"Research on Dehydrated Fruit Leathers: A Review"

NATALIA ANDREA QUINTERO RUIZ; SILVANA DEMARCHI; SERGIO GINER

Atenas

Congreso; 11th International Congress on Engineering and Food; 2011

National Technical University of Athens, School of Chemical Engineering

Justification. Fruit leathers are pectic gels obtained by dehydrating fruit purees to attractive flexible sheets which retain shape and are eaten as snack/dessert. Home preparation is usual to preserve fruit, while  some commercial brands offer thin bars of diverse composition, with or without real fruit solids. Investigations on fruit leathers began in 1978 and kept an irregular pace, except in the last five years. A review of the research on fruit leathers may benefit industry and consumers.  Objective. The aim of this work is to review published research on fruit leathers to summarize available information on formulations, sorptional equilibrium, drying kinetics and quality indices. Methodology. Not all fruits are succesfully converted into fruit leathers. Sucrose or glucose syrup are usually added to the puree to increase sweetness, solids content, and to reinforce the sugar-acid-high methoxyl pectin gelation, for which a pH below 3.5 is recommened. At low pH, carboxyl groups of pectin are undissociated and participate in hydrogen bonds that support gel structure.  Fluid-like formulations 5-10 mm thick are placed in trays and dehydrated in hot-air dryers at 60-80 ºC and 2-4 m/s. Drying times of 4-8 h were published. In longan fruit, a convective-far-infrared combined drying was also tested.Results. Research articles on fruit leathers do not always describe strict pectic gels:  pestil  is formulated with concentrated grape juice plus wheat starch, before drying. A durian leather of pH = 5.8 was reported, so its gelation mechanism may have not been pectic.  A mango product formed with soy proteins and skim milk was also termed a leather. Home dehydration enthusiasts in the USA coined the term ?fruit leather? but dried apricot paste, a leather-type product, has their roots in the Middle East. In any case, pureed fruit was used as main raw material to form product by pectic gelation, a concept that may be useful for food regulations.  Sorption isotherms of leathers are J-shaped, with low temperature effect. Drying kinetics requires differential equations for modelling the coupled mass and heat transfer with shrinkage. Product characteristics often reported are moisture content (13- 25 % w/w), water activity (0.52 -0.69), pH ( 3.4 -3.8 in pectic gels), surface color and sensory attributes. Few authors reported final thickness (1-2.7 mm). The glass transition temperature (Tg) was related with textural aspects. Loss of antioxidant capacity was reported for Granny Smith apple leathers, being 73% during drying at 50 ºC and 47% (of the remainder) after 7 months storage. The use of potassium metabisulphite (MBK) reduced those values to 57% and 16% respectively.  In this leather, Maillard browning was also inhibited by MBK (100 ppm SO2 in the final product), which extended the storage period to at least 7 months for product packaged with barriers to light, water vapor and gases. Implication. Fruit leather studies are useful to industry and society. These products add variety to a healthy diet and possess dietary fibre, vitamins and minerals while providing a good energy intake. Food research may also be improved in many ways.