Opuntia ficus‐indica (Figure 17.1) is a widely used plant from the kingdom of Plantae, order of Caryophyllales and family of Cactaceae, genus of Opuntia, and species of indica . Opuntia is a local plant of different environments. It grows in various dry and semi‐dry countries such as Mexico, Morocco, Tunisia, Eritrea, Ethiopia, Argentina, Peru, Bolivia, Brazil, the United States, Spain, Italy, Israel, Iran, and South Africa . Even this kind of cactus is cultivated in Angola, Australia, India, and Canada because Opuntia ficus‐indica has the capability to adapt to various environmental conditions. It can harvest in the fields of Mexico, where the temperature is above 5 °C, and also in Canada, where temperatures drop to −40 °C in winter . In Table 17.1, the applications of Opuntia in different countries are summarized .
Table 17.1 Production and uses of cactus plants and fruits in selected countries.
Source: Reported by FAO .
|Country (year)||Cactus production area/hectares||Production rate||Use|
|Argentina (2004)||Grown: 2 000; Wild: 200 000||Not reported||Fruit production|
|Chile (2003)||Grown: 1 100; Wild: 395||Not reported||Fruit production and Cochineal production|
|Eritrea (1999)||Not reported||4 794 tonnes in summer||Fruit production|
|United States of America (2001)||200||Not reported||Fruits and frozen cactus pear puree|
|Ethiopia (2004)||Grown: 30 000; Wild: 32 500||Not reported||Fruit production and for cultural heritage|
|Italy (2001)||3 000||70 000 tonnes/year||Fruit production|
|Morocco (2005)||45 000||14.3 tonnes/ha/year||Fruit production|
|Mexico (2004)||72 500||563 443 tonnes/year||Fruit production|
|Peru (2000)||Grown: 29 746.15; Wild: 35 000||6.15 tonnes of fruit/ha||Fruit production and Cochineal production|
|South Africa (2004)||Grown: 29 746; Wild: 35 000||15 000 tonnes/year||Fruit production|
|Tunisia (2002)||Around 450 000||Not reported||Forage production|
|Egypt (2002)||2 548||29 442 tonnes/year||Fruit production|
This type of Opuntia has different names depending on the region where it is found. For example, in Spain, the plant is renamed chumbera and the fruit as higo de las Indias (Indian fig), which is currently known as higo chumbo (chumbo fig). In Italy, it is called fico d'India; in France, figue de Barbarie; and in Australia, Africa (Morocco, Tunisia, Eritrea, and Ethiopia), and in the United States, prickly pear. Opuntia ficus‐indica includes four parts: root, stem, fruit, and flower. The fruit has a broad range of color (yellow, orange, red, and purple) and is sweet and juicy, full of pulp and mucilage with different skin thicknesses .
This plant is native to many countries and has many usages. In some countries like Mexico, it is eaten as fresh fruit  or canned. Syrup and toffee from fruit juice, cactus pear cheese made from the fruit, and cactus pear raisin are some of the other applications of this fruit . Also, in traditional usage, small farmers use cactus mucilage to purify drinking water, and fresh cladodes of pointier are used for wastewater remediation.
Opuntia, like other plants, has fibers with health benefits such as weight control, blood cholesterol, and diabetes control. Cactus fibers bind to dietary fat, reducing absorption of fats. This action reduces energy uptake and facilitates weight loss after approximately 45 days. It also reduces the level of blood cholesterol; scientists have stated that the effect of Opuntia fiber on plasma cholesterol could enhance fecal excretion of bile acids . Dietary fiber of Opuntia cladode is associated with reduction in the risk of certain types of cancer, such as colon cancer, and also decrease in symptoms of chronic constipation, diverticular disease, and hemorrhoids . Nopal cladode can be used for treating patients with type 2 diabetes. It reduces postprandial blood glucose, serum insulin, and plasma (gastric inhibitory polypeptide) GIP peaks, as well as increasing antioxidant activity in healthy people . Although the mechanism of this action is unknown, the value of the glucose can be controlled and influenced by a small amount of Opuntia extract (1 mg kg−1 body weight per day), indicating an important role for dietary fiber in hypoglycemic activity. Opuntia ficus‐indica cladodes can protect against damage to mice liver . The extracts of Opuntia ficus‐indica cladodes can confirm the detrimental effects of cytokine interleukin‐1β (IL‐1β) .
Hydrocolloids are water‐soluble macromolecules with a high molecular weight (HWS) which are extensively used as functional ingredients in the pharmaceutical and food industries . For instance, they are broadly utilized as thickening and stabilizing agents, edible coatings, wall material for encapsulation of phytochemicals, and for the manufacture of eco‐friendly packaging systems. In recent years, interest has been growing in finding new sources of hydrocolloid with appropriate characteristics (low price, ease of access, and proper functionality). Therefore, an extensive range of studies has focused on finding and characterizing new hydrocolloid sources which could serve as potential substitutes for commercial gums. The aims of the present chapter are to provide detailed information about the chemical composition, structural properties, functional characteristics, and rheological behavior of the mucilages extracted from different parts of Opuntia ficus‐indica plant as new sources of natural polymers to elucidate their potential applications in the food, pharmaceutical, and other industries.
Cladodes are re‐formed stems of the Opuntia ficus‐indica plant that have photosynthetic action instead of leaves (Figure 17.1a). They are a lush and segmented organ with an elongated form, usually 30–80 cm long and 18–25 cm wide, with the weight in the range 40–100 g. It is usually 1 to 3 old . Cladodes include two parts: the inner part, where photosynthesis occurs, is composed of chlorenchyma, and the inside part consists of white medullar parenchyma that is a source of water storage .
Cladodes are valuable for industry. They have broad uses in each stage of growth. When they are buds, they can be used as a vegetable. After 2–3 years, Opuntia cladodes become woody, and in this stage, they are used in flower production. In the end, the cladodes become wholly lignified and are burned as biofuel .
The age of the plant affects the chemical composition of cladodes . As shown in Table 17.2, in the young stem, protein, and ash are at their highest level, but fiber, fat, and nitrogen‐free extract reach their lowest value . Another study explains that protein is higher in the fleshy leaf, but the fiber trend is the same .
Table 17.2 Chemical composition of Opuntia ficus‐indica cladodes of different ages (g/100 g dry matter).
Source: Reported by FAO .
|Age (years)||Description||Protein||Lipid||Ash||Crude fiber||Non‐nitrogen extract|
|0.5||Young stems (nopalitos)||9.4||1||21||8||60.6|
|1||Fleshy leaf (penca)||5.4||1.3||18.2||12||63.1|
|2||Fleshy leaf (penca)||4.2||1.4||13.2||14.5||66.7|
|3||Fleshy leaf (penca)||3.7||1.3||14.2||17||63.7|
In fresh and young stems, the major component is water (91%). The other components are proteins 1.5%, lipids 0.2%, total carbohydrates 4.5% and ash 1.3%, and specially calcium 0.090%. In addition, 11 mg (100 g−1) vitamin C and 30 μg carotenoids have been detected .
Opuntia ficus‐indica cladodes contain functional compounds consisting of fiber, hydrocolloids (mucilage), pigments (betalains and carotenoids, flavonoids and flavonols), minerals, (calcium and potassium), several amino acids and amines, and vitamins with antioxidant activity, such as vitamin C . These compounds are necessary for a healthy diet and are used as ingredients to design new products. Similar to other vegetables, nopal contains a large amount of water and fiber content . Fiber is classified into two types: soluble and insoluble in water. The soluble form is contained mucilage, gums, pectin, and hemicellulose, and insoluble fiber consists of cellulose, lignin, and a large fraction of hemicellulose . The amount of fiber in Opuntia ficus‐indica cladodes is comparable with that in other fruits and vegetables such as mango, melon, apricot, grapes, spinach, artichoke, beet, eggplant, broccoli, and radish .
This plant has a large amount of water; its fat content is low, and it has about 0.3 and 5.6 g/100 g carbohydrate and protein, respectively . Cladode also has a considerable amount of minerals: 93 mg/100 g calcium and 166 mg/100 g potassium, but it is poor in sodium (2 mg/100 g). The young stems contain a modest amount of carotenoids (30 mg/100 g) and vitamin C (11 mg/100 g) .
Opuntia ficus‐indica fruit (Figure 17.1b) is a “false” berry fruit as it has an undesirable ovary that is simple and fleshy. This fruit comes in various shapes and sizes such as ovoid, round, elliptical, and oblong with flat, concave, or convex borders  and has a wide range of colors like red, orange, purple, yellow, and green with pulp having the same color. The fruit weighs from 100 to 150 g, depending on the origin, cultivar, and edaphic conditions.
The epidermis of the fruit is analogous to the cladode, including areoles and abundant glochids and spines that remain even after the fruit has over‐ripened; in cladodes, however, they do not remain. The pulp is the edible part of the fruit, and its main components are water (84%–90%) and reducing sugars (10%–15%) . In Table 17.3, the chemical composition of fruit pulp from Opuntia is summarized. The amounts of these constituents depend on the cultivation site, climate, and fruit variety [17,18]. The soluble solids content in fresh fruit (12%–17%) is higher than that present in other fruits, such as prunes, apricots, and peach . Cladodes are richer in pectin, mucilage, and minerals, whereas the fruits are rich in sources of vitamins, amino acids, and betalains. In addition, the seed endosperm consists of polysaccharides full of arabinan .
Table 17.3 Chemical components of fruit pulp from Opuntia ficus‐indica.
|Component||Amount of fruit pulp (%)|
Fruit ripening affects the composition of fruit. Opuntia fruits are non‐climacteric, so it is important to harvest them at the optimal grows stage for processing, marketing, and consumption. Size, changes in peel color, firmness of the fruit, depth of the flower cavity or receptacle, total soluble solids (TSSs) content, and reduction of glochids are important parameters for determining the best time for harvesting the fruit . The physical changes and composition of Opuntia fruits during ripening are summarized in Table 17.4. It seems the maximum values for fruit weight, diameter, and pulp are achieved in overripe fruit. In all stages of maturity, the pH of fruit is in the range 5.2–6.4, but the firmness decreases with increasing fruit age . The sugar, TSS, and vitamin C content increase considerably during the ripening process, whereas the firmness and acidity decrease .
Table 17.4 Physical and chemical changes of Opuntia ficus‐indica fruits during ripening.
Source: Reported by FAO .
|Ripening state||Mass (g)||Diameter (cm)||Receptacle depth (mm)||Pulp (%)||Firmness (kgcm−2)||T.S. (%)||Acidity (%)||pH||Vitamin C (mg/100 g)|
T.S.: Total solids.
The prickly pear fruit contains many seeds. They make up about 10%–15% of the edible pulp and usually are eaten with the pulp and can be thrown away as waste after pulp extraction. Several researchers have reported a major variation in the number of seeds, from 1–5 to more than 2000 per fruit . This variation is reported within or between species related to factors such as the age and size of the plant, and the number of flowers per plant. Seed gayness under natural or controlled storage conditions may depend on many factors, including seed type, maturity stage, activity and moisture content during storage, temperature, and degree of fungal or bacterial infection . Some fruits contain aborted seeds, which improve the proportion of edible pulp. Seeds represent significant variations in form, size, structure, characteristics, and color.
The fruit is the most attractive part of the plant due to its different size, low acidity, and adequate sugar content . Cactus fruit has a short shelf life of 3–4 weeks, thus limiting long‐term storage and worldwide distribution. Cactus fruit includes considerable amounts of ascorbic acid, vitamin E, carotenoids, fibers, amino acids, and antioxidant compounds such as phenols, flavonoids, betaxanthin, and betacyanin, which have health benefits such as hypoglycemic and hypolipidemic action, and antioxidant properties [23,24]. The fruit of O. ficus‐indica is a rich source of various nutrients with antiulcerogenic , antioxidant , anticancer , neuroprotective , hepatoprotective, and antiproliferative properties . Cactus peel and seeds can be used to produce cactus oil; peel lipids are fortified with essential fatty acids and liposoluble antioxidants. Amino acid content in cactus fruit is higher than in the cactus cladode. In cladode, carnosine, citrulline, ornithine, proline, and taurine were not detected. Cactus fruits contain high amounts of amino acids, especially proline, threonine, and serine .
Vitamins are important nutrients of cactus pear fruit constituent. Cactus fruit pulp contains ascorbic acid (12–81 mg/100 g), niacin (trace amounts), riboflavin (trace amounts), thiamine (trace amounts), and total carotenoid (0.29–2.37 g/100 g) . Total vitamins in fruit are more than in cladode. In the lipid fraction of the cactus fruit pulp, fat‐soluble vitamin E or tocopherols, and beta‐carotene were observed . The principal components of pulp oils are vitamin E homologue isoforms gamma‐ and delta‐tocopherol, which constitute about 80% of the total vitamin E content . Cactus pear has 180 to 300 mg kg−1 of vitamin C. This amount is higher than in other common fruits like apple, banana, or grape . Ascorbic acid is the third major vitamin in cactus pear fruit. It is important to note that the total vitamin C content of cactus fruits might have been minimized because of the presence of dehydroascorbic acid, which has not been considered so far . Vitamin K1 is found in all parts of the cactus pear fruit in the range 0.5 to 1 g kg−1 [30,31].
All parts of the cactus plant contain members of the polyphenol family, such as various flavonoids and phenolic acids. The pulp of Opuntia ficus‐indica fruit indicates 218.8 mg/100 g total phenol content . In addition, the high content of isorhamnetin glycosides (50.6 mg/100 g) is comparable to that of other flavonoids . Fruit seeds include a large amount of phenolic compounds in the range of 48–89 mg/100 g and containing feruloyl derivatives, tannins, and sinapoyl diglucoside . Fruit peel has a very high phenolic content (45.7 g/100 g). Many of these phenols are bioactive molecules; flavonoid derivatives such as kaempferol and quercetin are present in amounts of 0.22 and 4.32 mg/100 g, respectively.
Cactus pear fruit has health benefits because of the high levels of ascorbic acid, vitamin E, carotenoids, fibers, amino acids, and large amounts of glucose and fructose; it is also rich in phenols, flavonoids, betaxanthins, and betacyanins and has hypoglycemic and hypolipidemic actions, besides antioxidant properties . Generally, cactus contains a high amount of natural pigments betalains. Consumers are increasingly averse to synthetic colorants, and natural colorants, such as the red betacyanins and the yellow betaxanthins, demonstrate a good natural alternative to meet the growing demand of the food industry. The antioxidant properties of these betalain pigments are a further reason for their use in nutrition and health .
For the mucilage of Opuntia cladodes (OCM), the mucilage yield is 1.56% on the basis of fresh weight and 19.26% on the basis of dry weight. Climatic conditions such as cold or rainy weather have a strong effect on the amount of mucilage. This could be due to the ability of these polysaccharides to absorb water as a plant defense against stress conditions. Forni et al.  characterized the mucilage from Opuntia ficus‐indica peel (OPM) in two different ripeness stages and regions. They found that the average extraction yield of the extracted gum was very low (0.12% w.b.).
The average yield of mucilage extracted from nopal peel (NPE), nopal pulp (NPU), and pear peel under different extraction conditions (acid‐alcoholic and aqueous process) are presented in Table 17.5. It can be seen that the yield reported for mucilage extracted from NPE, NPU, and pear peel are 0.085, 0.085, and 0.48 (w/w %), respectively. Comparatively, the yield of pear peels mucilage is close to those of apple (0.41 w/w %) and citrus (0.8% w/w %) . The lower extraction yield reported for NPU mucilage is due to the large amount of fiber and of extractable low‐molecular‐weight substances (LWS) . As shown in Table 17.5, the extraction yield reported for the aqueous process is close to that of acid/alcoholic process, demonstrating that the gum can be extracted without application of non‐aqueous solvents. On the basis of Forni et al. , the extraction yield obtained by acid/alcoholic process is 0.12 (w/w %), which is lower than the value reported by Majdoub et al. , which may be attributed to their different moisture content. Furthermore, since the physicochemical properties of natural polymers are mainly dependent on environmental conditions, the observed difference also could be associated with the variability in growing conditions, the age of the plant, time of collection, and the contamination of the mucilage with other natural compounds . For instance, the age of nopal cladode affects the mucilage properties. In the cladode aged 50, 100, and 150 days, the amount of extracted mucilage is significantly different (P ≤ 0.05). The highest amount of mucilage is obtained in the first stage of maturity (50 days). This result shows that with increasing cladode maturity stage, the amount of mucilage decreased . This could be attributed to the fact that mucilage is part of the soluble fiber, and when the cladode becomes older, the amount of soluble fiber decreases . Despite low‐yield extraction of the mucilage NPU, it can be a good source of hydrocolloid because it can be harvested in large amounts during the whole year .
Table 17.5 Extraction yield of mucilage obtained from different parts of Opuntia ficus‐indica.
Source: Adapted from Majdoub et al.  with permission from Wiley.
|Yield from wet matter (wt%)||0.085||0.085||0.48||0.46|
|Yield from dry matter (wt%)||3.7||0.65||7.3||7.0|
|Yield from water‐soluble matter (wt%)||5.4||1.2||30||–|
NPU, nopal pulp gum (aqueous extraction); NPE, nopal peel gum (aqueous extraction); PPE, pear peel gum (aqueous extraction); PPI, pear peel gum (acid/alcoholic extraction).
On the basis of a recent study , the extraction yield of Opuntia ficus‐indica mucilage (OFM) extracted from the fruits was 3.8%, which is considerably more than those extracted from other parts of this plant mentioned above.
Mucilage is one of the main components in the different parts (cladodes, fruits, and peel) of Opuntia ficus‐indica. It requires enzymatic digestion and cannot be digested by humans . Mucilage is added in mucilaginous cells that are in the chlorenchyma and parenchyma inside the tissues . This kind of cell is plentiful in the parenchyma. This mucilage is responsible for the osmotic property of saving water . So, mucilage should be extracted from the plant cell.
The first step to evaluating the purity of the gums is the evaluation of its chemical composition . Various studies have been conducted to elucidate the chemical composition and structural properties of the mucilage extracted from different parts of Opuntia ficus‐indica [ 36, 40 43–52]. The results of these studies differ, which may be attributed to ineffective purification methods and/or contamination of the gum with other compounds .
The mucilage extracted from OCM contains, on average, 64.15% carbohydrate . Hence, carbohydrate is the main constituent of this gum. The amount of carbohydrate in this gum is lower than those reported for commercial ones like guar gum (71.1%) and gum ghatti (78.4%), and locust bean gum (85.1%–88.7%) [54–56]. Therefore, it seems that a purification process should be carried out to improve its purity. On the other hand, OFM is composed, on average, of 93.48% carbohydrate, 23.40% uronic acid, 7.65% moisture, 0.86% protein, and 9.00% ash. The carbohydrate content of OFM (93.48%) is higher than that reported for many hydrocolloids like guar gum (71.1%), locust bean gum (85.1%–88.7%), and gum ghatti (78.36%) [ 54– 56].
Monosaccharide composition is one of the most important factors that seem to have a considerable effect on the physicochemical and functional properties of the gums. For example, in galactomannan gums, the ratio of mannose to galactose is considered a detrimental factor, which mainly influences their physicochemical properties . With increasing galactose substitution, the solubility of galactomannans increases .
Pectin is a complex polysaccharide mainly composed of anionic D‐galacturonic acid units in an alpha‐(1 → 4) chain, which are interrupted occasionally by alpha‐(1 → 2)‐rhamnose residue . The monosaccharide composition of the gum extracted from different parts of Opuntia ficus‐indica is given in Table 17.6. Galacturonic acid and rhamnose are the main monosaccharides of these gums, therefore it can be suggested that the gums extracted from different parts of Opuntia ficus‐indica have a pectin‐like structure. However, the galacturonic acid content of these gums is under the limit of 65% recommended for the pectin used in food applications . Forni, Penci  reported that prickly pear pectin contains 64% galacturonic acid with a degree of methoxylation of about 10% and a neutral sugar content of 51%.
Table 17.6 Monosaccharide compositions of the gum extracted from different parts of Opuntia ficus‐indica.
Source: Adapted from Majdoub, et al.  with permission from Wiley.
NPU, nopal pulp gum (aqueous extraction); NPE, nopal peel gum (aqueous extraction); PPE, pear peel gum (aqueous extraction); PPI, pear peel gum (acid/alcoholic extraction).
Different results have been reported for the sugar composition of OCM. This mucilage is a neutral polysaccharide containing approximately 55 sugars composed of arabinose, rhamnose, galactose, and xylose. Neutral fractions of glucans and glycoproteins and acidic fractions that yield arabinose, galactose, rhamnose, xylose, and galacturonic acid constitute the water‐soluble polysaccharide of Opuntia ficus‐indica . The mucilage of Opuntia ficus‐indica cladode is composed of arabinose, galactose, rhamnose, xylose, and galacturonic acid. Chromatography techniques are used to determine the molecular structure and sugar components . Different amounts of the major neutral sugars such as L‐arabinose, D‐galactose, L‐rhamnose, and D‐xylose have been obtained from nopal mucilage. Also, the existence of D‐galacturonic acid is illustrated. Opuntia ficus‐indica mucilage is made up of 24.6%–42% of arabinose, 21.0%–40.1% of galactose, 8.0%–12.7% of galacturonic acid, 7.0%–13.1% of rhamnose, and 22.0%–22.2% of xylose . In a recent study, Lefsih et al.  employed gas chromatography‐mass spectrometry (GC‐MS) to determine the monosaccharide composition of the mucilage obtained from whole cladodes of Opuntia ficus‐indica. Their results showed that this gum had 31.8% galactose, 25.1% glucose, 23.2% galacturonic acid, 18.8% arabinose, and 1.1% xylose.
The amount of galacturonic acid in the carbohydrate isolated from NPU is lower than those extracted from NPEs and polysaccharide from peels (PPE). Comparatively, the galacturonic acid content of the gums obtained from different parts of OFI is lower than those reported for fruit pectins like quince pectin (78%), Argentina lemon (70%–80%), and apple pectin (77%) [ 36,62,63]. There is no considerable difference between the monosaccharide composition of PPI and PPE samples, which establishes that the difference in pH of the extraction process is not responsible for the discrepancy. In conclusion, the pectin obtained from the fruit peel is an anionic biopolymer with about one free galacturonic acid per three saccharide residues, whereas that extracted from pear nopal is an anionic polysaccharide containing about one free galacturonic acid per nine and twelve saccharide residues .
Majdoub et al.  extracted a water‐soluble fraction from the nopals of prickly pear of Opuntia ficus‐indica. On the basis of their results, this water‐soluble fraction contains two fractions including the fraction with high average molecular weight (Mw of 13 × 106 Da) and the other with LWS (4000 Da). The high‐molecular‐weight substances (HWS) fraction is a flexible polysaccharide with a pectin‐like structure; however, its composition considerably differs from that of peel pectin . Additionally, this fraction has a low amount of charged sugar. The acid equivalent weight of HWS is 840 g per mole of equivalent, indicating that this fraction has the potential capability to interact with divalent cations like Ca+2 and Mg+2 , as extensively described in the literature [ 37,65]. Most of the LWS fraction is protein (about 80%), whereas there is no protein in the HWS fraction.
Lefsih et al.  indicated that the pectin fraction extracted from cladodes of OFI is composed of three fractions, water‐soluble pectin (WSP), chelating‐soluble fraction (CSP), and acid‐soluble pectin (ASP). The galacturonic acid content of WSP, CSP, and ASP is 66.6%, 81.1%, and 44.3%, respectively. On the basis of the FT‐IR spectrum of this gum, there is a strong absorption peak at around 1618 cm−1, confirming the presence of galacturonic acid in the composition of WSP, CSP, and ASP. Neutral sugars of WSP contain galactose (11.4%), arabinose (9.2%), and glucose (6.9%), suggesting an arabinogalactan structure. ASP comprises 44.3% glucuronic acid and 6.6% rhamnose, indicating the occurrence of a rhamnogalactan backbone with homogalactan chains. CSP is a homogalacturonan pectin without a rhamnose residue.
On the basis of Cárdenas et al. , the molecular weight of the gum obtained from Opuntia ficus‐indica is 3 × 106 g mol−1, which is lower than the value reported by Trachtenberg and Mayer  (4 × 106 g mol−1) and considerably higher than that cited by Medina‐Torres et al.  (2.3 × 104 g mol−1). The chemical properties of the gums are considerably dependent on source, the age of the plant, growing conditions, and contamination of hydrocolloids with other cell compounds . Therefore, the differences in the compositional properties of the gum in the literature may be due to the difference in the isolation process and/or contamination of hydrocolloids with other cell compounds.
The protein content of OCM is 1.04% and is similar to values reported in most studies . The protein fraction in gums is a factor that determines their functional properties. For instance, film‐forming ability, emulsifying and stabilizing capacities, as well as foaming properties arise from the protein fraction [ 38 68–70]. The protein content of the mucilage extracted from cladodes is higher than the values reported for commercial gums like xanthan gum (2.12%), lower than guar gum (8.19%), and close to the values reported for locust bean gum (5.2%–7.4%) [55,71]. Proteins comprise both hydrophobic and hydrophilic functional groups and thus impart surface activity in hydrocolloids. The presence of protein in the composition of the mucilage extracted from cladodes indicates that this gum can be applied in emulsification systems. OCM has 20.08% ash , which can be changed by the age of the cladode and environmental conditions . The ash content of this gum is profoundly greater than those reported for locust bean gum (0.7%–1.5%), gum arabic (1.2%), xanthan gum (1.5%), and guar gum (11.9%) [ 55, 71]. Natural gums have various neutralized cations and metal ions, which can reinforce their viscosifying and gelling properties . For instance, carboxyl functional groups in the structure of biopolymers can serve as the binding site for calcium ions, improving their rheological properties and gelling capability . This gum has a considerable amount of calcium ion (3.10%) , which interacts with gum carboxylic functional groups located in its backbone . Due to the high amount of ash, it is very important to characterize the mineral profile of this gum.
The protein content in OFM (0.86%) is considerably lower than those of commercial gums like xanthan gum (2.12%), locust bean gum (5.2%–7.4%), and guar gum (8.19%) [ 55, 71]. The high amount of carbohydrate (93.48%) and low protein content (0.86%) indicate that OFM has a high level of purity. The ash content of OFM (9.00%) is lower than that of OCM but is considerably high in comparison to those reported for commercial gums like locust bean gum (0.7%–1.5%), xanthan gum (1.5%), and gum arabic (1.2%) [ 55, 71]. Due to the abundant amount of ash in OFM, it seems that its mineral characterization is important. Furthermore, since minerals profile have a considerable effect on emulsification ability (iron), viscosifying features and gel‐forming ability (calcium) , and enzyme activity (calcium and magnesium), and because of the adverse health effects of some ions at high concentration , it is necessary to evaluate the mineral profile of the gums. The mineral composition of OFM in comparison to some commercial gums is presented in Table 0. Surprisingly, the magnesium content of OFM (33991.90 ppm) is notably higher that those of commercial gums like gum arabic (2400 ppm), guar gum (760 ppm), and xanthan gum (1340 ppm). Similarly, the manganese content of OFM is considerably higher than the values reported for these commercial gums. Due to the high values of nutrients in the composition of Opuntia ficus‐indica fruit mucilage, it is expected that this new source of hydrocolloid can be applied as a value‐added by‐product in foodstuffs.
Table 0 Minerals of Opuntia ficus‐indica fruit mucilage (OFM) compared to other commercial gums.
|Mineral||OFM||Gum arabica||Guar gum a||Xanthan gum a|
|Calcium (ppm)||3 169||7 222||1 258||1 458|
|Zinc (ppm)||653.18 * ± 2.38||<4.0||12.1||9.0|
|Magnesium||33 991.90 * ± 119.64||2 400||760||1 340|
|Manganese (ppm)||653.18 * ± 2.38||9.5||4.6||6.0|
|Sodium (Na)||414.26 * ± 6.42|
|Potassium (ppm)||74.12 * ± 1.40||<MDL||<MDL||<MDL|
|Iron (ppm)||4 055.81 * ± 20.89||<MDL||<MDL||<MDL|
|Lead (ppm)||2.49 * ± 0.12||<4.0||<4.0||12.0|
MDL: Method detection limits.
* Values are means ± SD of triplicate determination.
The amount of uronic acid in natural polymers is commonly considered to be a measure of their polyelectrolyte nature. Uronic acids are composed of acidic functional groups like carbonyl and carboxylic acids. When the pH of the acidic gum solution reaches the acid dissociation constant, the acidic functional groups will be ionized, making it negatively charged. The uronic acid content of OFM is 23.4%, which shows that this gum has a polyelectrolyte nature. Matsuhiro et al.  reported that the amount of uronic acid in OFM composition increased after saponification of the mucilage, demonstrating that a notable proportion of uronic acid was esterified. The moisture content of OCM (12.43%) is well within the limit set for natural gums (about 15.0%) .
The sugar composition of OFM is presented in Table 0. On the basis of gas–liquid chromatographic analysis, OFM contains arabinose, rhamnose, xylose, and galactose with respective ratios of 1.0:1.7:2.5:4.1. Due to the high amount of galactose and xylose in OFM, it is expected that this gum falls in the category of galactoxylan gum; however, further analysis like 1D and 2D NMR spectroscopy should be performed to confirm its structure.
|Protein||0.86 ± 0.03|
|Uronic acid (Galacturonic acid) a||23.4|
|Ash||9.00 ± 0.20|
|Moisture||7.65 ± 0.80|
MDL: Method detection limits.
* Values are means ± SD of triplicate determination.
Although various studies have investigated the chemical compositions of OCM [ 3, 40, 64,73], little is known about the structural properties of OCM. The structural characteristics of OFM have been analyzed by 1H and 13C NMR spectroscopy . The 1H NMR spectrum of this gum contains several signals at 4.87, 4.52, 4.37, 4.06, and 3.88 ppm, which have been attributed to H1, H5, H4, H3, and H2, respectively, of α‐D‐galactopyranuronic acid units connected 1 → 4 [74,75]. The diagnostic signals at 5.25 and 1.28 ppm have been associated with the anomeric and methyl protons at position‐6 of a‐L‐rhamnopyranosyl residues, respectively. Additionally, this gum has three signals at 4.10, 3.75, and 3.42 ppm due to H2, H5, and H4 of rhamnopyranosyl residues, respectively. The integration values of the anomeric and methyl protons of rhamnopyranosyl residues and the anomeric proton of galactopyranuronic acid are 3.18, 1.37, and 1, respectively . The 13C NMR spectrum of OFM indicates several signals at 100.39, 70.06, 70.96, 78.90, 71.95, and 173.46 ppm, which is attributed to C1, C2, C3, C4, C5, and C6 of α‐D‐galactopyranuronic residue, respectively. Furthermore, the diagnostic signals at 99.16, 77.95, 70.96, 73.28, 70.20, and 17.85 are due to C1, C2, C3, C4, C5, and C6 of α‐L‐rhamnopyranosyl, respectively [ 75,76]. Overall, it can be suggested that OFM is composed of a quite regular repeating unit of a rhamnogalacturonan.
Gel permeation chromatography of OFM indicates a heterogeneous composition for this gum. Treatment with cetrimide demonstrates that this gum consists of two major fractions: first, the SF (15.6% yield), which is composed of the neutral monosaccharides including arabinose and galactose with respective ratios of 1.0:2.2 and uronic acids (16.0%). The second fraction, the insoluble fraction (44.3% yield), contains three neutral monosaccharides including xylose, rhamnose, and galactose with respective ratios of 1.0:2.5:2.8 and 28.0% of galacturonic acid .
One of the most important properties of hydrocolloids is their rheological properties, because in gums they are necessary to determine the functional properties of polysaccharides and permit better selection of the appropriate biopolymer according to the specific usage in the industry . In solutions prepared by OCM, the rate of deformation is increased when the viscosity is decreased. This means that mucilage solution acts as shear‐thinning fluids. In this case, the power‐law model is fitted to experimental data  as follows:
where k is the consistency coefficient (Pa sn), and n is the flow behavior index that is determined by linear regression analysis.
The steady shear viscosity of mucilage solution at low shear rates shows Newtonian behavior over a broad range of shear rates. This fact can be demonstrated by the modified cross model (Williamson model)  as follows:
where η is the steady shear apparent viscosity (Pa.s), λ is a structural parameter (relaxation time), p is an exponent due to shear‐thinning behavior, and η0 is the Newtonian viscosity at the low shear rate. At very low concentrations, η0, known as the “intrinsic viscosity,” shows the hydrodynamic volume of the polysaccharide coil. This index gives useful information about the conformation of polysaccharide molecules in solution. In most engineering applications, shear rates of operations are in the shear‐thinning region. Eq. 17.2 is generally used to explain polysaccharide macromolecular solutions in random coil configuration (non‐gelling polysaccharides) such as galactomannans, dextran, l‐carrageenan, and cellulose derivatives .
In a study , by increasing the OCM concentration, the apparent viscosity also increases by 10% at shear rate = 100, pH 4.8, and I = 0:1025 M, which is similar to the behavior of a xanthan solution at 3%. It illustrates that steady shear viscous flow properties of economically useful biopolymers such as xanthan gum and carboxymethylcellulose (CMC) are similar to those of OCM. This result underscores the economic and technical importance of this novel source of mucilage.
The apparent viscosity of the mucilage from OCM slightly depends on the temperature. For temperatures between 5 and 70 °C and the concentration range 3%–10%, increasing the temperature decreases the viscosity. This behavior can be explained with an Arrhenius‐type equation  as follows:
where E a is the energy of activation, R represents the universal gas constant, A is a fitting constant, and T is the absolute temperature. A high Ea value shows the high sensitivity of viscosity to temperature change, which means the viscosity varies greatly when the temperature increases or decreases. According to the data, at shear rate 100 s−1, pH 4.8, and ionic strength (I) 0.1025 M, the 10% OCM solution has the highest E a compared to the solution with 3% mucilage, which has the least E a. However, several biopolymers like xanthan have stable viscous properties at various temperatures .
The rheological behavior of the mucilage extracted from different parts of the Opuntia ficus‐indica plant has been investigated by several studies [ 51, 64, 66, 67]. The foods containing polyelectrolytes are commonly processed under different conditions such as mineralized solutions, and thus it is essential to elucidate the influence of salt addition on the rheological properties of the gums. Furthermore, evaluating the influence of cations on the flow behavior of gums is necessary to predict their rheological and functional properties [ 67,80]. As mentioned above, the gum obtained from different parts of the Opuntia ficus‐indica plant contains different amounts and types of uronic acids, which makes it dependent on salinity. Majdoub et al.  investigated the effect of divalent (Ca2+) and monovalent (Li+) cations on the flow behavior of the high weight fraction of the gum extracted from nopals of the Opuntia ficus‐indica plant (NOF). According to this study, the addition of cations to the gum decreases the viscosity. In pure water, a negative charge increases the electrostatic repulsion and thus produces a more expanded molecule. The mucilages extracted from the Opuntia ficus‐indica plant are negatively charged macromolecules and therefore produce a high degree of viscosity in deionized water, due to large repulsive forces. The effect of Ca2+ on the viscosity of NOF gum is more marked than that observed for Li+, which may be due to the interaction between carboxylic acid groups in the structure of the gum and calcium ions .
In several studies, it has been reported that complex formation between carboxylic acid groups and Ca2+ considerably increases the gum solution viscosity or establishment of gel in the presence of sufficient interactions. Lefsih et al.  reported that when Ca2+ is increased from 40 to 80 mM, the viscosity of OCM improves, but when the Ca2+ concentration is further increased, its viscosity decreases. Despite the increased viscosity, no gel formation occurs in the presence of Ca2+, which is due to the high degree of branching of this pectin. Lefsih et al.  investigated the influence of saponification on the rheological properties of OCM. They found that saponification can change the physicochemical properties of the gum and improve its solubility, and as a result reinforces its viscosity and gelling capability . The authors also indicated that the viscosity of mixing OCM (82%)/carrageenan (18%) solution increased when the pH increased. This trend is due to an increase in the dissociation rate of sulfate of carrageenan and carboxyl groups of pectin which interact with divalent cations. This mixture also is sensitive to temperature. An increase in the mixture temperature decreases the viscosity, which is related to the depolymerization of pectin with increasing temperature.
pH is another effective parameter of the rheological properties of the mucilage from OCM. At shear rate 100 s−1, pH 4.8, and I = 0.1025 M, increasing pH increases the viscosity. In the alkaline zone, viscosity values tend to remain constant. This can be caused by the ionization of the carboxyl groups in mucilage above pH 7.0. The pH affects the hydrodynamic and the mucilage flow properties. These changes are as a result of the conformational changes in the molecule structure.
The effect of various concentrations of CaCl2 on the dynamic rheological behavior of OCM was investigated by Lira‐Ortiz et al. . In OCM, both the dynamic storage modulus (G′), and the viscous modulus (G″) depend on the frequency. In the range of frequency tested (0.1–100 rad s−1), OCM solutions showed gel‐like behavior, with the storage modulus higher than the loss modulus and a slight frequency dependence. They also reported that when the Ca2+ concentration was increased, the values of G′ and G″ increased, indicating a more pronounced solid‐like structure in the presence of Ca2+.
Other findings demonstrated that at low concentration of OCM solutions (<3%), G″ is always higher than G′. This indicates that at this concentration, OCM usually shows viscous properties instead of gel formation. Otherwise, at high mucilage concentrations (>3%), G′ exceeds G″, which indicates the inclination of OCM to form macromolecular networks with important elastic properties. This kind of behavior has been seen in xanthan gum at concentrations greater than 1% . Xanthan solutions (1% and 2%) are compared with Opuntia cladode mucilage solution at 10% . The elastic and viscous responses of mucilage at different concentrations are also dependent on temperature, pH, and ionic strength. The effect of temperature is analyzed on the viscoelastic properties for 5% (w/w) OCM solution .
One of the most important functional properties of hydrocolloids is the control of emulsion shelf life . A convenient way to analyze the relative effectiveness of the emulsification efficiency of the gums is to determine the emulsifier concentration needed to produce an emulsifying system with the minimum mean droplet size . The surface–volume mean diameter versus concentration plot of OFM is shown in Figure 17.2. It can be seen that OFM can serve as an emulsifier and stabilize an oil‐in‐water emulsion. The good stabilizing property of polysaccharides is attributed to their high molecular weight and gelation ability .
There is little information and data on the dynamic rheological properties of the mucilage extracted from different parts of Opuntia ficus‐indica [ 64, 67,84]; however, there is a comprehensive research article on the evaluation of the oscillatory behavior of the mucilage obtained from another species of Opuntia (Opuntia albicarpa Scheinvar) . Typical mechanical spectra related to the mucilage of Opuntia ficus‐indica at various temperatures and gum concentrations are presented in Figure 17.3 . As seen in Figure 17.2a, in the frequency range tested, both the storage modulus G′ and the viscous modulus G″ exhibit a slight frequency dependence, which is typical behavior for random coil polysaccharide solutions . It is evident that the solution concentration has a considerable influence on this behavior. At low gum concentrations (3%), the value of G″ always exceeds G′, revealing typical viscous behavior. However, when the mucilage concentration is high, G′ exceeds G″, reflecting typical weak gel‐like behavior.
The frequency dependence of G′ and G″ at various temperatures is shown in Figure 17.2b. When the solution temperature increased, the storage and loss moduli declined, showing weaker structures at higher temperature. It can be seen that there is a crossover point between G′ and G″ at a temperature of 35 °C, demonstrating a conformational change in the mucilage. The authors also reported that the elastic and viscous moduli of mucilage solutions are mainly dependent on both pH and ionic strength.
Majdoub et al.  investigated the steady shear and dynamic rheological behavior of two fractions obtained from Opuntia ficus‐indica mucilage at a concentration of 30 g L−1. They reported that both fractions exhibited remarkable shear‐thinning behavior. It also was found that Carreau's model can be used as an appropriate model to describe the steady shear behavior of mucilage solutions. The Cox–Merz rule states that there is an empirical relationship between the apparent viscosity (ηa) and the dynamic viscosity (η*) . According to this rule, when strong interactions are absent in polymer solutions, the apparent and dynamic viscosity curve should be identical. Majdoub et al.  indicated that in the shear range 0.001 to 1000 s−1, there is a perfect superposition between the curves of steady flow and the complex viscosity, demonstrating that Opuntia ficus‐indica mucilage is a biopolymer with random coil conformation and irregular chains.
The mucilage from Opuntia ficus‐indica cladode (OCM) can be used as an edible coating to extend the shelf life of fruits and vegetables. For example, strawberry fruits were coated with pure mucilage extract and mucilage extract with 5% w/w glycerol as a plasticizer . During storage, the texture of the fruits is likely to soften owing to several factors involving a loss in cell turgidity pressure, loss of extracellular and vascular air, and the degradation of the cell wall and consequent loss of water by cell breakdown . This coating can affect the firmness of strawberries during storage at 5 ± 0.5 °C for nine days. Mucilage has a hydrophilic disposition which can hinder water transfer, thus retarding dehydration and therefore extending the firmness of the coated fruit .
Kiwifruit slices were coated with OCM, Tween 20, and glycerol, and the samples were stored at 5 °C for 12 days. Results showed that the firmness of both coated and uncoated samples decreased after storage, but the reduction in coated kiwi slices was significantly less than in the uncoated (P ≤ 0.05) ones, while no differences were observed between mucilage and Tween 20 and glycerol‐coated slices, and after storage, no significant difference was observed between coated and uncoated fruits .
There are many other reports on the beneficial effects of coating with Opuntia ficus‐indica cladode mucilage on different fruits and vegetables such as Dottato fig fruit (shelf life and carotenoid content), red delicious apple (shelf life), banana (color preservation during drying), and eggplant (preservative effect on anthocyanins during spray drying) [ 86,87].
There are very few studies on the filming properties of OCM. In a study, mucilage film‐forming dispersions were prepared under different pHs (3, 4, 5.6, 7, and 8) and calcium concentrations (0% and 30% of CaCl2, relative to mucilage's weight). The results showed that the addition of calcium increased the water vapor permeability of the films. Calcium and pH affected the mechanical properties of the films; the largest tensile strength (TS) was observed without calcium, whereas the highest elongation (%E) was observed with calcium. The highest differences among films were observed at pHs 5.6 and 7 for TS and at pHs 4 and 8 for %E. No effect of pH and calcium was observed on transparency and color parameters . In another work, OCM was incorporated into gelatin and beeswax, and its filming properties were evaluated . In a recent study, the mucilage from OCM was used to prepare biodegradable films containing 40% (w/w) of glycerol, sorbitol, polyethylene glycol (PEG) 200 or PEG 400. The physical, thermal, mechanical, and barrier properties of the obtained films were investigated . The results showed that the minimum water vapor permeability was observed in the films containing sorbitol. From a mechanical point of view, the TS of OCM‐based film comprising PEG 200 and sorbitol was about two times higher than the films plasticized by glycerol. The author suggested that the positive effect of plasticizers used on the physicochemical properties of OCM films may be due to their structural properties that promote various interactions with the polysaccharides.
In some studies, the mucilage obtained from Opuntia ficus‐indica was used as an encapsulation agent to preserve bioactive compounds such as betalains and anthocyanins [90,91]. Otálora et al.  encapsulated betalains extracted from purple OCM by OFM, OFM, and maltodextrin with the spray‐drying technique to stabilize these pigments. After the microcapsules were manufactured, their structural and thermal properties were investigated by scanning electron microscopy (SEM), thermal analysis (TGA–DSC), and tristimulus colorimetry. Moreover, the storage stability of the tested pigments was analyzed at 18 °C and various relative humidity levels. The results demonstrated that the incorporation of OCM in the formulation enhanced the encapsulation efficiency, decreased the moisture content, and produced spherical microparticles with greater uniformity of size. The authors stated that the developed microencapsulates can be introduced as a promising functional additive (natural colorant) in food systems. In another study, Gutiérrez et al.  attempted to protect anthocyanins from Solanum melongena L with Opuntia ficus‐indica mucilage. The spray‐drying technique was used for encapsulation of the pigments. The protective effects of the mucilage were evaluated using the ratio of the percent of radical scavenger activity (RSA) to the anthocyanin concentration [C]. It was found that at the end of the storage period (four months) that the %RSA/C was preserved in 73% of wet samples and in 64% of spray‐dried samples. On the other hand, the tested parameter was 30% for the wet samples without mucilage. Furthermore, they reported that the acidified mucilage (0.4%) led to modification of the rheological behavior of mucilage from non‐Newtonian to Newtonian behavior, enhancement of the spray‐drying process, and preservation of the %RSA/[C A] in 96% of samples after drying.
Opuntia ficus‐indica cladode mucilage was used as a flocculation agent to remove bacterial contamination . It was also used to spin nano‐ or microfibers to fabricate membranes used for removing different heavy metals (such as copper, arsenic, and chromium) from wastewaters [92–94]. In a recent work, Adjeroud et al.  investigated the effect of applying mucilage of Opuntia ficus‐indica on copper removal efficiency by the electrocoagulation‐electroflotation (EC‐EF) process. Surprisingly, the mucilage improved the EC‐EF performance. The authors also found that at the predetermined operating conditions, the mucilage concentration had a considerable influence on the efficiency of copper removal. In another study, Asha et al.  used mucilage of Opuntia ficus‐indica as a natural coagulant. Several parameters including mucilage concentration, time, temperature, and agitation speed were optimized to evaluate the efficiency of the mucilage in the removal of chromium. The results indicated that Opuntia ficus‐indica mucilage functioned well as a coagulant to decrease the concentration of chromium in synthetic wastewater.
Another finding showed that the mucilage extracted from Opuntia ficus‐indica can be employed as eco‐friendly adsorbents to remove arsenic from water. For this purpose, first the formulation of the adsorbent biobeads was optimized, and then its ability to remove arsenic was tested. The results revealed that the biocomposite formulated with 1.25 mg L−1 of gelling mucilage, 4% of sodium alginate, and 0.75 mol L−1 of calcium chloride could remove up to 63% of arsenic from water .
Lamghari El Kossori et al.  compared the influences of Opuntia ficus‐indica, gum arabic, citrus pectin carrageenan, alginic acid, and locust bean gum on the in vitro digestibility and viscosity of casein. The results of their study demonstrated that the decrease in digestibility of the casein‐based diet in the presence of Opuntia ficus‐indica mucilage is more pronounced than with gum arabic, citrus pectin carrageenan, or alginic acid. The effect has been associated with the interaction of the fibers with the enzymes or casein .
Opuntia ficus‐indica has been used for centuries as a food resource and in traditional folk medicine for its nutritional properties and healing benefits, particularly in diabetes, obesity, cardiovascular diseases, and cancer. It is widely distributed in America, Africa, and the Mediterranean basin. It has great economic potential because it can grow in arid and semi‐arid areas and has beneficial properties due to its high content of antioxidants (flavonoids, ascorbate), pigments (carotenoids, betalains), and phenolic acids. Other phytochemical components (biopeptides and soluble fibers) have been characterized and contribute to the medicinal properties of Opuntia spp. Mucilage obtained from different species of cactus is an interesting functional biopolymer widely applied in the food, cosmetic, and pharmaceutical industries. Different parts of Opuntia ficus‐indica have a considerable amount of mucilage which is easily extractable. Because of its viscoelastic characteristic and its ability to form a molecular network, Opuntia ficus‐indica mucilage could find applications in food packaging as edible films or coatings. A future challenge is the need to evaluate the influence of different purification techniques, drying, and other processing conditions on the chemical composition, rheological behavior as well as the structural and functional properties of the mucilage extracted from different parts of Opuntia ficus‐indica.