Optimising the drying process for grape (Vitis vinifera L.) using pre-treatments to maximise quality retention

Abstract

Grape (Vitis vinifera L.) is a valuable fruit, but highly perishable as it has high water and sugar content. Drying grape is an option for shelf-life extension. Reducing moisture content can prevent micro-organism growth, deteriorative reactions and reduce weight and volume of the product. After drying, dried grape, so called raisin, represents a rich source of carbohydrate and organic compounds in concentration, especially phenolic compounds that are beneficial for consumers’ health. However, the commercial practice of using chemicals for pre-drying treatment such as potassium carbonate has raised concern about food safety. The research reported here aimed to examine an alternative method to increase drying effectiveness, while avoiding potassium carbonate use. Five pre-drying treatments (2% olive oil blanching, 2.5% K2CO3 plus 2% olive oil dipped, needled, halved and no-treatment control) were compared at two drying temperatures of 60 °C and 70 °C. The drying characteristics and appropriate models were investigated. Product quality including colour, pH, total soluble solids and acidity, total phenolic contents were assessed. The results show that drying time of oil blanched-grapes was reduced by 42% at drying temperature of 60 °C and 50% at drying temperature of 70 °C compared to chemical treatment and more than 60% compared to the no-treatment control. The oil blanching treatment retained the highest total phenolic content and it is applicable into food processing. Statistical indicators (determination coefficient, sum square error and root mean squared error) showed the Logarithmic model best described the drying kinetics for all treatments. Additional research is needed to test the convective drying method with a thick bed layer in order to scale up into industry. The low-cost oils such as canola, sun flower oils should also be examined to reduce the application of olive oil.