Stabilization of fat fraction in walnut-based freeze dried products

Aim of this work was to create an innovative walnut product able to increase the shelf life of the walnuts themselves and consequently facilitate their industrial use. Main purpose was to protect the walnuts against rancidity, making the fat fraction less exposed to oxidizing agents and preserving all the well-known positive compounds of the fruit (polyunsaturated fatty acids, vitamins, fiber and antioxidants). To this end an encapsulation process was developed consisting of three different stages: roasting and milling of walnut kernels for paste production, adding of different coating materials for emulsion formation, drying of emulsified samples by lyophilization. Different encapsulation agents (maltodextrin DE12, tragacanth gum, starches, and whey protein concentrates with 80% and 50% proteins on dry basis) were tested, either individually or in combination, in order to assess the effect exerted by different formulations and technological treatments on the product stability. Preliminary tests were carried out to select type, concentration, and combination of coating materials. The fittest samples were therefore examined in terms of residual moisture, pH, total acidity, color, peroxide value, conjugated dienes and trienes. Phenolics and compounds resulting from the oxidation process were detected using an ultra-high performance liquid chromatograph coupled to quadrupole-time-of-flight mass spectrometer (UHPLC/Q-TOF). Analysis were carried out immediately after freeze-drying, and on freeze-dried products after 15 day 60°C storage. Chemicals found in the fresh lyophilized and in the stored samples were identified and compared through the loading plot: flavonoids (flavanones and isoflavones) and phenolic acids, proved to be the most detected differential compounds. The formulation consisting in walnut paste added with 0.2% tragacanth gum, 15% whey protein concentrates with 50% protein/d.m., and 18.3% maltodextrin DE12 allowed to obtain a freeze-dried product with good texture, dry (with less than 0.8% moisture), and with good stability of the fat phase.