Improving hazelnut shelf-life through an innovative roasting process

Hazelnut roasting is conventionally carried out in a single-stage toaster, in order to develop the typical roasting flavor, color and texture. Due to their high lipid content (about 60%) with 7-15% linoleic acid, roasted hazelnuts are prone to lipid oxidation, with negative effect on their shelf-life and of hazelnut-containing products. A two-step process with a preheating step under moist conditions, followed by high temperature dry roasting is suggested in the literature, in order to improve product oxidative stability. The aim of this work was to implement the two-step roasting process and to evaluate the modification kinetics of hazelnut characteristics under accelerated storage conditions. Besides, the ultrastructure of raw and toasted hazelnuts was studied, in order to better understand the effect of the innovative roasting process. In order to study the influence of operative conditions in the two-step process, a Box-Behnken experimental design was applied. Results showed that among considered factors (first step treatment time, second step treatment time and temperature), the treatment time of the second step was the only one significant (p<0.05), affecting instrumental crispness and color of hazelnuts. One sample (identified as WET) of the experimental design was then used for accelerated storage experiment, together with another sample (identified as DRY) roasted in the same operative conditions, but without vapor addition in the preheating step. The moist conditions applied in the first step of roasting did not modify texture properties of the final roasted hazelnuts. During accelerated storage, a definitely lower lipid oxidation rate of WET sample was observed, attributable to a more compact ultrastructure in comparison to that of DRY sample, as evidenced by scanning electron microscopy analysis. In WET sample, cells are quite tightly packed and oleosomes are visible as discrete units with a cellular organization similar to that of raw hazelnuts, thus limiting oxygen diffusion in the fat matrix. In conclusion, the innovative two-step roasting process contributes to extend hazelnut shelf-life, minimizing ultrastructure modifications, probably because the use of vapor in the first roasting step makes water evaporation from nuts milder.