Effect of roasting conditions and packaging on physicochemical properties of wild almond (Amygdalus scoparia L)during storage

Roasting is one of the most important thermal processing to improve the physicochemical and organoleptic properties of nuts. Wild almond, especially roasted wild almond is very sensitive to oxidation due to its high content of unsaturated fatty acids. Lipid oxidation can be inhibited by using a suitable packaging material and modifying the atmosphere of the packaging. In this research, the effects of roasting degree (light roasting at 50°C/60 min, medium roasting at 100°C/45 min and dark roasting at 200°C/ 30 min), as well as the presence/ absence of nitrogen gas in two types of pouches including polyethylene (PE) and oriented polypropylene/ cast polypropylene (OPP/CPP) were investigated. The effects of above mentioned treatments were examined using Completely Randomized Design on the moisture, total fat, firmness, peroxide value and conjugated trienes, fatty acid profile and sensory properties during two months storage at 37°C.  The wild almonds used in this research were collected randomly from the Dasht-e-Arjan region in Fars province (Iran). The debittering process of almonds was done by soaking in 4- 6 % (w/v) of NaCl solution for 12 hours at three consecutive days. A certain amount of debittered wild almonds with a uniform size were roasted at 50°C for 60 minutes (light- roasting), 100°C for 45 minutes (medium- roasting) and 200°C for 30 minutes (dark- roasting) in a laboratory toaster (SSN-2004x, Iran). After cooling at room temperature, samples of 100 g were packed using gas flushing packing machine (90SM4, Shadmehr Co., Iran) in three different conditions: a) air atmosphere (in transparent PE pouches, 70- 75 mm thickness), b) modified atmosphere flushed with N2 gas (in transparent PE pouches, 70- 75 mm thickness), c) modified atmosphere flushed with N2 gas (in two-layer transparent OPP-CPP pouches, 25 and 50 mm thickness of the first and second layer). The samples were stored for two months at 37°C and 7-8% R.H. At definite time intervals, the samples were analyzed for the moisture content by oven drying, total oil content by the soxhlet method, protein content by the Kjeldahl method, and fiber content through chemical gravimetric method. Extraction of oil from the nut samples was carried out using hexane, as described by Kornsteiner et al. (2009). Peroxide value (PV) was determined by the iodometric assay according to IUPAC standard method 2.501. Conjugated trienes were calculated according to IUPAC Official Method 2.205 based on measuring absorbance of a solution containing 0.01-0.03 g of oil in 25 ml of isooctane. The fatty acid composition of wild almond oils was determined by gas chromatography equipped with flame ionization detection. Hardness of the wild almond was evaluated using a Texture Analyzer (CT3-Brookfield, USA). Sensory evaluation of samples was carried out by 30 trained panelists using a five point hedonic scale. Statistical analysis of completely randomized design with three replications and means comparison in 95% confidence level was employed using the SPSS-19 software.  Analysis of variance indicated that the independent effects of roasting conditions, packaging and storage time and the interaction of roasting conditions and storage time on the mentioned parameters were significant (p<0.01). With roasting at higher temperatures, the moisture content and hardness decreased significantly while the oil, protein and fiber content of wild almond kernels and the peroxide value (PV) and conjugated trienes (CT) of their oils increased significantly. However, the increase in protein and CT for medium and low roasting conditions and the increase in fiber for medium and high roasting conditions were not significant (p>0.05). Moreover, increasing the roasting temperature of wild almond led to a significant increase of palmitic acid and a significant decrease of stearic acid whereas other fatty acids did not change significantly. There was a remarkable increase and then a rapid decrease in the fiber, fat and protein content of all roasted samples during the first and second months of storage, respectively. Whereas the PV and CT of oils increased and the moisture content and hardness decreased significantly during the two- months storage. For each roasting temperature, wild almonds packaged in pouches under N2 had lower PV and CT of oils and the OPP/CPP pouches under N2 had better performance in this regard and retaining moisture, fat, protein, fiber content and texture. At the end of storage with PV of 24.6 (meqO2/kg oil) and K268 of 1.92 the dark-roasted almond sample packaged with PE/air atmosphere was the least stable, and the light-roasted sample packaged with OPP/CPP under N2 with PV of 5.3 (meqO2/kg oil) and K268 of 1.57 was the most stable form against oxidation. According to the results of sensory analysis, the highest overall acceptability score was attributed to the samples roasted at 100°C for 45 minutes.