The new research, published in the journal LWT - Food Science & Technology, reports that adding trehalose to dehydrated pear cubes could improve aroma retention by 15 per cent. "The best retention of aroma compounds in dehydrated pear puree was obtained by trehalose addition, regardless of the dehydration process applied (freeze drying or foam-mat drying), although much higher retention of flavour volatiles was obtained in freeze-dried purees," wrote lead author Draenka Komes from the Faculty of Food Technology and Biotechnology at the University of Zagreb. "The aforementioned products are free of preservatives, maintain their natural flavour and colour and have an agreeable texture and good rehydratability properties. Therefore, they could be widely used as ingredients in bakery products, ice cream or yoghurt," she said. Fruit pieces, used increasingly in food products, can contribute sweetness and colour to the end brand, as well as adding mouthfeel and texture, particularly relevant for fat replacement. In addition, fruits act as natural humectants which can extend the shelf life of many foods and contain a host of phytochemicals linked to benefits for human health. However, according to background information in the article, Komes and colleagues report that dehydration of fruit pieces leads to a reduced quality of the product resulting from significant losses of aroma compounds. The Croatian researchers investigated the potential of sugar solutions of sucrose and trehalose on the aroma retention of pear purees and cubes. They report that the best retention of the aroma compounds was observed when freeze-drying and trehalose were used in combination. Indeed, total aroma retention was 46 per cent for the trehalose, while retention was 38 and 31 per cent for the sucrose and control pear cubes, note the researchers. Three potential mechanisms behind the beneficial effects of trehalose were suggested by the researchers - water replacement hypothesis, glass transformation and chemical stability hypotheses. The first theory assumes that the sugars replace water in the dried product. "Trehalose has superior effects in "destructuring" the network of water and in slowing down its dynamics. These two properties could play a key role in the understanding of the microscopic mechanisms of bioprotection," wrote the researchers. Glass transition - properties of the material change from a crystalline (glass) to a liquid - refers to the ability to prevent the loss of small volatile compounds such as esters during drying and storage. "By binding water molecules more tightly, the glass formed by trehalose could hinder molecular motion more effectively," stated Komes. "When the matrix is in the glassy state, volatiles are encapsulated in the amorphous glass and low mobility leading to the increased stability of the material being preserved. Above the glassy state, temperature collapses and sometimes crystallization takes place… and the encapsulated volatiles are released." Further research is needed, said the researchers, to obtain complete information on how trehalose influences aroma retention. This is not the first time that bioprotection with trehalose has been proposed. Last year scientists from the Nestlé Research Center reported findings from their extended program with the University of Bristol on the molecular physics of carbohydrates. Lead researcher Dr. Job Ubbink told FoodNavigator.com that the collaboration used Positron Annhilation Lifetime Spectroscopy to study the free volume in trehalose, and showed that changes in this free volume are directly connected with molecular structure and mobility of water in the crystalline and amorphous states. "We provide structural evidence for this mechanism, which also might provide incentives to further explore the use of trehalose and other carbohydrates in bioprotection," he said. By optimising the molecular packing of carbohydrates, said Dr. Ubbink, scientists can influence and thereby reduce the rate of oxidation of food ingredients such as flavours and polyunsaturated fatty acids (PUFAs). Source: LWT - Food Science & Technology Published on-line ahead of print, doi: 10.1016/j.lwt.2006.12.011 "Aroma of dehydrated pear products"
Authors: D. Komes, T. Lovric and K. Kovacevic Ganic