Pre-shelling parameters and conditions that influence the whole kernel out-turn of steam-boiled cashew nuts
Abstract
This work investigates the effect of moisture content (MC), nut size distribution and steam exposure time (SET) on the whole kernel out turn (WKO) of cashew nuts during shelling using a 3 x 5 x 4 factorial experiment. Three nut sizes: small (18–22 mm), medium (23–25 mm) and large (26–35 mm); five levels of MC: 8.34%, 11.80%, 12.57%, 15.40%, 16.84% (wet basis) and four levels of steam exposure time (SET): 28, 30, 32, and 34 min were considered. Nuts were conditioned with warm water to the desired moisture content of 8.34%,11.80%, 12.57%, 15.40% and 16.84% (wb); and steam-boiled at 700 kPa for 28, 30,32, and 34 min. The pre-treated nuts were shelled using a hand-operated cashew nuts shelling machine. The results showed that the single effect of MC, steam exposure time (SET) or nut size distribution is not enough for estimating WKO; it is rather by an interaction of these parameters. The optimum WKO of steam-boiled nuts was 91.74%, 90.94% and 87.98% for large, medium and small sized nuts at MC∗SET combination of 8.34%∗30 min, 11.80%∗32 min and 8.34%∗30 min, respectively. Pre-treatment of cashew nuts by steam boiling was found to improve whole kernel out-turn of the cashew nut. Whole kernel out-turn decreased as MC increased, thereby limiting the need for moisture adjustment when nuts are to be processed by steam boiling.
Keywords
- Whole kernel out-turn;
- Steam exposure time;
- Cashew nuts;
- Moisture content;
- Nut sizes
1. Introduction
Cashew nut is one of the prized edible nuts in the world; others include almond, pistachio, peanut, hazelnut, and walnut. It refers to the ash-green or greyish-brown kidney-shaped seed at the base of cashew apple which weighs between 4–6 g and measures about 24 × 22 × 17 mm in length × width × thickness (Agnoloni and Giuliani, 1977, Oloso and Clarke, 1993, Balasubramanian, 2001 and Ogunsina and Bamgboye, 2007). Fig. 1shows the internal structure of a cashew nut featuring the kernel, the testa and the shell. The kernel is the edible portion which is widely eaten as a snack-food for accompanying drinks at cocktails or an ingredient for confectioneries and bakery products. The testa shields the kernel and separates it from the shell inside the internal cavity where the kernel develops. The shell is a layer of three protective tissues namely: the epicarp, the external integument of the nut; the mesocarp, which contains cashew nut shell liquid and the endocarp, which limits the internal cavity. Cashew kernel is reported to be rich in fat (46%), protein (21.2%) and carbohydrates (22.3%); providing 596 kCal of energy per 100 g of intake. In addition, it contains substantial amounts of essential amino acids, vitamins and minerals which are seldom found in daily diets (The Wealth of India, 1985). The high-oleic-acid in cashew kernel oil, apart from making it an excellent frying oil, makes it useful as fruit polish, vegetable-based lubricant, feedstock for the oleo-chemical industry or in food systems wherein high oxidative stability is prime (Holland et al., 1991, Davis, 1999 and Janick and Paull, 2008). The tannin content of the seed coat (testa) is a useful resource in the leather manufacturing industry (Andrighetti et al., 1994). Resin extracts from cashew nut shell liquid is a valuable material for manufacturing acid-resistant paints, inks, varnishes, insecticides, fungicides, lacquers for decorating vases and friction powder for automobile brake linings and clutch discs (Laurens et al., 1997, Panda and Panda, 1991 and Echendu et al., 1995). The conversion of processed cashew nut shells into alternative fuels by pyrolysis has also been reported (Ogunsina et al., 2009). Cashew nut may therefore be considered to be a crop of high economic value.
- Figure 1.
Schematic drawing of a longitudinally sectioned cashew nut.
Commercial trade in cashew nuts began in the early 1920s when India pioneered its processing and marketing as an industry. Today, cashew nut production and processing is a major source of foreign revenue in India, Vietnam, Brazil, Mozambique, Tanzania and other countries along the sea coast wherein cashew tree has found favourable conditions for growth (Agnoloni and Giuliani, 1977 and Andrighetti et al., 1994). Nigeria ranks among the world’s top producers of cashew nuts with about 660,000 tonnes in 2009 (FAOSTAT, 2009).
The processing of most edible nuts often requires conditioning or subjecting them to one form of thermal treatment or the other to make their shell brittle and amenable to cracking (Kahyaoglu and Kaya, 2006 and Shakerardekani et al., 2011). For cashew nuts, thermal treatment involves roasting the nuts in air or in oil during which the caustic liquid in the mesocarp of the shell gets burnt or dislodged. Afterwards, the nuts are tumbled in wood ash or sawdust to absorb residual liquid film on the shell, cooled and cracked having turned brittle (Andrighetti et al., 1994 and Azam-Ali and Judge, 2001). However, the products of the reaction that occurs between sugars and amino acids during roasting of edible nuts lead to certain desirable sensory attributes such as flavour, colour and taste (Nikzadeh and Sedaghat, 2008) which limit industrial application of cashew nuts. Roasted cashew nuts therefore yield kernels of low export value but it is usually preferred by local consumers. In addition, cashew nut roasting, though requires minimum investment, generates thick acrid fume that pollutes and makes the environment very uncomfortable (ITDG, 2001, ITDG, 2005 and Azam-Ali and Judge, 2001). With the growth of the cashew nut industry and several applications of cashew kernels in food systems, export quality demands that processed nuts should be whole and white or ivory in colour to attract premium price and this is rarely achievable by roasting. An alternative thermal treatment which overcomes these limitations involves steam-boiling (Fig. 2) the dried nuts at about 100 psi for about 25–30 min and cooling it naturally for 12–24 h (Balasubramanian, 2006). This preserves the white or ivory colour and improves whole kernel out turn (WKO); hence a preferred method in industrial processing of cashew nuts for export.
- Figure 2.
Cashew nut processing by steam boiling method.
Several attempts to mechanize cashew nut shelling have recorded little success, low whole kernel out turn being a major problem for the industry. Processing procedures had hitherto been based on the experience of processors rather than scientific understanding of the process (Kahyaoglu, 2008). Researchers in the past found that the behaviour of nuts during processing largely depends on processing parameters such as moisture content, size distribution and the method of thermal treatment among others (Araújo and Ferraz, 2006, Balasubramanian, 2001, Jain and Kumar, 1997 and Oloso and Clarke, 1993). Such previous works on walnuts, macadamia, pistachios, hazelnuts, almonds and cashew nuts focused on the effect of thermal treatment on the chemical properties, colour and texture of the kernel (Kosoko et al., 2009, Hebbar and Ramesh, 2005, Wanlapa and Jindal, 2006, Ozdemir and Devres, 2000, Dogan and Cronin, 2004 and Saklar et al., 1999). For cashew kernels, wholesomeness is a major quality index which is largely determined during shelling; an understanding of the parameters that may affect WKO is therefore important as local and foreign demand for cashew kernels keeps increasing worldwide. This study provides information on the effect of thermal treatment by steam-boiling on WKO of cashew nuts which has not been reported so far. It focuses on the effect of moisture content, nut size and steam exposure time on the WKO of steam-boiled cashew nuts.
2. Materials and methods
2.1. Source of materials and sample preparation
Freshly harvested cashew nuts were obtained from farm gates in Iseyin, Iwo and Oro areas of Oyo, Osun and Kwara states (Nigeria) at moisture content (MC) of 8.34% on the average. Extraneous materials such as leaves, stones, immature and spoilt nuts were removed from the lot. The nuts were graded into three size ranges based on their major axial dimensions: large (26–35 mm), medium (23–25 mm), and small (18–22 mm) by the method of Andrighetti et al. (1994). The samples were kept on raised pallets for good ventilation until the time of use according to common practice in the industry.
2.2. Experimental design
The experiment was a 3 × 5 × 4 factorial experiment considering three nut sizes: large, medium and small; five levels of MC: 8.34%, 11.80%, 12.57%, 15.40%, 16.84% (wet basis) and four levels of steam exposure time (SET) 28, 30, 32, and 34 min.
2.3. Moisture content adjustment
The amount of water required to condition samples to the desired MC was determined from the equation below.
equation1
2.4. Determination of whole kernel out turn
All nuts were shelled using a hand-operated cashew nut shelling machine manned by one trained regular operator (Ogunsina and Bamgboye, 2011, Kosoko et al., 2009, Balasubramanian, 2006 and Ajav, 1996). For each batch, whole kernels were separated and the weight was determined. Whole kernel out turn was estimated as the percent ratio of the weight of whole kernels (Ww) to the total weight of kernels recovered (WT).
equation2
2.5. Statistical analysis
The data were subjected to analysis of variance using SAS (2002). Least Significant Difference test was used to separate means that were significantly different from each other at p < 0.05.
3. Results and discussion
The WKO of cashew nuts that were pre-treated by steam-boiling is presented in Table 1. The general trend was that WKO decreased as MC increased within the range of experimental values. This may be due to an increase in the fracture resistance of the nut-shell as MC increased. The shell was tough and after rupture, it did not yield or split open to release the embedded kernel; exerting more force caused the total nutshell failure. Although WKO ranged from 38.98% to 96.96%, its value for any treatment combination was higher than 32%–62.02% obtained for raw cashew nuts. This indicated an improvement in WKO of cashew nuts when pre-treated by steam boiling. Ogunsina (2010) in reva pious investigation with cashew nuts that were pre-treated by hot-oil roasting observed that the individual effect of pre-treatment method, nut size or MC alone cannot be used to determine WKO; rather, it is a product of interaction between these parameters.
- Table 1.
Whole kernel out-turn of steam-boiled cashew nuts.
With steam-boiling as a pre-treatment method, the main effect of either MC or SET alone on WKO viz-a-viz the interaction of MC∗SET on the WKO of three different size ranges of cashew nuts is presented in this report. The data for large sized cashew nuts are presented in Table 2. It was observed that at MC of 8.34% and 32 min of steam exposure (i.e. 8.34%∗32 min), the highest significantly different value of WKO (p < 0.05) was 96.96%; and this was not significantly different from 91.74% obtained at 8.34% ∗30 min. Although WKO of 96.96% is numerically higher than 91.74%, in the context of this investigation the numerical difference cannot be attributed to SET. It therefore makes no difference if large size cashew nuts at 8.34% MC are steam-boiled for 30 mins of SET. This is 6% reduction in steam-exposure time per batch of nuts; and implicitly, energy, labour and overall costs. Going through other MC∗SET interactions, the highest significant (p < 0.05) WKO was lower than WKO at 8.34%∗30 mins. This shows that the optimum WKO is achievable when large size cashew nuts are at 8.34% MC and subjected to SET of 30 min. The general trend depicted by this data further shows that as MC increased, WKO reduced; regardless of the SET. For cashew nuts with 34 min of SET,there was first an increase in WKO as MC increased from 8.34% to 11.8%, afterwards there was a decrease and later again, an increase. This irregular trend was similarly observed for raw nuts, but WKO of steam-boiled nuts was consistently higher than that of raw nuts. A linear decrease in WKO was observed for nuts with 32 min of SET at all MCs. It was also observed that WKO ranged from 80% and above until the moisture content was above 12% for all durations of steam exposure. Afterwards, WKO decreased, tending towards 60% and below. However, within the range of these experimental values, for any level of SET, raising the MC above 8.34% may cause a significant reduction in WKO during shelling.
- Table 2.
Least significant difference test for the WKO of large sized cashew nuts as influenced by steam exposure time and moisture content. -
- a–d
- Means within the same cell with different letters are significantly different (p < 0.05). 28, 30, 32 and 34 min are steam exposure times (mins); SET = 0 for raw nuts.
