Structuring plant cell materials using pre-treatments. Effects on texture and carotenoid accessibility

Licentiate thesis, 2009

A high consumption of fruits and vegetables is considered beneficial and to support good health. To promote higher fruit and vegetable intake, attractive and nutritious fruit- and vegetable-based ready-to-eat products should be available on the market. A key property of an attractive food product is a palatable texture. Also, it is crucial in the design of healthy products to retain high nutrient content, and to create high bioavailability of the nutrients. Both texture and bioavailability of nutrients are linked to the microstructure of the product, which in turn is affected by processing conditions. Crushed tomatoes and carrot pieces are commonly used as ingredients in food products, and were evaluated in this work. Pre-treatments such as low and high temperature blanching, different crushing intensities, crushing/blanching order, and the addition of calcium were investigated. A boiling step was added to produce samples representative of ready-to-eat products; effects of pre-treatments were studied both before and after boiling. Textural properties of crushed tomatoes were evaluated in terms of Bostwick consistency index and vane viscosity, and the texture of carrot pieces was evaluated by compression and tensile tests. The microstructure was visualised by light, confocal, and transmission electron microscopy. Pectic enzyme activity and pectin degree of methoxylation were studied to increase the understanding of changes in material properties. In crushed tomatoes, carotenoid content was measured using HPLC and carotenoid accessibility was estimated using a static in vitro digestion model. Thermal pre-treatments were observed to have significant effects on crushed tomatoes and carrot pieces. Low temperature long time blanching activated pectic enzymes, which resulted in crushed tomatoes of low consistency and firm carrot pieces after boiling. The textural changes of carrot pieces could be linked to changes in the microstructure; hardness retention of carrot pieces achieved by low temperature blanching was accompanied by retention of a cell wall structure similar to that of raw carrots. In addition, changes in microstructure of crushed tomatoes were visualised that were suggested to affect carotenoid accessibility. In crushed tomatoes, crushing before blanching and high crushing intensity increased the contact between pectinmethylesterase and pectin, which affected the consistency. In vitro lycopene accessibility from crushed tomatoes was significantly affected by processing conditions; a combination of intense crushing and thermal treatment was needed to increase in vitro lycopene accessibility.