Fracture Resistance of Cashew Nuts as Influenced by Pre-Shelling Treatment

Abstract

The fracture resistance of raw and pre-treated cashew nuts during uni-axial compressive loading was investigated. Cashew nut samples were subjected to two pre-shelling treatments, namely: steam boiling and roasting in hot cashew nut shell liquid. Two loading rates of 2.5 and 50 mm/min and two loading orientations (longitudinal and transverse) were considered for fracture resistance of pre-treated cashew nuts using a 50 kN capacity Instron testing machine. The data obtained were subjected to analysis of variance. The average values at 2.5 mm/min were 342 and 318 N for raw nuts, 321 and 242 N for roasted nuts, and 341 and 309 N for steam boiled nuts during longitudinal and transverse loading, respectively; whereas corresponding values at 50 mm/min were 784 and 763 N for raw nuts, 517 and 464 N for roasted nuts, and 436 and 398 N for steam boiled nuts, respectively. In each of the pre-treatment methods and loading rates, more force was required to crack cashew nuts during longitudinal loading than transverse loading; and for each loading rate, pre-treated nuts generally required less force than raw nuts.

Keywords:

• Cashew nut
• Fracture resistance
• Pre-shelling treatment
• Steam boiling
• Hot-oil roasting

INTRODUCTION

Cashew nut (Anacardium occidentale) is known globally for the prized exotic snacks that the kernel provides when roasted, fried, salted, spiced, or sugared.[1, 2] It is a valuable ingredient for confectionery and baked food products[3, 4] because it contains essential amino acids and minerals that are seldom found in daily diets.[5– 8] Cashew nut processing involves cleaning/grading, pre-shelling treatment (roasting or steam boiling), shelling, separation, drying, peeling, grading/sorting, and packing[1, 2, 6, 8] with shelling being a particularly tedious unit operation. This is especially due to the peculiar kidney-shape of the nut, the presence of a tough, leathery outer shell, the corrosive cashew nut shell liquid in the mesocarp of its shell, and brittleness of the embedded kernel.[3, 9] Presently, cashew nut shelling is still a manual process in most places because 25–40% of the kernels get broken with the existing mechanized systems.[9– 13] Therefore, low whole kernel out-turn is has been a limiting factor in most countries with high potentials for production and processing; whereas enormous revenue is obtainable when raw cashew nuts are processed into kernels.[14] About 70% of the weight of the cashew nut is constituted by the shell,[4] meaning that the bulk of exported raw cashew nut is garbage. In the last three decades, cashew nut production in Nigeria increased from 25,000 metric tons in 1980 to 660,000 metric tons in 2008,[15] making Nigeria second among the top ten cashew nut producing countries in the world. However, foreign earnings from cashew nut is relatively low because 90% of the total production is exported raw to India where the bulk of world cashew nut is processed.[16]

Understanding the force-deformation characteristics of biomaterials, such as nuts and kernels, helps in identifying the conditions that will favor or facilitate easy removal of the kernel from the shell with little or no mechanical damage.[17, 18] Fracture resistance of biomaterials gives insight into their fracture mechanics and provides a basis for rational design or modification of shelling devices and dehullers. Previous works on cashew nut,[19] macadamia nut,[20– 23] bambara groundnut,[24] palm nut,[25] balanite,[26] shea nut,[27] pistachio nut,[28] walnut,[25] pumpkin,[29] and dika nut[30] established that significant information on the fracture of any biomaterial in a nutshell may be obtained by subjecting it to quasi-static compressive loading in different orientations. For instance, Liang et al.[31] found that force-deformation characteristics of walnuts can be used to predict the effect of moisture content on the magnitude of compression for nutshell fracture. Oloso and Clarke[19] studied the fracture resistance of pre-damaged roasted cashew nuts using sunflower oil as the roasting medium and using a loading rate of 50 mm/min. The failure pattern was characteristic of brittle fracture; compressive force reduced sharply as deformation increased. The limitation of this work was that cashew nut shell liquid (CNSL) is the correct roasting medium for cashew nuts in industrial practice. Liu et al.[22] investigated the behavior of pre-damaged macadamia nuts during quasi-static compression between two rigid plates and found that nuts cracked in the longitudinal direction had a lower fracture resistance than the nuts that were cracked in the transverse direction. Borghei et al.[32] found that large size walnuts at 6% moisture content (w.b.) require higher cracking force and experienced greater deformation than small ones during compression tests. Khazaei et al.[33] found that the cracking force, absorbed energy, and power required for almonds ranged from 139–1526 N; 70–2093 MJ; and 0.015–5.121 W, respectively. Loading rate had a significant (p < 0.01) effect on cracking force and required power, but its effect on absorbed energy was not statistically significant. Aydin[34] found that the rupture strength of almonds decreased with increasing moisture content and rupture strength was more during transfer loading. Pliestic et al.[35] investigated the average forces required for cracking filbert nuts in three loading orientations and found that the maximum force for nut cracking occurred in a longitudinal direction and the minimum force was in the transverse direction. For all loading orientations considered, cracking force decreased with increase in moisture content. Arslan and Vursavus[36] studied the effect of processing conditions on the physical and mechanical properties of three hard-shelled varieties of almonds and found that conditioning hastened nutshell rupture during lateral loading. Sharifian and Derafshi[37] found that the force, energy, and power required for walnut rupture were higher during longitudinal loading than during transverse loading. Ogunsina et al.[30] found that quasi-static uni-axial compression of the dika nut was characterized by two regimes of force-deformation behavior prior to nutshell fracture: a non-linear, non-elastic transient stage and a pronounced elastic portion preceding a brittle failure. The average compressive force required for cracking the dika nut increased with nut diameter; and cracking force was lower during transverse loading than during longitudinal loading, but both were not significantly different from each other. Similarly, the stiffness modulus and the energy required for failure were significantly different (p < 0.05) and lower for nuts compressed along the transverse axis than along the longitudinal axis. However, no published literature was found on the fracture resistance of cashew nuts under uni-axial compression considering the different methods of pre-treatment in common industrial practice. This investigation focuses on the effect of different methods of pre-shelling treatments and loading orientation on the fracture resistance of cashew nuts with the view to minimize kernel breakage during shelling.

MATERIALS AND METHODS

Samples Preparation

Raw cashew nuts were obtained from plantations in Iseyin, Iwo, and Oro areas of Oyo, Osun, and Kwara states. The lot was cleaned manually to remove foreign and extraneous materials, shriveled apples, and immature and broken nuts. The lot was sun-dried in a thin layer to a moisture content of 8.3% (w.b.) and graded into three sizes: small (18–22 mm), medium (23–25 mm) and large nuts (26–35 mm). Five (5) kg of raw cashew nuts were steam-boiled at 700 kPa for 30 min and cooled at room temperature (27 ± 1°C) in desiccators.[2, 8] Another 5 kg from the same lot was conditioned from an initial moisture content of 8.3 to 17.6 g/100 g nuts (w.b.) and roasted in a pre-heated CNSL bath for 1.5 min at between 185–190°C.[2, 19] The roasted nuts were discharged on saw dust to mop residual coating of CNSL on the surface of the shell. The nuts were then cooled at room temperature in desiccators. For each treatment, there were five replicates. The moisture content of samples was determined following ASABE method S410.1.[38]

Fracture Resistance Test

Samples of the graded and pre-treated cashew nuts (raw, roasted, and steam-boiled) were subjected to uni-axial compression using the Instron universal testing machine (UTM) (model 3369, Instron, Norwood, MA, USA), each with 50 kN capacity (Fig. 1). Each nut was wrapped in a polyethylene bag to protect the spindle of the UTM from CNSL loaded between two parallel hardened stainless steel plates at a constant rate until the nut ruptured. Two loading orientations (longitudinal and transverse) and two loading rates (2.5 and 50 mm/min) were considered.[19, 33] The magnitude of the applied load on the sample and deformation during compression was read on the Instron UTM. There were five replicates for each run. From the force-deformation curves generated by the Instron UTM software, the cracking force and slope in the apparent elastic region (i.e., stiffness modulus, S) were determined. It was assumed that the deformation of the shell alone prevailed until the load cell came in contact with the kernel. Provided that the shell was still intact thereafter, the nut behaved as a composite biomaterial with two elastic bodies in parallel compression. This change resulted in a deviation of the slope of the force-deformation curve, and this point was taken as the limit for estimating stiffness modulus of the shell. The results were analyzed using SAS 2001.[39]

Table 1  Average fracture resistance of cashew nuts at two loading rates

	Loading rate
Loading orientation/Pre-shelling treatment	2.5 mm/min	50 mm/min
Raw nut
Longitudinal	342 ± 45	784 ± 340
Transverse	318 ± 51	763 ± 300
Roasted nut
Longitudinal	321 ± 20	517 ± 223
Transverse	242 ± 33	464 ± 170
Steam-boiled nut
Longitudinal	341 ± 28	436 ± 72
Transverse	309 ± 23	398 ± 70

Figure 1 Typical loading of a cashew nut sample between two parallel compression plates on Instron UTM (3369 Series, 50 kN capacity) during the investigation. (Color figure available online.)

RESULTS AND DISCUSSION

The fracture resistance of raw, roasted, and steam boiled cashew nuts at 2.5 and 50 mm/min loading rates in the longitudinal and transverse directions are shown in Table 1. The average values at 2.5 mm/min were 342 and 318 N for raw nuts; 321 and 242 N for roasted nuts, and 341 and 309 N for steam boiled nuts during longitudinal and transverse loading, respectively; whereas corresponding values at 50 mm/min were 784 and 763 N for raw nuts, 517 and 464 N for roasted nuts, and 436 and 398 N for steam boiled nuts, respectively. The fracture resistance of cashew nuts in the two loading orientations were lower than the values reported for palm nuts,[25] macadamia nuts,[20, 22] and dika nuts[30] showing that these nuts are more brittle than cashew nuts. At 2.5 mm/min loading rate, the average least fracture resistance (242 N) was exhibited by roasted nuts during transverse loading, while the highest average value (342 N) was obtained for raw nuts during longitudinal loading. The average least fracture resistance (398 N) was exhibited by roasted nuts at 50 mm/min loading rate during transverse loading, while the highest average value (784 N) was obtained for raw nuts during longitudinal loading. As indicated in Table 1, the fracture resistance of pre-treated cashew nuts during longitudinal loading was generally more than during transverse loading. This may be due to the relatively smaller surface area over which the compressive force of the Instron UTM was distributed during longitudinal loading. Ogunsina and Bamgboye[40] had earlier found that cashew nut shell thickness tends to be less at the distal ends, hence, more prone to fracture in the transverse than in the longitudinal axis. Besides, most nuts/seeds, such as peanut, almond, walnut, and dika nut, regardless of heat-treatment, experience minimum resistance to fracture and fail more readily along their natural suture and symmetry line.[34, 40, 41] Furthermore, pre-treated cashew nuts were found to require less force than raw nuts; the least fracture resistance was exhibited by roasted nuts. For most biomaterials, fracture resistance reduces when they are subjected to pre-treatment by heat. During hot-oil roasting of cashew nuts, there is rapid moisture removal from the nutshell within the short-span of the associated high temperature condition; this makes the shell brittle and more susceptible to fracture during compression. For steam-boiled cashew nuts, the CNSL bearing cells in the mesocarp of the shell become crusty after cooling; this reduces the tough nature of the shell and makes fracture resistance less than for raw nuts.

For all pre-treatments and loading orientations, the force required to crack cashew nuts was less when the loading rate was slow (2.5 mm/min) than when it was fast (50 mm/min); however, it was observed that the embedded kernel in most nut samples that were loaded at 2.5 mm/min got crushed. This may be due to the fact that compression occurred progressively over a relatively longer period of time than at 50 mm/min during which loading rate was fast and nutshell fracture was sudden. Oloso and Clarke[19] had earlier obtained a cracking force of 99.8 N for roasted nuts that were pre-damaged with random saw cuts inflicted on the shell flanks prior to compression and loaded at 50 mm/min. Although, pre-damaged nuts will most likely crack along the axis of injury and this was reported to solve the problem of nuts alignment during centrifugal cracking, the wholesomeness of kernels was quite low and this has serious labor and cost implications in industrial practice.[2]

In Fig. 2, the characteristic failure pattern shown on the force-deformation curves was of a brittle fracture consisting of a short, initial, non-linear, and non-elastic portion, then a more pronounced, apparently elastic portion, which terminated abruptly. The nutshell failed catastrophically, splitting symmetrically along the natural suture line, thereby releasing the kernel. The apparently linear elastic portion of the curve represents the primary resistance of the nutshell to facture, while the upper limit marks the bio-yield point, or the point of nutshell rupture. This failure pattern had earlier been reported for roasted cashew nuts,[19] macadamia nuts,[21, 22] walnuts,[29, 37] palm nuts,[25] and dika nuts.[30]

Figure 2 Typical force deformation curves of cashew nut on the basis of size and pre-shelling treatment. (Color figure available online.)

CONCLUSIONS

The fracture resistance of pre-treated cashew nuts has been investigated. Pre-shelling treatment of raw cashew nuts reduced the fracture resistance. The average values at 2.5 mm/min were 342 and 318 N for raw nuts, 321 and 242 N for roasted nuts, and 341 and 309 N for steam boiled nuts during longitudinal and transverse loading, respectively. The corresponding values at 50 mm/min were 784 and 763 N for raw nuts, 517 and 464 N for roasted nuts, and 436 and 398 N for steam boiled nuts, respectively. These values show that in each of the pre-treatment methods and loading rates, more force was required to crack cashew nuts during longitudinal loading than transverse loading; and at the two loading rates that were considered, pre-treated nuts generally exhibit less fracture resistance than raw nuts.