Influence of thermal treatments on microscale release of nutraceutical compounds in syneresis liquids from frozen blood orange purées

• Thermal treatments influence serum composition in ‘Tarocco’ orange purées.
• A complementary biochemical and microscopy approach was applied.
• Fruit blanching increases carotenoids and particle components in the serum fraction.
• Fruit blanching decreases the anthocyanins content in the serum fraction.
• Purée pasteurization increases viscosity and soluble components in the serum fraction.

Phytochemicals release from blood orange (Citrus sinensis (L.) Osbeck cv Tarocco) fruit into the liquid fraction of thermally treated homogenate products was studied by means of a complementary biochemical and micro-structural approach. Thermal treatments were applied either before (10 min steam-blanching, BL) or after puréeing (30 min autoclave pasteurization, PAST) of thawed (24 h, 4 °C, NB) IQF fruit. NB, BL and PAST homogenates (200 g), packed in sealed plastic vessels, were frozen and stored at −20 °C; liquid fractions were separated from the pulps by syneresis upon thawing (24 h at 4 °C). Physico-chemical and colour parameters and total carotenoids of the samples, as well as anthocyanin content, viscosity and turbidity values of the separated liquids were measured, while histochemical profiles of fruit, purées and syneresis liquids were investigated by bright-field and fluorescence microscopy. PAST purées released a lower amount of syneresis serum (52%), compared to BL (57%) and NB purées (59%). Crystals, plastids, droplets and amorphous components were identified as constituents of syneresis liquids from frozen purées. Blanching of the fruit increased the release of membrane-bound carotenoids, insoluble pectinous materials and hesperidin crystals from the pulps into the liquid fractions, but decreased the anthocyanin content of syneresis liquids. In contrast, homogenate pasteurization reduced the amount of particulate components into the liquid fraction, but increased viscosity (+9%) and the release of more soluble fractions. These results indicate that starting from the same raw fruit, by acting on thermal processing variables, different homogenate products could be obtained, characterized by different partitioning of bioactive molecules between liquid and particle-bound fractions.

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