral aromas (Macleod & de Troconis, 1982) and play a significant role to produces aromas in raisins (Javed et al., 2019;Wang et al., 2017).Likewise, they can be synthesized via two different pathways: the 2-C-methyl-D-erythritol-4phosphate (MEP) and the mevalonate (MVA) pathway.MEP is sometimes referred to as the 1-deoxy-D-xylulose-5-phosphate (DXP) pathway.Fig. 3C briefly describes both distinct synthesis pathways and their corresponding processes.

Odor Activity Value
The Odor Activity Value (OAV) has been commonly utilized in order to identify the active VOCs that contribute significantly to the aroma profile (Zhu et al., 2018).OAV, also known as an odor value or odor unit, is an assessment of odor intensity based on the ratio of a volatile compound's concentration to its odor threshold value.The reported aroma compounds with OAVs > 1 from different studies were generally categorized into six groups based on their sensory characteristics: floral, fruity, green, roasted, fatty, and chemical aromas.
The 20 aromas active compounds were recognized as OAVs in airdried TSRs, CSRs, and FSRs which made a significant contribution to raisin aromas.Among these, β-damascenone was the most prominent compound that had a higher OAV in all three varieties.A characteristics comparison of the three varieties revealed that the fruity, floral, green, and chemical OAVs were almost the same, whereas the roasted OAVs had higher in FSRs, followed by TSRs and CSRs with 57.69, 15.11, and 8.85 (Fig. 4A), respectively (Wang et al., 2015).A follow-up study used a traditional sun-drying procedure to yield 25 odor-active compounds from three grape varieties with varying levels of flavor intensity.Overall, CSRs were the richest flavoring varieties because it was having the potent fragrance of fruity, green, and roasted aromas as compared to TSRs and ZSRs (Fig. 4B).Rose oxide is a VOC that contains OAVs ≥ 1 and is responsible for the significant floral aroma in ZSRs.Besides that, hexanal and 3-ethyl-2,5-dimethylpyrazin mainly accounted for the roasted and green aromas, respectively (Wang et al., 2017).These two experiments were carried out by our research team utilizing the same procedure for volatile organic compound (VOC) extraction and identification.Therefore, comparison analysis revealed that sun-drying is more effective than air-drying based on CSRs and TSRs.In order to provide additional evidence, Javed et al. (2021) carried out a comparison study on aroma compounds that are changed from fresh grapes to raisins by drying, both with and without pre-treatment solutions (lipid, KOH, and Na 2 CO 3 ).Out of the 30 odor-active compounds, they also found that the predominant contributors to floral and fruity aromas were β-damascenone, while the primary contributors to green flavors were hexanal and (E,E)-2,4-nonadienal. Further, results concluded that the air-dried procedure retained the green, floral, and chemical fragrances, whereas the sun-dried method produced more roasted and fatty odors (Fig. 4C).The roasted aroma of grapes is brought about by the high temperature under the sun-drying process, which alters their green scent.In another research study, the pre-drying process for Gök Üzüm raisins was carried out using wood ash and potassium carbonate.The main odor-active compounds were β-damascenone (fruity), β-ionone (floral), rose oxide (floral), and cedrol (chemical), and the OAVs of these compounds were higher in the raisin treated with wood ash than in the raisin treated with potassium carbonate (Supplementary Table 1).Therefore, it was speculated that the wood ash pretreatment provided Turkish raisins with stronger fruity, floral, green, and chemical fragrance (Fig. 4D) (Karakus et al., 2023).
Furthermore, the VOC changes experienced by air-and sun-dried TSRs during the storage were monitored.Similar patterns of aroma changes were seen with both treatments when the raisins were stored in plastic and woven bags (Fig. 5).After 0 to 10 months of storage, the OAVs for fruity and flowery fragrances were found to be the same in sunand air-drying; however, after 12 months, they were shown to be more substantial (Fig. 5).As per the findings, air-drying preserved the fragrant qualities of fruits and flowers, but sun-drying produced a more roasted aroma in plastic bags as compared to woven bags (Javed et al., 2019).

Conclusion
Raisins are an excellent fruit selection for an individual's diet due to their reasonable price, accessibility, nutrition, and convenience.Raisin processing should be emphasized because the sensory quality of the final product has a significant impact on customer satisfaction.Up to now, over 120 volatile organic compounds (VOCs) have been documented; these are primarily extracted using a highly sophisticated method called solid phase microextraction (SPME).Further raisin aromas were influenced by various factors such as grape varieties, harvesting period, climate, drying process, storage, and extraction methodology.Different extraction methods were utilized to find the VOCs in raisins; the most successful approach was SPME.Moreover, it was greatly influenced by the drying process; high-temperature drying method or sun-drying process did not preserve the fresh aroma, instead, it resulted in a more roasted and fatty flavor.In contrast, it was found that air-drying, shadedrying, and freeze-drying methods maintain floral and green aromas.Regarding raisin cultivars, the red cultivars (CS and FS) were richest in green, fruity, and roasted fragrances, and had higher levels of terpenoids than the green variety (TS).Yet, there is no study regarding the effect of modern drying technology on raisin aromas.Therefore, future studies are needed in order to optimize modern technology to meet stringent raisin quality in terms of aromas.The consequences of modern drying techniques on the aromas of raisins have not yet been studied.Therefore, further research is necessary to optimize modern drying technology to meet the high standards for raisin quality in terms of fragrances.

Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Hafiz Umer Javed reports was provided by Beibu Gulf University.Hafiz Umer Javed reports a relationship with Beibu Gulf University that includes: employment and funding grants.Hafiz Umer Javed has patent NA pending to NA. we dont have any conflict If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 3 .
Fig. 3. Proposed pathways for the generation of the raisin aroma.

Fig. 4 .
Fig. 4. OAVs in different studies.A (Three raisin varieties dried by air); B (Three raisin varieties dried by air); C (Comparison of fresh, dried, and pre-treated raisin); D (Wood ash and potassium carbonate as a pre-dipping solution).

Table 1
Nutritionals values of Vitis vinifera per 100 g.
According to the National Nutrient Database from the United States Department of Agriculture (USDA, 2018).H.U.Javed et al.

Table 2
Factors affecting the quality and aroma of raisins.VOCs were found in TS, FS, and CS raisins, respectively Floral aromas in ZSRs > CSRs > TSRs Fruit aromas in ZSRs ≥ CSRs > TSRs Roasted aromas CSRs > ZSRs > TSRs Green aromas CSRs > ZSRs > TSRs Javed et.al., C had the highest levels of glucose, fructose, malic acid, shikimic acid, and succinic acid.Drying at 40 • C preferred the accumulation of various phenols while at 30 • C favored the accumulation of anthocyanins, flavonoids, and flavanols.Raisins produced at 30 • C had higher levels of volatile ketones and acids, with 2,6-Zicui raisins that were dried at temperatures from 30 • C to 50 • C. The study revealed that raisins dried at 30 • C exhibited higher accumulations of acids and ketones.Specifically, the concentration of 2,6-dimethyl-4-heptanone, a compound known for its sweet odor, was significantly higher in raisins dried at 30 • C (measuring 70.34 μg/L) compared to raisins dried at 40 • C and 50 • C Gu et al., 2022ld(2.14kg)thantheother harvests.Waste decay was also higher in 30 days-LHS Maximum drying rate and weight average were recorded in 20-ALHS.Sensory attributes like color, texture, flavor, and overall acceptability scored higher in 20-ALHS, with values of3.97, 3.89, 3.87, 4.09, and 3.96, respectively.Quality parameters such as TSS/TA, ascorbic acid, and sugar content increased in all raisin varieties with each harvest, peaking in 20-ALHS.The highest sugars/acids ratio was reached on D21 for Merlot and D28 for the other three varieties.Carbonyl compounds decreased consistently over the harvesting period.Free-form alcohols, esters, and terpene concentrations increased during ripening and decreased in overripe stages.Gu et al., 2022• C + lyophilized had the best sensory quality in terms of smoothness, gloss, color, odor, chewability, juiciness, flavor, and overall impression as compared to the higher temperature D-limonene, the main terpene (63.0-90.8%),was notably increased by high humidity.colleagueshave found 44 free-form VOCs in

Table 2
(continued ) After IFD, CFS, and AD, dried samples lost 7, 7, and 6 compounds, respectively, compared to fresh samples.The total loss rates of volatile compounds in the dried samples were 82.73 % in CFD, over 71.22 % in IFD, and over 28.78 % in AD.Storage time impacted VOC content, with 15 VOCs disappearing partially or completely during processing or storage.Tea from Sampo and Sambu cultivars stored in plastic and aluminum bags maintained satisfactory flavor and VOC content for at least 3 months post-processing.In contrast, tea from Samyl had an unpleasant grassy off-flavor in all samples.

Table 3
Volatile compounds reported in raisins.