Download files
Abstract
Blueberries are known for high level of antioxidant activity with health benefits attributed to their high phenolic compounds content, especially, anthocyanins. Given their seasonality and short shelf life, drying is one of the options making them available throughout the year. However, anthocyanins and other phenolic compounds can be degraded to a large extent during processing, resulting in loss of fruit functionality. So far, freeze drying is regarded as the most suitable method to preserve the functional properties in blueberries. The high operation cost being the drawback of this method, alternative air drying and pre-treatment techniques were investigated aiming at finding a more cost effective method that can give high quality products comparable to freeze dried ones.
The study examined the effect of temperature and relative humidity (RH) on the drying kinetics as well as on phenolics, anthocyanin content and antioxidant activity of the blueberries. Low temperature drying was performed at 30 °C and 15% RH using a heat pump dryer. High temperature drying was performed at 60, 70 and 80 °C with RH of 10, 15 and 20% using a cabinet dryer. The effects of pre-treatments including osmotic dehydration, mechanical abrasive wax removal and blanching were also investigated.
The results showed lower functional properties of all air dried blueberries compared with the freeze dried fruit. A significant reduction of antioxidant content and activity was observed following low temperature drying in comparison with high temperature drying due to a prolonged drying time. However, the wax abrasive and blanching pre-treatments aided in reducing degradation of the phenolic compounds and preserving antioxidant activity by reducing drying time and inactivating enzymes that are responsible for anthocyanin degradation, respectively. Osmotic dehydration, in contrast, caused a greater loss of beneficial compounds and activities after dipping in a sucrose solution.
The analysis by HPLC and LC/MS showed high concentration of anthocyanin and a phenolic compound namely, chlorogenic acid, in fresh southern highbush (Star, Sharpe, Crunchy) and rabbiteye blueberries (Climax, Brightwell and Powderblue). Theanthocyanins detected in all blueberry cultivars were monoglycosides of 5 anthocyanidins including delphinidin, cyanidin, petunidin, peonidin and malvidin. Among these, rabbiteye species were more abundant in malvidin followed by cyanidin derivatives whereas southern highbush species contained high concentration of malvidin and delphinidin. Peonidin glycosides had the lowest proportion within total anthocyanins in all blueberry cultivars. After drying, the levels of individual anthocyanins decreased in different manner depending on the cultivars. However, the proportion of each of them remained similar to that in the fresh berries.
The blueberries dried using abrasive and blanching pre-treatment were also selected to be tested for their anti-proliferative properties (against cancer cells) given their high antioxidant content. Colon cancer (HT-29) and colon normal (CCD-18Co) cell lines were used in the study. All extracts showed selectivity for colon cancer cell line in comparison with normal cell line. The dried blueberries showed adequate antiproliferative activity against HT-29, with however lower values than fresh and freezedried blueberries.