Saffron Drying; How To Preserve The Color, Aroma, And Taste Of The Spices

Introduction

Most foods are highly perishable; therefore, awareness of shelf-life is vital if products are stored in a warehouse or by the consumer for substantial periods. Drying is a traditional and valuable method for keeping solid foods safe for long periods. When food products are exposed to drying conditions, the native physical state of the food product is altered, leading to changes in the quality and safety of the food product. Drying involves the removal of excess water from the food matrix until a "safe" moisture level is achieved, at which minimal or no physical, chemical, or microbiological reactions occur.

Moisture content in saffron is a critical parameter for preserving its characteristics (Alonso et al., 1993). To preserve saffron for a more extended period, it should be dried. Postharvest processing, such as drying methods and storage conditions, determines the stability of saffron, which directly affects the market value of the product. Drying operations, therefore, need to be precisely controlled and optimized. This is necessary to produce a high-quality product with the highest level of color and flavor while maintaining microbiological safety. During the dehydration process, the stigmas lose 80% of their weight. A lower moisture content, at least below 12%, maintains the quality of saffron for a longer time (International Standard ISO 3632).

Drying must be done correctly to achieve the correct moisture content level. If the stigmas are not dried soon after picking, they are attacked by molds. On the other hand, if saffron is dried too much, it breaks easily, turns into powder, and loses weight below the trade requirements. The equilibrium moisture content depends on the environment's temperature, relative humidity, and the plant's species, variety, and maturity (Iglesias and Chirife, 1982; Rahman, 1995).

The dehydration process not only plays a vital role in preserving saffron, but it is a critical step in developing the principal substance responsible for saffron's aroma, safranal (Del Campo et al., 2010). Fresh stigmas are not able to impart their organoleptic characteristics to food. A dehydration treatment, which is necessary to convert Crocus sativus L. stigmas into saffron spice, brings about the physical, biochemical, and chemical changes required to achieve the desired attributes of saffron (Carmona et al., 2005). The most apparent characteristic of saffron is its deep red color. The high gloss of fresh stigmas becomes dull upon drying, and a solid yellow extract passes into water upon wetting the dried stigmas. Water activity (aw) is an important parameter to assess the shelf-life of foods. Many desired and undesired physical changes in food can be correlated to the water activity of the system. Furthermore, water activity can significantly impact foods' color, taste, and aroma. The loss of saffron color is reduced at a medium water activity of 0.430.53 (Tsimidou and Biliaderis, 1997), and the development of safranal is promoted.

Drying methods

Drying by any method requires considerable skill to ensure a high-quality spice. The main attributes determining saffron's market value and quality are color, aroma, and taste. The compounds responsible for these attributes are crocins, safranal, and picrocrocin. The quantities of these compounds in saffron depend mainly on how the stigmas are dried. The drying process differs from country to country (Ordoudi and Tsimidou, 2004), and the different conditions of drying and aging affect the saffron constituents (Carmona et al., 2005). Low-temperature drying processes are favorable for maintaining the high bioactivity of desired biocomponents in the final product. A minimal change in nutritional values is targeted during low-temperature processes.

Generally, there are two forms of saffron dehydration: traditional and industrial drying. Three different procedures are employed depending on the temperature used for dehydration: sundried or at room temperature with ventilation (India, Iran, and Morocco), mild temperature (Greece and Italy), and high temperature (Spain) (Carmona et al., 2005).

Traditional methods

The traditional method of drying saffron varies from country to country and depends on the availability of required equipment (Acar et al., 2011). Natural sun drying is the most common drying method for saffron worldwide. Solar drying, in sun or shade, has been used for many years because of its simplicity and low investment cost, although it results in a photochemical decrease in color intensity. Natural sun-drying occurs outdoors, and the product is directly exposed to sunlight and at risk of contamination. Therefore, the ultraviolet solar light can degrade or isomerize the carotenoids in saffron.

These drying methods are still used in Iran, India, and Morocco for saffron stigmas drying. In Iran and Morocco, the stigmas are handled gently and spread thinly on a large cloth. They are then exposed to the sun for several hours or placed in shade for 710 days. Drying is completed before the stigmas break or crumble. Air-dried saffron retains its purplish-red color, fragrance, and aroma. In India, the stigmas are solar-dried for 35 days until their moisture content is reduced to 8%10%. However, these methods raise some problems, such as long drying times and microbial contamination of the dried materials (Gregory et al., 2005).

Artificial drying methods

Some modern drying methods have replaced the age-old drying method of fine mesh screens held over burning coals (Raines Ward, 1988). Artificial drying methods are carried out at higher temperatures and have been employed in saffron processing in some countries, such as Spain, Greece, and Italy (Carmona et al., 2006). High-temperature drying requires a strong heat supply to remove moisture from the food sample. The heat can be supplied in many ways such as microwave, radio frequency, hot gas stream including air, superheated steam, etc. The hot gas stream is the most frequently used heat source for large-scale commercial industries due to its increased availability, easier heat recovery, and cheaper cost than other heat sources. The saffron is dried by hot air flow or placed on a heater for this method. According to Carmona et al. (2005), the highest coloring strength was obtained when saffron was subjected to higher temperatures and shorter times. These findings may be supported by the fact that samples dehydrated at elevated temperatures were more porous than those dry at room temperature.

In Italy, the process is carried out by spreading the fresh stigmas on a sieve placed B20 cm above live oak-wood embers. Halfway through the process, the stigmas are turned over to ensure homogeneous drying. The process is considered finished when saffron stigmas retain between 5% and 20% moisture and possess a certain amount of elasticity when pressed between the fingers (Tammaro, 1999). Saffron dried over charcoal maintains its organoleptic qualities better; its purplish red color, fragrance, and aroma will be retained (Zanzucchi, 1987).

Drying saffron in Spain is accomplished by placing a layer of fresh stigmas less than 2 cm thick on a sieve with a silk bottom. The sieve is placed over the heating source, which can be a gas cooker, live vineshoot charcoal, or, less often, an electric coil. In CastileLa Mancha, in central Spain, the most used source is a gas cooker, followed by embers from kermes oak or occasionally electrical sources (Alonso et al., 1998). The process is finished when the sample has lost between 85% and 95% of its moisture after being dried at 50C80C for 3060 minutes. At the halfway point of the process, when 1015 minutes have passed, the entire mass is turned over with the help of another sieve of the same type, which is again placed over the heating source to finish the drying process.

In Greece, when stamens and stigmas are dried together, the stamens' pollen pollutes and causes the red saffron to deteriorate. It is therefore recommended to separate them before drying. The drying process starts by placing a thin layer of freshly harvested stigmas (45 mm) on 40350 cm trays with a fine silk screen. These trays are piled on frames with shelves 2530 cm apart. The frames are placed in a dark or storage room with a controlled temperature and then heated with a firewood stove. During the first few hours of drying, the temperature is maintained at 20C and then raised to 30C35C. The drying process is stopped when the moisture content of the product reduces to 10%11% after 12 hours. If saffron's red (stigmas and styles) and yellow (stamens) components are still together after drying, they can be separated at this stage. At the same time, all foreign substances (soil, hairs, threads, etc.) are removed from the dried saffron product. The pure dried saffron is kept in hermetically sealed glass vases or tin cans at 5C10C.

Despite some advantages of industrial dryers, including the achievement of hygienic conditions, quality control, reduction of product loss, and decreased process duration, the energy requirement of drying technology is one of the critical problems that should be overcome. The drying process generally consumes energy and releases carbon oxides into the environment. Therefore, it is crucial to ensure good quality of the dried products, high energy efficiency, and low environmental impact (Aghbashlo et al., 2013).

Hybrid photovoltaic thermal solar dryer

The energy required for drying, primarily for hot air production, is mainly supplied by fossil fuels. The global demand for fossil fuels, the consequent price increase, the insecurity of their supply, and environmental concerns have increased interest in using renewable energy sources such as solar power.

Solar energy in its first form (sunlight) can be converted into heat by thermal solar collectors or electricity by photovoltaic solar cells. A new technology has been developed to combine both types of conversions: the solar photovoltaic/thermal collector (PV/T). Hybrid PV/T systems, or PVTs, convert solar radiation into thermal and electrical energy. These systems combine a solar cell, which converts sunlight into electricity, with a solar thermal collector, which captures the remaining power and removes waste heat from the PV module (Fig. 17.1). Results of various studies have shown that such hybrid systems are more efficient than both individual photovoltaic and thermal systems.

 FIGURE 17.1 Schematic diagram of heat pump-assisted hybrid PV/T solar dryer. From Mortezapour, H., Ghobadian, B., Khoshtaghaza, M.H., Minaei, S., 2014. Drying kinetics and quality characteristics of saffron dried with a heat pump-assisted hybrid photovoltaic-thermal solar dryer. J. Agr. Sci. Tech. 16, 3345.

In a hybrid PV/T solar dryer, a photovoltaic panel provides the thermal energy required for moisture removal from the products and electrical power for a fan to circulate air through the dryer. Mortezapour et al. (2012) used a hybrid PV/T system for drying saffron stigmas. To improve the quality of the final product, a heat pump was also added to the system, making it suitable for heat-sensitive materials such as saffron stigma. The sides and back wall of the solar air collector were constructed from wood and insulated by glass wool. A glass sheet was used as the transparent front cover of the solar collector, and a photovoltaic panel was fixed at the middle of the collector sides, with equal distances from the wooden back wall and the top glass cover, to work as the solar irradiance absorber plate. The arrangement of the dryer's components and the heat pump system, which were connected by glass wool-insulated round ducts, created a system in which the drying air was circulated in a closed cycle through the evaporator, solar air collector, condenser, auxiliary heater, and drying chamber, respectively. A fresh air valve allowed ambient fresh air to enter the dryer's duct and mix with the drying air when the temperature and relative humidity exceeded their desirable set values. New saffron stigma (moisture content of 80% wb) was spread on a tray and placed inside the drying chamber. Results showed that drying time decreased by 62% as the air temperature increased from 40C to 60C. Utilizing the heat pump system with the hybrid solar dryer improved the drying rate and shortened the drying time by 60%. The color of the saffron was also enhanced when the drying temperature increased and the heat pump system was applied. Saffron dried with heat pump-assisted hybrid PV/T had excellent aromatic properties with no significant changes in its bitterness.

As a result of these improvements, total drying time and energy consumption are decreased using a hybrid PV/T solar dryer. Applying a heat pump with the dryer leads to a further reduction in the drying time and energy consumption and an increase in the electrical efficiency of the solar collector. The average total energy consumption is reduced by 33% when the dryer is equipped with a heat pump. Maximum values for electrical and thermal efficiency of the solar collector are 10.8% and 28%, respectively. A maximum dryer efficiency of 72% and a maximum specific moisture extraction rate were obtained at an airflow rate of 0.016 kg s21 and an air temperature of 60C when using the heat pump (Mortezapour et al., 2014).

Infrared thin-layer drying

Infrared radiation (IR) is an increasingly popular method of supplying heat to dry, moist materials. Infrared drying involves heat transfer by radiation between a hot element and a material that needs to be dried at a lower temperature. The peak wavelength of the radiation depends on the temperature of the heated component (Fig. 17.2). IR heating presents advantages such as decreased drying time, high energy efficiency, and lower environmental impact. The energy of radiated waves is transferred from the source to the sample product without heating the surrounding air, leading to higher temperatures in the inner layers of the samples than the surrounding air and, thus, a high heat transfer rate (Celma et al., 2008). Akhondi et al. (2011) investigated the drying of saffron stigma with a laboratory infrared dryer. The influence of temperature on the drying rate of samples at various temperatures (60C, 70C, 110C) was studied. 

FIGURE 17.2 Infrared dryer setup schematic. 1, digital balance; 2, infrared heating tube; 3, dimmer; 4, fixed voltage power unit; 5, data logger; 6, hygrometer; 7, k-type thermocouple; 8, t-type thermocouple; 9, samples; 10, inlet cold air; 11, PC. From Ziaforoughi, A., Yousefi, A.R., Razavi, S.M.A., 2016. A comparative modeling study of quince infrared drying and evaluation of quality parameters. Int. J. Food Eng. 12, 19.

 

FIGURE 17.3 Schematic diagram of the freeze dryer used for the saffron dehydration experiment. From Acar, B., Sadikoglu, H., Doymaz, I., 2015. FFreeze-drying kinetics and diffusion modeling of saffron (Crocus sativus L.). J. Food Process. Preserv. 39, 142149.

 The drying time decreased with an increase in drying air temperature. According to Torki-Harchegani et al. (2017), the total crocin content increased when the IR dryer temperature increased to 90C, but the amount of crocin slightly decreased at higher temperatures. The entire safranal content of the samples decreased when the IR drying temperature increased from 60C to 70C and then continuously increased to 110C. The amount of picrocrocin also increased as the IR drying temperature increased from 60C to 100C. As a result, the maximum values of crocin and safranal were obtained in the samples treated at the highest IR drying temperature.

Freeze drying

The freeze-drying process can be considered a drying method for stigmas of the saffron flower (Acar et al., 2015). Freeze drying, also known as lyophilization, is a dehydration process typically used to preserve a perishable material or to make the material more convenient for transport. Freeze drying works by freezing the material and then reducing the surrounding pressure to allow the frozen water in the material to sublime directly from the solid phase to the gas phase (Fig. 17.3). Sublimation is when a solid (ice) changes down a vapor without first going through a liquid (water) phase. Controlled freeze drying keeps the product temperature low enough during the process to avoid changes in the dried product's appearance and characteristics. Therefore, heat-sensitive materials, fine chemicals, biotechnological products, and some pharmaceuticals, which might lose their quality (activity) in conventional evaporative drying, can be safely freeze-dried (Sadikoglu and Liapis, 1997; Sadikoglu et al., 2006). Compared with the other traditional dehydration processes, the highest quality dried product can be obtained by freeze-drying.

The freeze-drying process is multistage, relatively slow, and expensive (initial investment and operating costs are high).

Lyophilized saffron is a new product with a higher content of crocins, a consistent content of native compounds, and a minimum water content (about 4%) compared to traditional dried saffron. For this reason, it could be used as a standard substance to evaluate saffron powder's quality and water content. At the same time, the innovative method of lyophilization enables the production of saffron with shallow moisture content and, consequently, higher crocin content, longer shelf life, more excellent stability, and higher coloring power.

In this method, stigmas are placed in the tray of the freeze dryer at 240 C for 4 hours for complete freezing. The initial temperature of the freeze dryer is 230C and increases gradually to 5C without causing any melting or scorching of the stigmas. The drying chamber pressure should be kept at its minimum value (at least it should be well below the ice vapor pressure of the freeze-dried sample). This increases water vapor mass flux through the pores of the dried material due to Sublimation during the primary drying stage (Sadikoglu et al., 2003, 1998). The drying chamber pressure is set at 50 Pa and kept constant during drying.

FIGURE 17.4 A schematic diagram of microwave convective oven dryer. From Zarein, M., Samadi, S. H., Ghobadian, B., 2015. Investigation of microwave dryer effect on energy efficiency during drying of apple slices. J. Saudi Soc. Agric. Sci. 14, 4147.

Even though the initial investment and operation costs are high and the drying time is substantially longer than conventional drying methods, freeze-drying is considered an exemplary method for the dehydration of saffron stigmas. The original shape and structure of the sample can be preserved during the freeze-drying process. Freeze-dried saffron contains high amounts of safranal and crocin and has lower moisture than traditional and sun-dried saffron. Lower moisture content can prevent fading of the saffron's color by limiting crocin's degradation into crocetin during storage. The high cost of freeze-drying saffron can be compensated for by keeping the safranal and crocin contents in the final product.

 Microwave drying

The term "microwave" refers to electromagnetic radiation in the frequency range of 300 MHz to 300 GHz with a wavelength of 1 m21 mm (Feng et al., 2012). Microwaves are not forms of heat but energy manifested as heat through their interaction with materials. It is the propagation of electromagnetic energy through space busing time-varying electric and magnetic fields (Fig. 17.4). Microwave energy makes it possible to control the drying process more precisely to obtain greater yields and better quality products in the shortest possible time.

The mechanism for drying with microwave energy is quite different from that of conventional drying. Microwaves initially excite the outer layers of molecules. The inner part of the material is warmed as heat travels from the outer layers inward. Most of the moisture is vaporized before leaving the material. This results in very rapid drying without the need to overheat the atmosphere and perhaps causes case hardening or other surface overheating phenomena. In microwave drying, energy is transferred through the material electromagnetically, not as a thermal heat flux. Therefore, the heating rate is not limited, and the uniformity of heat distribution is greatly improved. This significantly reduces drying time, improving product quality (Schubert and Regie, 2006). In many cases, microwaves are at least 50% more efficient than conventional systems, resulting in significant cost savings.

Microwave drying can be used to improve the chemical profile of saffron in terms of safranal, which is responsible for its aroma. Microwave drying of saffron has several quantitative and qualitative advantages over conventional drying methods. In this method, heat conductivities or heat transfer coefficients do not play such an important role. Therefore, saffron can be heated in a microwave dryer in a shorter time, with lower drying temperatures and a more even temperature distribution. In microwave drying, treatments at lower microwave power and longer time benefit the quality of saffron. It only takes 3 minutes at 600 W to dry saffron when moisture is less than 12% and 6 minutes at 400 W.—As a result, drying saffron stigma at moderate temperatures in a microwave oven results in better quality in terms of higher color strength, aroma, and taste.

Effects of drying on color, aroma, and taste

Postharvest processing, such as drying methods and storage conditions, determines the stability of saffron, which directly affects the market value of the product. The main active compounds in saffron are crocins, a group of glycoside derivatives from the carotenoid crocetin; terpenic aldehydes known as safranal; and a glycoside terpenoid, picrocrocin. These compounds are responsible for saffron's coloring power, bitter taste, and aroma (Carmona et al., 2006). Many factors influence the quantities of these compounds in saffron. One of the most important factors is the dehydration treatment necessary to convert saffron stigmas into saffron spice (Carmona et al., 2006). The effect of temperature and other drying components on spice color and taste remains to be entirely determined. There is some evidence that the three types of compounds (crocetin esters, picrocrocin and its related compounds, and volatiles) could be interrelated, as crocetin esters can generate as much picrocrocin as their analogs, such as safranal and other volatile compounds (Carmona et al., 2007).

According to Raina et al. (1996), temperatures lower than 35C45C are required for excessive enzymatic degradation of crocetin esters, the compounds responsible for saffron color. They found that crocin pigment content was highest in saffron dried between 35C and 55C in solar or oven drying. Under these conditions, safranal was at its peak value except for the vacuum oven-dried samples.

On the other hand, using high temperatures does not degrade the crocetin ester compounds. It increases the coloring strength. It decreases other crocetin esters, such as the trans-crocetin (β-D-glycosyl)-(β-D-gentibiosyl) ester and trans-crocetin (β-D-gentiobiosyl) ester (Carmona et al., 2005). According to Carmona et al. (2005), the highest coloring strength is obtained when saffron is subjected to higher temperatures and lower times. These findings were supported by the fact that samples dehydrated at elevated temperatures were more porous than those dry at room temperature. In addition, there is evidence that high temperature promotes the production of compounds responsible for taste and aroma. Maghsoodi et al. (2012) also reported that more elevated amounts of safranal (aroma) and crocin (color) were obtained at high temperatures. However, there was no significant difference between the amounts of picrocrocin at different temperatures.

Results by Gregory et al. (2005) showed that a brief (20 minutes) initial period at a relatively high temperature (80C92C) followed by continued drying at a lower temperature (43C) produced saffron with a safranal content up to 25 times more than that of saffron dried only at lower temperatures. Evidence suggested that drying with significant airflow reduced the safranal concentration. The results indicated that high-temperature treatment allowed more excellent crocin pigment retention than in saffron dried at intermediate temperatures (46C58C). The biochemical implications of the various therapies and their potential for optimizing color and fragrance quality in the product are discussed.

The effect of mild temperature on the main components responsible for saffron quality during dehydration was studied by Del Campo et al. (2010). Based on their results, crocetin esters were not as labile as the bibliography mentioned before. Saffron coloring capability increased from 40 C without finding significant differences with 55C. Similar behavior was obtained for picrocrocin, which was higher at the highest temperature but without significant differences with the immediate inferior conditions. However, at higher temperatures (e.g., 55C), more volatile compounds, especially safranal, were generated during dehydration.

The results of chromatographic analyses by Cossignani et al. (2014) also showed that samples dried in milder conditions had the lowest content of secondary metabolites such as crocins, picrocrocin, and safranal. Moreover, samples dried at 60C for 55 minutes presented the highest contents of trans-crocin-4 and picrocrocin, while safranal was the most prevalent compound in saffron, which was dried at 55C for 95 minutes. A detailed study by Tong et al. (2015) determined that the highest quality saffron is obtained when fresh saffron is treated at higher temperatures (no more than 70C) for an extended period by electric and vacuum oven drying.

As mentioned before, microwave radiation as a mild drying method is an efficient way to improve the chemical profile of saffron in terms of safranal. The total safranal content increases when microwaves dehydrate saffron at moderate temperatures (Muzaffar et al., 2015). Microwave drying retains the maximum concentration of safranal compared to samples dried under shade. Therefore, microwave drying could be the best method for saffron stigmas to retain their aroma. According to Maghsoodi et al. (2012), microwave drying obtained the highest amount of safranal at 1000 W among saffron drying methods. Under these conditions, the amounts of crocin and picrocrocin also increased. Results by Tong et al. (2015) also showed that the chemical contents in saffron treated by microwave drying were higher than those treated by other methods and that the time spent in the drying process was also less. However, the antioxidant activity of these samples was not more substantial, meaning that other chemical compounds were formed in the samples treated by electric and vacuum oven drying processes.

The safranal content of the samples dried in a freeze-dryer was five times higher than those dried naturally under the sun. The crocin content of samples dried in a freeze-dryer was about 40% higher than those dried naturally under the sun. These results indicate that the safranal and crocin contents that define commercial saffron's quality and market value were considerably higher in samples dried in a freeze dryer than those dried traditionally under the sun (Acar et al., 2011).

Conclusion

To achieve saffron's best color, aroma, and taste, drying must be done correctly. A dehydration treatment changes saffron spice's physical and biochemical properties, which is necessary to achieve the desired attributes. The quantities of crocins, safranal, and picrocrocin in saffron depend mainly on the method used for drying the stigmas. In traditional methods using solar and air drying, the carotenoids in saffron can be degraded. However, saffron's purplish red color and aroma are retained when the air drying method is used. The organoleptic qualities and its purplish red color are preserved in artificial drying methods, such as over charcoal. Due to environmental concerns, a solar photovoltaic/thermal collector dryer has been developed to decrease saffron drying time and energy consumption. IR heating is another drying method with advantages, such as lower drying time, higher energy efficiency, and lower environmental impact. The maximum values of crocin and safranal were obtained in saffron dried at the highest IR temperature. In the microwave drying method, heat conductivity is not a critical factor, and saffron can be dried in a shorter time and at a lower temperature. Therefore, saffron will have higher color strength, aroma, and taste and be better quality. The total safranal content increases when saffron is dehydrated using microwave drying. Freeze-dried saffron has the highest quality when compared with other conventional dry products. Saffron processed in this manner contains high amounts of safranal and crocin and has lower moisture content. However, the initial investment and operational costs are increased, and the drying time is substantially extended.

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