LITERATURE REVIEW

2.1 CONCEPTUAL REVIEW

Cashew (Anacardium occidentale L.) trees appear more like bushes than trees because of their small stature (Figure 2.1). They are evergreen trees, with an average height of not more than 30 feet. The limbs of the tree spread fast and strike the ground to re-root themselves. The trunk of the tree is often short and irregular in shape, giving it an extremely dishevelled look. Cashew trees grow abundantly in the tropics and are extensively found near the equator, as they flourish in extreme heat. These trees have been cultivated for food and medicine for more than 400 years. Cashews trees and their products seem to have served nutritional, medicinal and wartime needs. More recently, they have been used in the manufacture of adhesives, resins and natural insecticides. The cashew tree has also become a source of valuable oil drawn from the shell.

The cashew kernel is a rich source of protein (21 %), fat (46 %) and carbohydrates (25%) and a good source of calcium, phosphorus and iron (Table 2.1). It has a high percentage of polyunsaturated fatty acids, in particular, the essential linoleic acid. The tart apple is a source of vitamin C, calcium and iron. The bark, leaves, gum and shell are all used in medicinal applications. The leaves and bark are commonly used to relieve toothache and sore gums, and the boiled water extract of the leaf or bark is used as a mouth wash. A paste of the bark ground in water is used in topical applications for the cure of ringworm; in this form, however, it acts as an irritant and is not applied to sensitive skin or to children. The root has been used as a purgative. Fibres from the leaves are used to strengthen fishing lines and nets, and as folk remedies for calcium deficiency, intestinal colic, and as a vitamin supplement. The water-resistant wood is used for boats and ferries, while the resin, in addition to its having industrial use, is used as an expectorant, for cough and as an insect repellent.

Figure 2.1 Cashew tree

The cashew nut kernel is constituted of three different portions - the shell, the kernel and the adhering testa (Figure 2.2). The primary product of cashew nuts is the kernel, which is the edible portion of the nut and is consumed in three ways: (i) directly by the consumer, (ii) as roasted and salted nuts and (iii) in confectionery and bakery products, for example, finely chopped kernels are used in the production of sweets, ice creams, cakes and chocolates, both at home and industrially, and as a paste to spread on bread.

Table 2.1 Composition of cashew kernel

Total nutrients

Amino acid composition

Fatty acid composition

Contents

Values

Amino acid

%

Contents

%

Moisture

5.9

Glutamic Acid

28.0

Oleic Acid

73.3

Total Minerals

2.4

Leucine

11.93

Linoleic Acid

7.67

Total Fiber

1.3

Iso Leucine

3.86

Palmitic Acid

0.89

Energy

785

Alanine

3.18

Stearic Acid

11.24

Protein

24

Phenylalanine

4.35

Lignoseric Acid

0.15

Total Fat

64

Tyrosine

3.20

Unsaponifiable

Matter

0.42

Saturated

12.9

Arginine

10.30

Carbohydrate composition

Unsaturated

(Oleic)

36.8

Glycine

5.33

Contents

%

Unsaturated

(Linoleic)

10.2

Histidine

1.81

Reducing sugars

1-3

Carbohydrate

41

Lysine

3.32

Non-reducing

sugars

2.4-8.7

Ca

53

Methionine

1.30

Starch

4.6-11.2

P

52.2

Cystine

1.02

Oil

34.5-46.8

Fe

5.3

Threonine

2.78

Vitamin contents

Thiamin

0.63

Valine

4.53

Contents

Values

Riboflavin

0.19

Tryptophane

1.37

Thiamin

0.56

Niacin

2.5

Aspartic Acid

10.78

Niacin

3.68

Beta-carotene

60

Proline

3.72

Riboflavin

0.58

Serine

5.76

Tocopherol

210

Pyridoxine

traces

Axerophtol

traces

Vitamin D

traces

Figure 2.2 Cross sectional view of cashew fruit

The relative importance of the above mentioned uses varies from year to year and country to country; however, it is estimated that at least 60 percent of cashew kernels are consumed as salted nuts. Separately packed cashew nuts are a good selling line, mainly as an appetiser to cocktail drinks. Salted cashews are part of the snack food market. They compete mainly with other nuts, although chips, salted popcorn and other savoury snacks can impinge on the nut market. The price of cashew nuts is much higher than that of peanuts or other snacks, so that the sale of cashew nuts is based on a strong taste preference by the consumer. Cashew nuts are generally considered a luxury product, and an element of their appeal may lie in this status.

Countries like India, Brazil, Vietnam and Nigeria boast of more than 90% of the world’s cashew crop collectively. As far as India is concerned, the coastal states of India are the main cashew producers. It is grown in Kerala, Karnataka, Goa and Maharashtra along the west coast and Tamil Nadu, Andhra Pradesh, Orissa and West Bengal along the east coast. In cashew-producing countries, the nut is only one of the products enjoyed by the local populations. It can be dried, canned as a preserve, or eaten fresh from the tree. The fruit can also be squeezed for fresh juice, which is then fermented into cashew wine, which is a very popular drink in West Africa. In parts of India, the cashew apple is used to distil cashew liquor referred to as feni. In some parts of South America, local inhabitants regard the apple, rather than the nut kernel, as a delicacy. In Brazil, the apple is used to manufacture jams, and soft and alcoholic drinks. The cashew tree bears a false fruit known as the cashew apple from which the nut protrudes. The cashew apple is between three and five inches long and has a smooth, shiny skin that turns from green to bright red, orange or yellow in colour, as it matures. It has a pulpy, juicy structure, with a pleasant but strong astringent flavour.

The fruits are picked from the tree by hand to avoid bruising of the delicate flesh. They are then carefully washed and the nuts are removed for processing. Cashew apples are processed within two to three hours of picking, as they undergo rapid deterioration, when kept for a longer time. Currently only six percent of cashew apple production is exploited, as the producers have a guaranteed market only for cashew nuts. It is also extremely difficult to use the whole fruit commercially as the apple ripens prior to the nut and the quality of nuts detached from the green fruit, is unacceptable for commercialization. The development of processing options for the cashew apple, has also been limited by its high degree of perishability, and consequent difficulties in transportation from the growing areas to distant processing plants.

The cashew apple is very rich in vitamin C (262 mg/100 ml of juice) and contains five times more vitamin C than an orange. A glass of cashew apple juice meets an adult individual's daily vitamin C (30 mg) requirement. The cashew apple is also rich in sugars, and contains considerable amounts of tannins and minerals, mainly calcium, iron and phosphorus. Furthermore, the fruit has medicinal properties. It is used for curing scurvy and diarrhoea, and is effective in preventing cholera. It is also employed for the treatment of neurological pain and rheumatism. Until recently, the potential of the cashew apple had not been investigated due to its highly astringent and acrid taste, which is believed to originate in the waxy layer of the skin, and which causes tongue and throat irritation on consumption. Cashew apples can be made suitable for consumption by removing the undesirable tannins and processing them into value-added products, such as juices, syrups, canned fruits, pickles, jams, chutneys, candy and toffee. The recommended methods for removing the astringent properties of the cashew apple include steaming the fruit for five minutes before washing it in cold water, boiling the fruit in salt water for five minutes, or adding a gelatin solution to the expressed juice.

The cashew nut shell contains a viscous and dark liquid, known as cashew nut shell liquid (CNSL), which is extremely caustic. It is contained in the thin honeycomb structure between the soft outer skin of the nut and the harder inner shell. The CNSL content of the raw nut varies from 20 to 25 percent. CNSL is an important and versatile industrial raw material. There are more than 200 patents for its industrial application; in particular, it is used as a raw material for phenolic resins and friction powder in the automotive industry (brake linings and clutch disks). In drum-brake lining compounds, the cashew resins are used as fillers and binders. The advantage of cashew resins compared to synthetic phenolic resins is that, they are more economical and produce a softer material, giving a quieter braking action.

CNSL is also used in mouldings, acid-resistant paints, foundry resins, varnishes, enamels and black lacquers for decorating vases, and as insecticides and fungicides. In tropical medicine, CNSL has been used in treating leprosy, elephantiasis, psoriasis, ringworm, warts and corns. Like cashew nuts, CNSL also has an excellent international market. After extracting the CNSL, the cashew nut shells (Figure 2.3) can be burned to provide heat for the decorticating operation or can be used in the manufacture of agglomerates. Together with the testa, it may be used either in the manufacture of dyestuff or to provide durability to hammocks and fishing lines.

Figure 2.3 Cashew nut shells

Patel et al (2006) investigated the extraction of CNSL using supercritical carbon dioxide (SC-CO2). The effects of process parameters such as extraction pressure, temperature and flow rate of SC-CO2 have been investigated, and it was reported that the yield of CNSL increased with an increase in the pressure, temperature and mass flow rate of SC-CO2. It has also been reported that the oil obtained through super critical fluid extraction possesses a better quality, compared to the CNSL obtained through the thermal method.

CNSL separation from the fragmented honeycombed cashew shell material without employing thermal techniques, but with a pressure profile method using SC-CO2 as a solvent has been studied (Setianto et al 2009). An analysis with liquid chromatography of the extract obtained by this method, showed that it contained approximately 50 % anacardic acids, 29 % cardols, and 21 % cardanols including mono-, di-, and tri-ene constituents.

CNSL was evaluated for its potential use as an antibiotic in ruminants, by Watanabe et al (2010). It was found that the phenolic compounds present in the CNSL were responsible for its antibacterial activity. Hence it was concluded by the workers, that raw CNSL, rich in antibacterial compound, anacardic acid, can serve as a potential candidate feed additive with selective activity against rumen microbes.

Cheriyan and Abraham (2010) investigated the utility of enzymes, oxidoreductases and proteases for the bioremediation of CNSL, which contain phenolics, mainly cardanol (60–65%). It was observed that peroxidase reduces the colour of the CNSL solution by polymerization and precipitation, whereas laccase, papain, and fungal and bacterial protease degraded the phenolic constituents.

Andrade et al (2011) investigated the antioxidant activity of the technical CNSL using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging assay and the xanthine oxidase assay, as well as in vivo evaluation by the Saccharomyces cerevisiae assay. It has been concluded that technical CNSL exerts an important protective effect against oxidative stress in yeast when used in the 100–500 µg/ml concentration range. Properties of CNS oil have been found to be amazingly equivalent to those of petroleum fuels with a calorific value as high as 40 MJkg−1, the oil has a low ash content (0.01%) and the water content is limited to 3–3.5 wt% of oil (Das and Ganesh 2003).

Singh et al (2006) studied the fuel properties of cashew nut shell and its feasibility of gasification for open core down draft gasifier. Cashew nut shell was converted to a producer gas in an open core down draft gasifier, and its performance was evaluated in terms of the fuel consumption rate, calorific value and gasification efficiency, at different gas flow rates. It was also reported that the producer gas calorific value and volumetric percentage of its combustible constituents, along with the gasification efficiency increases with the increase in the gas flow rate. Studies reveal that cashew nut shells could be used as feedstock for the open core down draft gasifier.

A study of coconut and cashew nut shells, two typical biomass wastes has been made (Tsamba et al 2006). Both biomasses were thermally degraded through thermogravimetry, and their characteristics, such as the devolatilisation profiles and kinetics were analyzed, from 250 to 900 °C, in an inert atmosphere, at two different heating rates, and compared with wood pellet.

Tippayawong et al (2010) investigated cashew nut shells as potential replacement for fuel wood towards thermal applications in factories. They reported that the direct combustion of cashew nut shells was troublesome, due to its low efficiency and high smoke emission, and suggested an alternative thermal conversion of cashew nut shell. It is seen from the results, that cashew nut shells can be successfully used as feedstock for a gasifier.

The different unit operations involved in a small scale cashew nut processing mill in the Konkan region of Maharashtra, India (70°17' to 74°31'E Longitude 15°37' to 20°20'N Latitude) was elucidated by conducting the randomized sample survey of 122 registered small scale cashew processing mills in the region by Mohod et al (2010). The study revealed that, the small scale cashew nut mills in the Konkan region of Maharashtra (India) widely followed the steam roasting process due to better control over the process, which retains the valuable CNSL in the shell, as well as generates a huge quantity of shells for further utilization as a fuel.

Zirconium impregnated cashew nut shell carbon has been studied, to assess its capacity for the adsorption of fluoride from aqueous solutions by Alagumuthu and Rajan (2010). Field studies were also carried out with the fluoride containing water samples collected from a fluoride-endemic area in order to test the suitability of the sorbent in field conditions.

2.2 CASHEW SHELL NUT LIQUID

Cashew nut shell liquid (CNSL) is a dark brown viscous liquid present inside a soft honeycomb structure of the cashew nutshell and is a very important agricultural byproduct of cashew nut and cashew apple production, produced by the cashew nut tree (Anacardium occidentale). The shell of the nut is approximately 1/8 inch thick. Cashew nut shell liquid is the pericarp fluid of the cashew nut. Natural CNSL is a mixture of phenolic compounds with aliphatic side chains, and these are 70% anacardic acid, 5% cardanol, and 18% cardol [57] (Fig. 3.6). Several methods can extract CNSL: hot oil process, solvent extraction, mechanical extraction, vacuum distillation, or supercritical fluids processes: mainly hot-oil and the local roasting in which the CNSL flows out from the shell. CNSL is typically treated with high temperatures, which decarboxylates anacardic acid, yielding cardanol; additional distillation of CNSL removes cardol leaving cardanol as the primary component in CNSL (Fig. 2.3).

Figure 2.3: Major components of cashew nut shell liquid.

The composition of CNSL can vary based on the method used for extraction, and hence they have different chemical composition which can be classified into two main types: solvent-extracted CNSL (natural CNSL) and technical CNSL (tCNSL) [60]. Natural CNSL is obtained by the utilization of some solvent extraction technique (commonly soxhlet, supercritical carbon dioxide, or subcritical water) in order to obtain its constituents under mild conditions, without promoting any chemical modification. Technical CNSL was employed as a source of phenolic compounds for the synthesis of phenol/formaldehyde polymers. Nowadays, with the advances in the chemistry of these phenolic lipids, tCNSL appears as an economically feasible source of phenolic components. In the industry, CNSL is extracted by an automated process that employs high temperatures in order to open the shell and recover the cashew kernel [60]. The presence of aromatic rings in CSNL is an added advantage which results into thermal stability, making it to be used as a fire retardant [61]. The different constituents of CNSL, as aromatic, phenolic compounds, can react with formaldehyde to create condensation polymers, such as resole and novolac, which make excellent matrix resins for composites and also the unsaturation sites in the side chains of CNSL can also undergo addition polymerization using either free radical initiators or ionic initiators. A hexamethylenetetramine (HMTA) hardener is added to CNSL-formaldehyde resins as a way of improving the cure characteristics [57]. The presence of aromatic and phenolic compounds enables CSNL to cross-link with formaldehyde to form novalac and resole which are usable thermoset resins for biocomposites.

Novalac CNSL formaldehyde resins have long hydrocarbon chains; therefore the condensates are more flexible than the conventional phenolic resins. These resins can be used for surface coatings with or without oil modification when high chemical resistance is desired.

Natural fibers have been used to reinforce CNSL-based thermosets. Research elsewhere has utilized oil palm fibers, sisal fibers, kenaf and hemp bast fibers, jute fibers, and coconut coir fibers. Table 2.2 shows a comparison of the mechanical properties of selected CNSL-based composites.