Cactus Pear Mucilage (Opuntia spp.) as a Novel Functional Biopolymer: Mucilage Extraction, Rheology and Biofilm Development.

The investigation of novel, natural polymers has gained considerably more exposure for their desirable, often specific, functional properties. Multiple researchers have explored these biopolymers to determine their potential to address many food processing, packaging and environmental concerns. Mucilage from the cactus pear (Opuntia ficus-indica) is one such biopolymer that has been identified as possessing a functional potential that can be used in an attempt to enhance food properties and reduce the usage of non-biodegradable, petroleum-based packaging in the food industry. However, variations in the structural composition of mucilage and the different extraction methods that have been reported by researchers have considerably impacted mucilage's functional potential. Although not comparable, these factors have been investigated, with a specific focus on mucilage applications. The natural ability of mucilage to bind water, alter the rheology of a food system and develop biofilms are considered the major applications of mucilage's functional properties. Due to the variations that have been reported in mucilage's chemical composition, specifically concerning the proportions of uronic acids, mucilage's rheological and biofilm properties are influenced differently by changes in pH and a cross-linker. Exploring the factors influencing mucilage's chemical composition, while co-currently discussing mucilage functional applications, will prove valuable when evaluating mucilage's potential to be considered for future commercial applications. This review article, therefore, discusses and highlights the key factors responsible for mucilage's specific functional potential, while exploring important potential food processing and packaging applications.

Effect of pH on the Mechanical Properties of Single-Biopolymer Mucilage (Opuntia ficus-indica), Pectin and Alginate Films: Development and Mechanical Characterisation.

Pectin and alginate are well-established biopolymers used in natural film development. Single-polymer mucilage films were developed from freeze-dried native mucilage powder of two cultivars, 'Algerian' and 'Morado', and the films' mechanical properties were compared to single-polymer pectin and alginate films developed from commercially available pectin and alginate powders. The casting method prepared films forming solutions at 2.5%, 5%, and 7.5% (w/w) for each polymer. Considerable variations were observed in the films' strength and elasticity between the various films at different polymer concentrations. Although mucilage films could be produced at 5% (w/w), both cultivars could not produce films with a tensile strength (TS) greater than 1 MPa. Mucilage films, however, displayed > 20% elongation at break (%E) values, being noticeably more elastic than the pectin and alginate films. The mechanical properties of the various films were further modified by varying the pH of the film-forming solution. The various films showed increased TS and puncture force (PF) values, although these increases were more noticeable for pectin and alginate than mucilage films. Although single-polymer mucilage films exhibit the potential to be used in developing natural packaging, pectin and alginate films possess more suitable mechanical attributes.

Microstructural and Mechanical Properties of Calcium-Treated Cactus Pear Mucilage (Opuntia spp.), Pectin and Alginate Single-Biopolymer Films.

Pectin and alginate satisfy multiple functional requirements in the food industry, especially relating to natural packaging formulation. The continuous need for economic and environmental benefits has promoted sourcing and investigating alternative biomaterials, such as cactus pear mucilage from the cladodes of Opuntia spp., as natural packaging alternatives. The structural and mechanical properties of mucilage, pectin and alginate films developed at a 5% (w/w) concentration were modified by treating the films with calcium (Ca) in the calcium chloride (CaCl2) form. Scanning electron microscopy (SEM) showed the 5% (w/w) 'Algerian' and 'Morado' films to display considerable microstructure variation compared to the 5% (w/w) pectin and alginate films, with calcium treatment of the films influencing homogeneity and film orientation. Treating the alginate films with a 10% (w/w) stock CaCl2 solution significantly increased (p < 0.05) the alginate films' tensile strength (TS) and puncture force (PF) values. Consequently, the alginate films reported significantly higher (p < 0.05) film strength (TS and PF) than the pectin + Ca and mucilage + Ca films. The mucilage film's elasticity was negatively influenced by CaCl2, while the pectin and alginate films' elasticity was positively influenced by calcium treatment. These results suggest that the overall decreased calcium sensitivity and poor mechanical strength displayed by 'the Algerian' and 'Morado' films would not make them viable replacements for the commercial pectin and alginate films unless alternative applications were found.

Bioactive Betalain Extracts from Cactus Pear Fruit Pulp, Beetroot Tubers, and Amaranth Leaves.

Natural food items and the additional benefits they provide have received considerable attention in recent years. Betalains are nutritious pigments which have valuable biological properties, e.g., antioxidant and free radical scavenging activity. In this study, aqueous betalain extracts were obtained from different coloured cactus pears (purple, red/pink, and orange), amaranth, and beetroot, with and without the addition of ascorbic acid, microwave-heated, and freeze-dried and subsequently analysed by thin layer chromatography (TLC). Beetroot samples without the addition of ascorbic acid (AA) had lower phenols, flavonoids, and ascorbic acid content than beetroot samples extracted with the addition of AA. Amaranth had significantly higher contents of antioxidants than all the other plants. Results for phenolic compounds showed that there were no significant differences between cactus pear cultivars, however, significant differences were seen between the two beetroot samples (microwave-heated with and without AA) as well as amaranth. For flavonoid compounds, amaranth had significantly higher values than all other samples. The lowest flavonoid content was found in beetroot without AA (0.49 mgCE/g). For ascorbic acid, significant differences were noticed between amaranth (71.71 mg/100 g) and samples from cactus pear and beetroot. TLC results showed that purple and red cactus pear samples had the most vivid colours, a reflection of the high betacyanin and betaxanthin contents in the cultivars. Moreover, extracts from cactus pear, beetroot, and amaranth were classified according to a decision tree which was designed by the Code of Federal Regulations/Food Additives Regulation of the EU. The classification of betalain pigment extracts as colouring foods was achieved through enrichment factor calculations and the colourant decision tree. The results showed that the betalain pigment extraction method used is inexpensive, time-saving, energy-saving, non-toxic, and chemical solvent free and yields high concentrations of betalains.

Relationship between weather conditions and the physicochemical characteristics of cladodes and mucilage from two cactus pear species.

Climate change, limited water resources and expected population increases would require crops which contribute toward more resilient, more productive, more sustainable and climate-smart food systems. The cactus pear is a drought-resistant and sustainable food source to humans and livestock alike. Cactus mucilage has multiple applications in the food and packaging industry. It is eco-friendly, economical, functional and has multiple health benefits. However, the researchers observed umpteen variations in extracted mucilage yield and viscosity every time the cladodes were harvested, making the standardisation of formulations troublesome. We aimed to examine the effect of weather conditions on the physicochemical characteristics of cactus pear cladodes and mucilage extracted over two seasons to understand these observed variations in mucilage characteristics. Forty cladodes, ten from each of Opuntia ficus-indica Algerian, Morado and Gymno-Carpo and Opuntia robusta Robusta were harvested every month from February to August in Bloemfontein, South Africa. Daily weather data were obtained, weight and moisture contents determined on cladodes and yield, viscosity, pH, conductivity and malic acid content determined on extracted mucilage. Pearson correlation coefficients were calculated between the weather conditions, cladode properties, and mucilage properties. Contrary to common belief, neither increasing cladode weight as they grow, nor rainfall were the leading causes of mucilage inconsistencies. However, the correlations showed a relationship between environmental temperatures, cladode pH and conductivity, and mucilage viscosity and yields. In hot summer weather, the pH was lower, which led to an abundance of positive ions in cladodes. The H+ ions neutralise the negative charges along the outstretched mucilage molecule, causing it to coil up, reducing the viscosity of the mucilage. Thus, environmental temperatures rather than rainfall or cladode maturity influenced the physicochemical characteristics of mucilage. The findings should make an essential contribution in predicting the physicochemical characteristics of mucilage for specific food-related functions by observing the weather conditions.

Mucilage powder from cactus pears as functional ingredient: influence of cultivar and harvest month on the physicochemical and technological properties.

Cactus pear mucilage is a novel, inexpensive hydrocolloid that is used to add nutrients, body and texture to functional food products. However, varied physicochemical and technological properties of powders when cultivar and harvest month of cladodes differ could lead to differentiation in the application of mucilage powders. To that end, three Opuntia ficus-indica (Algerian, Morado and Gymno-Carpo) cultivars and one Opuntia robusta (Robusta) cultivar were harvested over a 6 month period and evaluated in this study. February mucilage powders were the most porous with highest oil absorption and oil holding capacity, lowest water holding and swelling capacity, and lowest ability to increase viscosity. August mucilage powders had the smallest impermeable particles, highest water holding and hydrophobic properties, as well as the best emulsifying capacity, stability and ability to increase viscosity. Opuntia robusta produced brighter, darker green, more viscous mucilage while Opuntia ficus-indica powders were dull, light yellow-green with a lower viscosity and emulsifying capacity. Overall, the mucilage powders were easy to pour and free-flowing yet were microbiologically safe due to low water activity and pH levels. Robusta mucilage was successfully applied in mayonnaise products to replace up to 50% egg yolk and 30% oil. It was concluded that mucilage powders might contribute to the textural and nutritional quality of food products. The acceptance of mucilage powder as an active functional and nutraceutical food ingredient will also lead to the development of cactus as a commercially viable crop in arid and semi-arid areas where few other crops can survive.

Cultivar and Harvest Month Influence the Nutrient Content of Opuntia spp. Cactus Pear Cladode Mucilage Extracts.

Mucilage extracted from cactus pear cladodes is a hydrocolloid gum. It is a novel, natural, low-kilojoule, cost-effective texture-modifying ingredient in functional food products. Yet, the cultivar with the most optimal nutrient content and the preferred harvest times are as yet unknown. For this reason, mucilage from three Opuntia ficus-indica (Algerian, Morado and Gymno-Carpo) and one Opuntia robusta (Robusta) cultivar were investigated to determine their nutrient content over six months. Nutrients that contribute energy (10.2 kJ/g) were low. The mineral content was high (ash 17.7/100 g), particularly calcium (3.0 g/100 g) and phosphorous (109.5 mg/kg). Low insoluble acid-detergent fibre (1.4 g/kg) and neutral-detergent fibre (2.1 g/kg) values indicated that mucilage was mostly soluble fibre. Calcium oxalate crystals were not detected in dried mucilage. Opuntia robusta powders had higher protein, extractable fat and potassium content, while Opuntia ficus-indica mucilage powders had higher polyunsaturated (Linoleic and α-Linolenic acid) fat content. O. robusta Robusta mucilage, harvested after the fruit harvest (February) had the lowest energy content and the highest mineral and protein content. Mucilage powders were highly soluble, low-kilojoule and mineral-rich. This is a functional ingredient that is produced from an easily cultivated crop, as cactus pears grow in areas with poor soil, extremely high daytime temperatures and limited water supplies.

Acceleration of yoghurt fermentation time by yeast extract and partial characterisation of the active components.

Water soluble autolysate of yeast, usually utilised for microbial growth support, was used as additive in yoghurt fermentation. The yeast extract (YE) resulted in a decrease of fermentation time by 21% to reach a pH of 4·6. However, the YE resulted in unacceptable flavour and taste. By size exclusion chromatography, a fraction of the YE was obtained that could account for the observed 21% decrease in fermentation time. The fraction contained molecules of low molecular weight, consisting of minerals, free amino acids and peptides. The acceleration of the yoghurt fermentation was ascribed to the short peptides in the fraction. It is proposed that the application of this extract in industrial yoghurt manufacture would result in savings for both the industry and the consumer.

The effect of variety and location on cactus pear (Opuntia ficus-indica) fruit quality.

Little is known about the performance of South African cactus pear varieties in different agro-ecological regions. Effects of locality on internal quality parameters of available cactus pear varieties were examined. With only one exception, no significant differences among the mean replication values for the different parameters between the different locations were observed. The differences between mean values for most individual parameters at the three localities were highly significant. Highly significant differences between the mean values for the measured characteristics were observed, not only among the locations (except for the pulp glucose values), but also for the influences of genotype and interaction between locality and genotype. Significant variations existed between mean values of the different characteristics between localities. Genotype x environmental interactions were noted. It was concluded that Meyers is the most appropriate cultivar for economical purposes in South Africa.