Many mucilaginous seeds around the world have been introduced for use as accessible, cost‐effective, and natural sources of hydrocolloids. These hydrocolloids are used as thickening, gel‐forming, foam‐, and emulsion‐stabilizing agents in a wide range of foods and pharmaceuticals. Due to the strong bonds between the mucilage layers and their hard seed cores, often a severe mechanical stress is required for quick and efficient separation of mucilage layers from their seed cores. The conventional method for isolating/extracting hydrocolloids from mucilaginous seeds includes a hydration process, followed by applying severe mechanical shear stress by high‐speed mixers or with rotating blade stirrers. Therefore, a heterogeneous mixture of crushed seeds and mucilage may be produced. Several stages of time‐consuming and energy‐intensive centrifugation or filtration are required to separate the crushed seeds and impurities, followed by drying and grinding. Application of mechanical shear stress often leads to high levels of seed damage, resulting in crushed seed cores, which, on the one hand, makes the use of centrifugal force inevitable, and on the other hand, results in the entry of a large number of impurities from the crushed seeds into the isolated hydrocolloids. To address this issue, in the recent years, some attempts have been made to use emerging technologies and new approaches for isolation of seed gums aiming at improving the extraction process of seed mucilage. This chapter aims to review the conventional and emerging isolation methodologies and provide an overview of the current state of hydrocolloid isolation techniques and compare their advantages and disadvantages for mucilaginous seeds.
Mucilaginous seeds are often covered with a polysaccharide‐rich outer layer with specific physiological functions. When placed in either cold or hot water, the outer layer absorbs water and swells several times of its original volume, forming a gel‐like structure. Mucilage is defined by the Oxford Dictionary  as “a polysaccharide substance extracted as a viscous or gelatinous solution from plant roots, seeds, etc. and used in medicines and adhesives.” Seed mucilage is primarily a hydrocolloid, being the main fraction of the outer layers of seeds, which after isolation can act as viscosifying and gelling agents in food and non‐food systems.
From the plant taxonomy point of view, only 2.7% of the plants that produce seed mucilage are in Magnoliids and Nymphaeales, and 18.2% are monocots. However, most plants that have seeds covered with mucilage are in the families of eudicots (dicots). Overall, most seed‐mucilage‐producing species are found in Poaceae and Asteraceae families . Physiological and ecological studies on seed coat mucilage have provided important information about its roles in different stages of the plant lifespan. These functions include providing a moist environment and maintaining the metabolic activities of the seed, supporting seed development, improving seed dispersal by attaching seeds to soil, lubrication, or adjusting the seed density. In dry climates, seed mucilage can prevent seeds from drying or initiate DNA repair mechanisms, thereby conserving the soil seed bank . Mucilage also may regulate oxygen permeation and seed dormancy. Seed mucilage is thought to promote seed germination in favorable environments. During the seedling stage, mucilage may loosen the radicle as it penetrates the soil and be degraded by soil microflora and therefore promote seedling growth .
Over the past few decades, a large number of research reports have been published on the isolation, characterization, and physicochemical properties of different mucilages isolated from mucilaginous seeds. Considering the emphasis of this chapter on mucilage isolation, the main extraction methods and production yields of a number of seed mucilages are reviewed and presented in Table 18.1 and subsequently discussed.
Table 18.1 Gum isolation conditions and mucilage production yields of selected mucilaginous seeds.
|Seed||Main mucilage isolation steps||Yield and main findings||References|
|Fenugreek (Trigonella foenum graecum) seeds||Seeds were mixed with water (1:10), kept cool for 3–4 h, and centrifuged for 20 min at 500 rpm. The supernatant was mixed with acetone (1:3), and the precipitate was then washed and oven dried.||The yield of acetone precipitated mucilage was 53%.|||
|Basil (Ocimum basilicum L.) seeds||
||Time and temperature played a key role in extraction yield.
A maximum yield of 18% was obtained at 50 °C, pH = 7, and water‐to‐seed ratio of 30 to 1.
||The yield was about 15.42% (crude) and 12.34% for pure mucilages.|||
||Yield ranged from 7.86 to 20.5 g/100 g and was significantly affected by temperature and water‐to‐seed ratio.|||
||Yield ranged from 7.97 to 20.5%.|||
|Quince (Cydonia oblonga) seeds||Fresh seeds were freeze‐dried and ground. The seeds were then soaked in water (1:50) while stirring for 24 h at 30 °C. The mix was centrifuged, and the supernatant was filtered using a cheesecloth and freeze‐dried.||The yield was 8% (db).|||
|Chia (Salvia hispanica L.) seeds||The effects of the seed‐to‐water ratio (1:20, 1:30, and 1:40), pH (4–8), and temperature (4, 40, 80 °C) were investigated. Seeds were mixed with water and stirred for 2 h using a magnetic stirrer. Then the aqueous suspension was dried at 50 °C for 10 h. The dried mucilage was separated from the seed cores by rubbing over a No. 40 mesh screen.||The yield was up to 6.97%.|||
||Hot water was efficient for most species but 0.2 M KOH was required for P. ovata.|||
||The yield of cold fraction 4.5%, hot fraction 3.2% and KOH soluble fraction 9.2%.|||
||Yield range was 7%–15.18%.
Temperature followed by water‐to‐seed ratio were the main driving forces of mucilage yield.
||The production yield of dry husk was ∼28%.|||
|Chia (S. hispanica L.) seed||
||M1: yield 3.8% and
M2: yield 3.7% (d.b.).
M1: protein 18.85% and
M2: protein 6.79%.
|Golden flaxseeds (Linum usitatissimum)||Seeds were soaked in water at a seed‐to‐water ratio of 1:18 at different temperatures (30–90 °C) with continuous stirring over a magnetic plate for 2 h for each of two consecutive cycles of extraction. The soaked seeds were filtered, and the water containing the extracted gum was treated with three volumes of 95% ethanol to precipitate the gum. The precipitated gum was collected by centrifuging at 4000 g for 10 min and vacuum‐dried at 50 °C.||Yield range was 2.1%–8.4%.
Yield increased with temperature from 30 to 90 °C. The protein level of the 90 °C mucilage was 3 times that of 30 °C.
|Tamarind (Tamarindus indica L.) seed mucilage||Seeds and water mixed in a 1:10 weight ratio and stirred on a hot plate stirrer for 10 min, then water was added to take up to the 1:40 weight ratio in relation with the initial weight of seeds and kept with constant stirring. The mixture was heated and kept at 80 °C for 60 min, cooled to room temperature and kept for 24 h and then centrifuged for 8 min at 524 g. The supernatant as mucilage was separated and stored at 4 °C.||Yield range was 29.83%.
Milled seeds were used for mucilage extraction; therefore, impurities would be high.
|Cress (Lepidium sativum) seeds||Whole seeds were mixed with water (1:30), and mucilage was isolated at room temperature, pH 7, with a soaking time of 15 min. Separation of the gum from the swollen seeds was achieved by passing the seeds through an extractor with an abrasive rotating plate that scraped off the mucilage layer. The mucilage was dried at room temperature.
Mucilage purification by 3 volumes ethanol 96% or isopropanol 99.5% was done and the mucilage dried at room temperature.
|The purified mucilage had a stronger and a more elastic network structure than the crude gum.|||
|Flixweed (Descurainia sophia) seeds||Variable parameters were water‐to‐seed ratio 15–45, pH 4–8, temperature 50–95 °C, and extraction time 1–5 h. The seed–water slurry was stirred with an electric mixing paddle until complete hydration. The seed cores were separated from the liquid using a basket centrifuge at 1200 rpm lined with a 1 mm mesh. The mucilage was recovered from the extract with precipitation using three volumes of 95% ethanol. The precipitates were collected and dried overnight in a vacuum oven.||Yield range was 2.55%–10.45%. The temperature was the dominant factor, and the optimum conditions to achieve 10.45% were extraction time of 2.9 h, 94.32 °C, pH 7.55, and a W/S ratio of 44.2.|||
|Ocimum americanum L. Seeds||Seeds and water (1:300–10:300) were mixed and allowed to swell while stirring using a mixer at 3000 rpm for 30 min. The mixture was passed through a muslin cloth, and the isolated mucilage solution was then poured into an aluminum tray and dried in an oven set at 45 °C.||Yields at 30 and 60 °C were 13.8% and 23.16%, respectively. The temperature was key; increasing temperature reduced the extraction time and increased the yield.|||
|Sage (Salvia macrosiphon) seeds||Whole seeds mixed with water (water‐to‐seed ratio of 25:1–85:1) at pH 3–9, and temperature ranging from 25 to 80 °C. Extraction was carried out in three stages; first, the seeds were swelled in water, then separation of the mucilage from the swelled seeds was done by passing the seeds through a laboratory juice extractor with a rotating disk. The collected crude gum was finally filtered and dried overnight at 70 °C.||Yield range was 7.04%–13.69%.
Water‐to‐seed ratio and the temperature had the greatest impact on yield, in that order.
Mucilage seeds purchased in bulk often include other seeds, dry stems, debris, stone, and weed seeds. This unwanted foreign matter is removed before mucilage extraction either during seed collection, at the production site, or otherwise before mucilage extraction. Cleaning may be done by sieving, using a pneumatic suction system, or manually. Often the selection of an appropriate cleaning method would depend on production size, availability of mechanical cleaning systems, and labor and equipment costs.
Generally, in biopolymer extraction from grains, size reduction is carried out to reduce the process time. Milling often helps when the biopolymers of interest are mixed with other seed compounds. However, the unique morphological structure of mucilaginous seeds in which a distinctive outer layer forms the mucilage that is not mixed with other seed sections located in the hard seed cores would make whole seed milling inappropriate for mucilage extraction, and should be avoided when possible. This is due to the fact that if whole mucilaginous seeds are crushed into a homogenous mill by whole seed crushing, then during mucilage isolation, other non‐mucilage components will enter the mucilage, and then the amount of non‐polysaccharide compounds (i.e., impurities) may substantially increase. In a study, freeze‐dried quince seeds were ground, and the milled seeds were used for mucilage isolation . The resultant mucilage had a high protein content of 19.6%, which could be related to extraction of other seed components as a result of grinding hard seed cores which are rich in non‐polysaccharide compounds including protein. Therefore, in mucilage isolation, attempts would be made to minimize the damage to the hard seed cores of mucilaginous seeds during pre‐treatment or mucilage extraction.
Conventional methods for isolation of mucilage from mucilaginous seeds can be divided into wet and dry methods as follows.
Previous research has mainly used the so‐called wet or soak‐shear method, in which cleaned mucilaginous seeds are soaked in water and left for some time to allow for partial or full hydration. During soaking, the soluble mucilage fraction, and any other soluble compounds, is solubilized and transferred into the aqueous phase. Often the swollen mucilage is strongly tied to seed cores, and therefore strong shearing forces are required to remove the mucilage layer. Different laboratory‐type equipment including blade mixers and juicers with rotating bowls have been used to apply an adequate shear force (Table 18.1). After shear application, a two‐phase mixture including hydrocolloid dispersion and solid hard seed cores is obtained. Whole seed cores can be removed rather easily, but broken seed cores would be difficult to separate and often become a source of impurities in the final mucilage powder. Factors controlling mucilage isolation using the wet method will be discussed in the following section.
A wide range of seed‐to‐water ratios (i.e., 1:10–1:85) has been applied and reported for the separation of different mucilages (refer to Table 18.1 for details). The seed‐to‐water ratio can impact the production efficiency, production yield, and physicochemical properties of the isolated gums such as rheological properties and color. Lower seed‐to‐water ratios can result in higher yields; however, from the economics of processing point of view, higher seed‐to‐water ratios are favored as these would result in energy saving and reduce the required capital and operating costs per production unit of mucilage gums. Therefore, the seed‐to‐water ratio should be maximized as any added water needs to be separated during drying, and this is likely to increase the production costs substantially.
A wide range of temperatures (4–95 °C) has been used for the extraction of mucilage gums (Table 18.1). The physicochemical properties of each mucilage play a key role in determining the best temperature profile for its extraction; for example, if a mucilage is largely water soluble at room temperature, then high temperatures would not be required for mucilage extraction. On the other hand, some mucilages have low solubility in cold water, and hence for these seeds, high temperatures would be used to obtain greater yield values. Also, high temperatures may reduce the extraction time for all mucilage gums with low or high solubility at low temperatures. Muñoz et al.  evaluated the impact of temperature, pH, and the seed‐to‐water ratio at a constant extraction time of 2 h on the production yield of chia mucilage and found temperature to be the major factor with the highest impact on mucilage yield, where a maximum yield of 7% was reported at 80 °C while this value was halved at room temperature . In this research, mucilage extraction was performed using a magnet stirrer without applying any strong shear forces; therefore, it may be concluded that increasing the solubility of chia mucilage at higher temperatures has been the major driver for doubling the yield at 80 °C as compared to room temperature. The effects of hydration time and temperature on Salvia macrosiphon hydrocolloid extraction yield (%) was examined, in which the mucilage production yields at 25 and 65 °C were not significantly different . This could be due to the use of high shear forces that are likely to scrape off the gel‐like mucilage from the hydrated and swollen seeds, thus minimizing the impact of mucilage solubility.
It is worth noting that some reports (e.g., for basil mucilage) have indicated that high temperatures in combination with long extraction times may have adverse effects on yield due to the degradation of polysaccharides .
Acidic solutions with pH as low as 3 and alkali pH as high as 10 have been studied for the extraction of gums from mucilaginous seeds. It has been reported that when the impact of the seed‐to‐water ratio, pH (4, 6, and 8), and temperature on production yield was investigated, the influence of pH was found to be minor as compared to the major effect of temperature . However, the rheological properties of the mucilages extracted at different pHs were not compared. For the extraction of basil mucilage, pH values of 5, 7, and 9 were compared, the conclusion being that the greatest yield (i.e., 16.2%) was obtained at pH 7 . A substantial reduction in yield (i.e., about 10%) was observed for both a low pH of 5 and a high pH of 9 in mucilage isolation trials. Razavi et al.  studied optimal gum extraction conditions for basil mucilage by varying the temperature (25–85 °C), pH (4–10), and the water‐to‐seed ratio (50:1–80:1) and reported that temperature had a more positive (linear) influence on the extraction yield than the other two variables. On the other hand, the lowest and the highest yields were found at pHs 4 and 8, respectively.
It is noteworthy that exposing hydrocolloids to low‐ and high‐pH treatments during extraction can also result in macromolecular degradation that is likely to reduce some functional characteristics (e.g., rheological properties) of the isolated seed mucilages. Hence, using very low‐ or high‐pH isolation techniques should be avoided, especially when high‐temperature processes are applied for long times.
The mucilage layer of mucilaginous seeds often forms thin polysaccharide‐rich layers that cannot be separated easily due to the strong bonding to their hard seed cores. For extraction of mucilage using the wet method, often dry seeds are placed in water and allowed to soak. During soaking, the soluble fraction transfers into the water while insoluble and swellable fractions of mucilage layers take up water and gradually swell, and a rather thick gel‐like layer is formed that can be separated using shear forces. Soaking times from minutes to hours have been examined for different seeds (Table 18.1). For some extraction procedures, soaked seeds are left overnight in order to reach maximum hydration; for example, for tamarind mucilage extortion, the seed‐and‐water mixture was first heated at 80 °C for 60 min, and then the mixture was kept at 20 °C for 24 h to ensure the release of the mucilage . From the process design point of view, the optimum condition would use a minimum soaking time to reduce the total production time and hence increase production capacity.
For the extraction of mucilage of basil seeds, increasing the soaking time from 0.5 to 5 min has had substantial impacts on the yield, and this positive influence gradually continued further but leveled off after 15 min of soaking . Also, the extent of the impact of time can be affected by other factors such as shear force and temperature. For example, at higher temperatures, shorter soaking times are required as thermal energy reduces the reaction time due to greater extraction rates.
In the mucilage extraction process, during soaking, if the hydrocolloid macromolecules get diffused into water in a reasonable period of time, then shear force application will not be required; however, for some mucilage seeds, even after long soaking times at high temperatures, a strong gel may still remain attached to the seed cores, and hence application of strong shear forces using a rotating abrasive disk or plate  or high‐shear mixers or blenders  may be necessary. In Salvia macrosiphon mucilage seeds, due to the strong bonds of the swollen mucilages to the seed cores and the low solubility of the mucilage in water, application of strong shear forces was inevitable. Applying high shears using blender blades resulted in breakage of the majority of Salvia macrosiphon seeds, and this has been reported as a challenge resulting in the presence of high levels of impurities and darkening of the color of the mucilage powder . Simultaneous application of heating and mixing during soaking has been used in a number of reports. In brief, application of a shear force to minimize the use of high temperatures and long soaking times has been recommended, but the force should be kept to low to medium levels to avoid any major seed damage that can cause leakage or transfer of other seeds constituents and seed coat debris into the final mucilage powder.
The mucilage layer and its physical characteristics, in particular, its adhesion to hard seed cores and the capability to flake as a result of applying a shear force play a key role in using the dry isolation method for separation of the mucilage layer. When different dry mucilage seeds are directly exposed to shear forces (e.g., using blades of a coffee grinder), they will behave differently. In some seeds such as Plantago ovata , this leads to scraping off the dry mucilage layer and produces small flakes that can be further separated from the seeds by a simple dry sieving or pneumatic separation (Figure 18.1). However, in other mucilaginous seeds such as garden cress or basil seeds, applying shear forces on dry seeds does not result in separating the outer mucilage layers, and hence the dry method cannot be employed successfully for most mucilaginous seeds.
This method is regarded as a green technology in which energy consumption is low and almost no water is required. Water is not used in this isolation technique; therefore, because processing occurs in limited‐water‐activity environments, the risk of microbial growth and contamination is also minimized. In terms of processing time and capital costs, dry separation may be regarded as the best isolation method if the mucilage seeds are found suitable for this technique. However, for most mucilaginous seeds, the dry method is not able to separate the mucilage layer, and one has to use other isolation methods. One of the methods which help to improve the susceptibility of the mucilage layer to scraping (shear force) is to perform pre‐treatments such as freeze drying prior to dry rubbing or scrapping. The aim of any pre‐treatment method would be to amend the seed and in particular, its mucilage layer so that the dry isolation method can be applied. In freeze drying, initially the seeds are soaked in excess water, and after partial or complete swelling, the seeds are frozen and then dried in a freeze drier. Swelling and subsequent freeze drying would convert the dense and thin mucilage layer to a much thicker and more porous structure, and hence the mucilage layer may be rubbed off easily by scraping or applying a shear force as used for isolating mucilage from chia seeds .
The crude mucilage isolated by either the wet or dry method is often polysaccharide‐rich matter, but may include considerable amounts of protein, fat, and ash. Mucilage polysaccharides are mainly water‐soluble or water‐swellable fractions. In order to remove non‐polysaccharide components from polysaccharide solutions, purification may be conducted . Aqueous dispersions of each mucilage may be mixed with organic solvents (e.g., ethanol or ethyl alcohol) two to three times the volume of the seed‐and‐water mixture, and as a result polysaccharide molecules are precipitated where clusters of whitish cotton‐like fibers are formed [ 5, 18]. These fibers can be filtered or separated by passing them through a cheesecloth, sieving, or centrifugation. The separated fibers then are dried at room temperature or using oven drying to the correct moisture content.
In mucilage isolation using the wet method, mucilage seeds are often mixed with large quantities of water, and this is to allow maximum swelling and hence production yield. In the wet method, after extracting mucilage into the aqueous solution, it has to be isolated either by drying or mixing with large quantities of organic solvents. This latter stage poses a major challenge due to the usage of large quantities of high‐quality water or organic solvents; high energy is needed to remove the added water or recover the organic solvents. These operation units are costly and increase production costs substantially. Another challenge is the impurities of the mucilage powder originating from non‐mucilage constituents mainly as a result of breakage of hard seed cores and release of non‐polysaccharide compounds into the extracting mixture. New approaches and innovative technologies are in high demand to optimize the mucilage isolation process, improve mucilage quality, and reduce production costs.
This section will focus on ultrasound‐assisted isolation of mucilage, as the application of ultrasound waves has been the main emerging technology reported for mucilage isolation.
Over the past few decades, there has been a dramatic rise in the application of new technologies in food processing. One of these emerging technologies is the ultrasonic technique, whose application in food processing is relatively new. Ultrasound technology is based upon oscillating mechanical waves at a frequency greater than the upper limit of the human hearing range (>16 kHz) . High‐intensity or power ultrasound (intensity higher than 1 Wcm−2 and in the range of 10–1000 Wcm−2) with a low frequency (in the range of 18–100 kHz) could change the material properties by creating uneven pressures, high shear stresses, and very high temperatures of up to 4000 °C at nanoscale . Generally, power ultrasound can be applied in various food unit operations, for example, for accelerated extraction from plant materials , and batch and continuous ultrasound units are now available for large‐scale processing for a wide range of unit operations (e.g., extraction, cleaning, disintegrating, degassing and deagglomeration, emulsification, disinfection, depolymerization of organic matters, disintegration of biogas substrate, and dispersal of solid particles in liquids) . Ultrasound has been well studied and is highly regarded for improving food processing efficiency, reducing cost by offering alternative operations, producing higher‐purity products, and saving process time and energy .
The time and energy needed for the separation, centrifugation, and purification steps of the conventional wet method have been reported as the main challenges for isolation of high‐quality hydrocolloids from mucilaginous seeds in commercial production settings . Moreover, high impurity and dark color are other problems that can reduce the quality of the hydrocolloids produced from mucilaginous seeds using the conventional isolation methods. Therefore, Farahnaky et al.  made the first attempt at ultrasound‐assisted isolation of wild sage seed (Salvia macrosiphon L.) mucilage, and isolation of mucilaginous hydrocolloids from wild sage seeds as an innovative technology was performed successfully. Mucilaginous hydrocolloid samples were isolated using a probe‐type ultrasonic system under different conditions (i.e., time, 1–20 min; temperature, 5–60 °C; and ultrasound power, 30–150 W), and some of the physicochemical characteristics of the isolated mucilage were studied in terms of yield, color (i.e., lightness), chemical composition, rheological properties, and intrinsic viscosity in comparison with the conventional method. The ultrasound method increased yield, lightness, and purity, and in particular, reduced the mucilage protein content of the isolated hydrocolloids. Rheological measurements showed that an increase in the intensity of ultrasound waves causes a decrease in the consistency coefficient and an increase in the flow behavior index, and thus hydrocolloid solutions tend to show a more Newtonian behavior. The critical concentrations of S. macrosiphon seed gum isolated with the conventional method and the ultrasonic treatment were 0.06 and 0.2 g/dl, respectively, indicating degradation of the mucilage biopolymers during ultrasound‐assisted isolation. It was concluded that ultrasound waves can be a suitable method for isolating hydrocolloids from S. macrosiphon seeds.
In another study, ultrasound‐assisted isolation was used to determine the optimum processing conditions to achieve the maximum extraction yield and viscosity of the gum extracted from quince seed using an ultrasound bath . Response surface methodology was used for the experimental design, and the impact of the extraction temperature (25–55 °C), time (3–10 min), and pH (6–8) were studied on the extraction yield (%) and rheological properties of the extracted mucilage. To extract the mucilage, the quince seeds were first cleaned, and mixed with distilled water at a seed‐to‐water ratio of 1:25 (v/w). The seed–water mixtures were placed in an ultrasonic water bath operating at a frequency range 50–60 kHz with an input power of 100 W and equipped with an automatic temperature controller. In order to separate the hard seed cores from the resultant gum, the slurry was centrifuged at 4000 rpm for 10 min. The resulting supernatant was finally dried at 60 °C for 16 h. The quince seed mucilage powders extracted by the ultrasound‐assisted method had 9.84% moisture, 12.59% ash, 71.6% carbohydrate, 3.16% fat, and 2.81% protein, all on a dry weight basis. The authors further concluded the mucilage isolated by the ultrasound‐assisted method had a higher purity due to lower ash and protein content as compared to the quince mucilage isolated by the conventional wet method. The production yield varied from 7.02% to 16.29% for different ultrasound‐assisted isolation conditions, but under the optimum operating conditions as determined by the highest yield and greatest viscosity (i.e., 38.03 °C, pH 6.35, and sonication time of 7.68 min), a production yield of 14.09% was obtained.
Previous publications have confirmed the successful application of ultrasound waves for the isolation of mucilage from sage seed (Saliva macrosiphon) and quince seed [ 29, 30]. Bakhshizadeh‐Shirazi  investigated the extraction of mucilage gum from some seeds using the ultrasound‐assisted extraction method, and in the next section of the chapter, a rather detailed report is presented. The purpose of this study was to make an effort to generalize this method by investigating its efficacy on another mucilaginous seed, that is, basil seed (Ocimum basilicum L.) – as one of the mucilaginous seeds that is found abundantly and besides its health benefits is considered a potential source of accessible and cost‐effective hydrocolloids that can be used in different food formulations as a thickener or stabilizer  – and monitor its response to this new procedure of gum isolation. Furthermore, some physiochemical properties of the mucilage obtained from the conventional and ultrasound‐assisted methods are compared.
Soon after basil seeds are mixed with water, their outer pericarp swells, and a mucilaginous or gelatinous mass is formed due to the presence of branched polysaccharides surrounding the hard seed cores . In ultrasound‐assisted isolation research  based on the maximum water uptake, the optimum conditions, that is, temperature and soaking time for basil seeds, were investigated and found to be 56.03 °C and 39.7 min, which resulted in 51.88 (g/g water‐to‐seed ratio). This water‐to‐seed ratio was used for mucilage extraction by both the conventional and ultrasound‐assisted methods. For the conventional extraction method (the control), the soaked seeds were subjected to shear forces for 30 s at room temperature using a blender equipped with rotating rough blades that scraped off the mucilage layer. The high mechanical shear forces transformed the swollen seeds into a high‐viscosity heterogeneous mixture that was then centrifuged at 2700 g for 20 min at 25 °C in order to separate the mucilage dispersion from the crushed seeds. Ultrasound‐assisted isolation of basil seeds from the swollen seeds was performed using the procedure described by Farahnaky et al. , for different sonication times (1–20 min), ultrasound powers (30–150 W), and temperatures (5–60 °C) using an ultrasonic processor (HD 3200, Bandelin, Germany) operating at a constant frequency of 20 kHz and amplitude of 100% equipped with a high‐grade titanium tip. After ultrasound application, hard seed cores were separated by passing the ultrasound‐treated mixture through a sieve, and the mucilage dispersion was finally dried at 50 °C.
Figure 18.2 shows step‐by‐step scrapping off of the mucilage layer by ultrasound waves, indicating a gradual removal of hydrocolloids from the outer to inner layers. As Figure 18.2A reveals, seed mucilage decreases in thickness until all mucilage layer is isolated and a hard seed core is left. The process must be stopped at this stage since there is no more mucilage to be isolated, and applying more ultrasonic waves to the mixture of hard seed cores and the isolated mucilage may lead to negative consequences. This is due to the fact that excessive use of ultrasound waves not only separates small particles from the hard seed cores and transfers them into the solution, causing darkening of the final mucilage powder, but also increases the mucilage impurity. High‐intensity ultrasound also has negative consequences on mucilage rheological characteristics, which will be discussed in the next section.
Figures 18.2B,C compare two mucilage dispersions after applying the conventional and ultrasound‐assisted isolation treatments. The color of the mucilage dispersion isolated by ultrasound waves is lighter with no impurities; however, the conventional method produced a solution with a darker color and contained unwanted particles caused by breaking of seed cores during high shear blending. In the ultrasound‐assisted method, no shear force is applied at the macroscale, and therefore the mucilage seeds are not physically damaged and seed core contents do not get transferred into the mucilage dispersion; as a result, a purer mucilage powder with a lighter color is produced.
The impact of ultrasound power, time, and temperature on mucilage production yield is presented in Figure 18.3. Time was the dominant factor, and yield had a direct but nonlinear relationship with time and power. The number of cutting actions during ultrasound application is directly proportional to time, while higher powers lead to increased fluid and seed turbulence and also a greater cavitation effect in mucilage layer, which in turn leads to increased yield.
As seen in Table 18.2, no major differences are found between the samples in terms of moisture and ash content. An important factor to notice is the protein content of the samples, which is considered to be the main criterion of gum purity and entrance of impurities through seed damage. The highest protein level was related to the sample isolated by the conventional method, in which the hard seed cores were crushed, and as a result, in addition to the intrinsic protein of the gum, a small amount of protein from hard seed cores entered the mucilage powder and increased the protein content to 8.71%. The protein content of the mucilage isolated by low‐intensity ultrasound waves was 4.12%, which shows a significant purity improvement in the samples isolated by various methods, confirming the efficiency of the ultrasound method in producing a high‐purity‐grade mucilage. The protein content of the mucilage isolated by the ultrasound‐assisted method increased with ultrasound power. This increase in protein content at medium and high ultrasound intensities is due to partial damage to the surface of the hard seed cores after complete separation of the mucilage layer, which causes entry of separated small particles from seed cores into the mucilage solution, leading to an increase in gum impurity and particularly protein content. However, the protein content of the mucilage produced by high‐intensity ultrasound is still significantly lower than that of the conventional method. Similar results were obtained in the previous research, which was conducted on Salvia macrosiphon .
|Isolation method||Moisture (%)||Ash (%)||Protein (%)|
|Conventional||7.01 ± 0.06 a *||5.86 ± 0.08 a||8.71 ± 0.45 a|
|Low ultrasound level||7.02 ± 0.04 a||5.07 ± 0.06 c||4.12 ± 0.29 d|
|Medium ultrasound level||6.97 ± 0.07 a||5.39 ± 0.16 b||5.22 ± 0.28 c|
|High ultrasound level||6.99 ± 0.08 a||5.76 ± 0.07 a||6.12 ± 0.36 b|
* Values are the averages of three replicates ± SD. Different letters in each column show a statistical difference between the samples (α < 0.05).
The trends seen for the lightness of mucilage powder is opposite to the yield value (Figure 18.4); that is, with an increase in process time and ultrasound power (or the process intensity), the mucilage lightness decreased. Also, temperature, especially at high ultrasound intensities, had the same impact with a much milder slope. In fact, after isolating all mucilage sections surrounding the hard seed cores, if ultrasound application continues, then hard seed cores are damaged and small solid particles are separated, which can lead to darkening of the gum solution and ultimately the corresponding dried mucilage powder. The L‐value for the basil seed mucilage isolated by the conventional method was 50, which shows a darker color compared to the mucilage obtained by the ultrasound isolation treatments. The average L‐value of all samples isolated by the ultrasound method was 56. In addition to the chemical composition of mucilaginous seeds and their intrinsic protein, the mucilage lightness is dependent on the adhesion level of the mucilage layer to the seed cores. That is, in long‐time ultrasound‐assisted isolation processes, the easier the mucilage layer is isolated from the seed cores, the greater seed damage, and the darker the mucilage powder. Thus, comparing the mucilage powders of basil and sage seeds, it was observed that the L‐value means for all mucilage powders derived from same ultrasonic treatments were 56 and 70, respectively. This variation is due to the intrinsic differences between the two seeds and their mucilages. The protein content of basil seed mucilage in this study was twice of that of sage seed mucilage.
Regardless of the isolation method, all mucilage samples (including mucilages isolated by the conventional method, low‐, medium‐, or high‐intensity ultrasound methods) showed shear‐thinning behavior. Figure 18.5 clearly reveals that with increasing sonication time and ultrasound power, the consistency coefficient of mucilage solutions reduced significantly; that is, power ultrasound application decreases the viscosity of mucilage dispersions, which is in line with the previous reports on ultrasound‐treated hydrocolloid solutions [33–36]. The conventional isolation method produced a mucilage with the highest viscosity. In this context, it should be noted that considering the yield results, gum isolation could be completed at low to medium intensities, and there is no need to apply high ultrasound intensities to achieve complete mucilage isolation. Therefore, the negative impact of ultrasound waves on mucilage viscosity can be minimized by using the optimum condition of isolation. A different trend was found for flow behavior index; that is, increasing sonication time and power caused increases in the flow behavior index toward 1. That is, the samples' rheological behavior approached the so‐called Newtonian behavior. This would be due to the size reduction and also the decrease in the degree of branching of biopolymer molecules by ultrasound waves. The results were in accordance with the sage seed (Salvia macrosiphon) study  which was discussed previously.
Despite a large number of research reports on mucilage isolation, characterization, and application in different food and non‐food systems confirming diverse but excellent rheological properties for mucilage seed gums, there is still only a very limited number of seed mucilage powders available on the market at the commercial level. This could be due to a number of reasons, and among them, the lack of an economically viable isolation technique is regarded as a major obstacle to the commercial production of seed mucilage. Emerging technologies are regarded as a way forward that can help develop adequate industrial procedures for the production of mucilage gums that are economically viable, require low capital and operating costs, and produce high‐quality mucilage powders.
Ultrasound‐assisted isolation showed great potentials to address some of the main challenges that mucilage production is facing. A number of research reports have confirmed ultrasound waves' ability to act like thousands of microscopic scissors, providing a set of cutting tools that can be fine‐tuned by adjusting process parameters. This isolation method minimizes process time, reduces water and energy usage, and can improve gum quality by reducing impurities and improving color and rheological properties. However, some of the challenges for industrial production of mucilage gums remain to be solved and can be the subject of future research, including: