Natural pre-treatment in raisins production: Effects on quality during storage

BRS Vitoria raisins were produced with a pre-treatment using a natural surfactant, i.e., extra-virgin olive oil (EVOO), to (i) better elucidate its action on the grape surface using scanning electron microscopy; (ii) evaluate its influence on the oleanolic and ursolic acid content of raisins through HPLC-DAD; (iii) assess raisins characteristics during 180-day storage at different temperatures by using physicochemical and microbiological analyses with multivariate-based chemometric methods (MASLT). Grapes which presented adequate physicochemical characteristics and high concentration of anthocyanins originated raisins similar to those reported in the literature. The pre-treatment accelerated the dehydration process (about 26%) mainly because of the non-uniform redistribution of the surface waxy components accompanied by the formation of microholes and cracks. The small amount of EVOO used did not affect the oleanolic acid content in raisins, yet this compound proved to resist drying, which means that there is no loss of wax during the pre-treatment. For both untreated (RW) and pre-treated raisins (RP), the main changes triggered over time were not related to microbiological characteristics. Most of the changes appeared to occur right during drying and not during storage for RW, whereas for RP the physicochemical characteristics that mostly correlated with the storage changes were total monomeric anthocyanins, total acidity, and total sugars; also, they tended to reach an equilibrium and equaled RW at the end of 180 days, especially at 35 °C. These parameters proved to be the main characteristics related to the changes triggered in raisins. For both raisins, sugar crystals were formed in the flesh, but they were not visible to the naked eye or perceived by consumers. The results show that it is highly desirable to have a high-quality product at the beginning of storage, which was possible thanks to the EVOO pre-treatment used.