Processing of berries – Effects on functionality, stability and bioactivity of anthocyanins
Abstract: Berries are rich natural sources of anthocyanins, water-soluble pigments in plants that have been linked to beneficial effects against chronic diseases, such as cardiovascular diseases, type 2 diabetes and neurodegenerative diseases. However, anthocyanins as well as other polyphenolic compounds are often sensitive to degradation during processing, storage and digestion. The stability of anthocyanins is affected, for example, by exposure to high temperatures, oxygen and increasing pH. Non-thermal and mild processing approaches have been revealed as useful tools to extend the shelf-life of berries and preserve phenolic compounds during processing. The overall aims of this thesis were first to evaluate different mild drying techniques and the fractionation of bilberry press cake from juice production toward obtaining phenolic-rich ingredients for incorporation into value-added food products. Second, to assess the recovery of anthocyanins in semi-dried berries and in coated whole berries, and for future analyses of compounds derived from breakdown or metabolism of anthocyanins, by developing a sensitive LC-MS/MS method. The recovery of anthocyanins was measured after non-thermal treatments and simulated gastrointestinal digestion of i) strawberry samples pre-treated with pulsed electric fields (PEF) prior to osmotic dehydration (OD) and ii) blueberry samples coated with chitosan and procyanidin. The third aim was to investigate the protective effect of digested anthocyanin extracts against H2O2-induced oxidative stress in yeast cells (Saccharomyces cerevisiae). Hot air drying and microwave drying applied to bilberry press cake resulted in a small and similar reduction in the content of anthocyanins. Milling of bilberry press cake into powders with small particle size (< 500 µm) provided a powder with the highest apparent content of phenolic compounds. Considering the non-thermal treatments, the application of PEF-assisted OD of strawberries and the use of edible coatings to blueberries maintained or enhanced the stability of anthocyanins during in vitro gastrointestinal digestion. Digested strawberry samples osmotically dehydrated with trehalose and digested blueberries coated with chitosan and stored for 14 days, had the highest recovery of anthocyanins. A protective effect of anthocyanins from berry extracts was observed only when the yeast cells were pre-incubated with digested extracts. The results imply a biological effect, i.e. a changed phenotype during growth induced by digested anthocyanin-rich extracts, rather than a chemical effect. In summary, berries can be tailored by mild processing to produce fortified ingredients and stabilise (poly)phenols during digestion. Also, the changes in their initial structures, occurring during digestion of berry extracts, are crucial to consider. Further studies on the bioactivities of anthocyanins and their transformed derivatives are needed to clarify the protective effects of digested anthocyanin-rich extracts.
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