Concentration of passion fruit juice on an industrial pilot scale using osmotic evaporation
Introduction
For economic reasons (reduced transport and storage costs), fruit juices are routinely concentrated. This is especially true in the case of tropical fruit juices for which centres of production and consumption are normally far apart geographically. Classical thermal concentration techniques lead to subsequent losses of aromatic compounds and vitamins. Especially for tropical fruits, which are usually valued for their distinctive aromas, these losses are a serious marketing problem. For passion fruit, Casimir, Kefford and Whitfield (1981) have reported important losses of the initial aromatic compounds when classical concentration was applied, even when an aroma recuperation unit was used.
Additionally, technological improvements to thermal concentration methods, while lessening the damage they cause, have more or less reached their peak. Despite improvements, thermal processing continues to lead to an inevitable loss of flavour and nutrients, and the resulting concentrates tend towards the low-quality end of the market. Meanwhile, a demand for fruit juices with better conserved nutritional and sensory qualities is increasing in industrialised countries (Ganlmann, 1993). During the last three decades, efforts have been made to develop new technologies such as cryoconcentration and reverse osmosis that would more satisfactorily conserve the original qualities of thermosensitive aromatic fruit juices. Nevertheless, these methods have been relatively less used in industry because of the difficulties in reaching juice concentration levels beyond 40 g TSS/100 g Gostoli, 1998, Jariel et al., 1996.
Osmotic evaporation (OE) is a relatively new technology based on the use of a hydrophobic microporous membrane to separate two liquid phases that differ greatly in terms of solute concentration Deblay, 1991, Hogan et al., 1998, Lefebvre, 1988. The membrane’s hydrophobic nature prevents penetration of the pores by aqueous solutions, creating air gaps within the membrane. The difference in water activity (Aw) between the two sides of the membrane induces a partial pressure gradient in the vapour phase. Vapour is transferred across the pores from the high-vapour pressure phase to the low one. This transfer is isothermal. OE can be carried out at low temperatures without the need for a pressure differential, thus improving preservation of volatile compounds (Barbe, Bartley, Jacobs, & Johnson, 1998).
Osmotic evaporation has been studied mainly in the laboratory, using sucrose solutions, orange, grape or tomato juices Courel et al., 2000, Durham & Nguyen, 1994, Sheng et al., 1991. Some work on an industrial level has been carried out on the concentration of wine production in Australia Johnson et al., 1989, Thompson, 1991 but little is known implement OE on a process line. The aim of this study is to evaluate the potential of OE for concentrating clarified passion fruit juice on an industrial scale, taking into account the relevant impact on the overall product quality.
Section snippets
Passion fruit juice and quality evaluation
Processed in the PASSICOL S.A. plant (Chinchiná, Colombia), raw passion fruit juice was clarified, using a crossflow microfiltration plant, fitted with a 0.2 μm ceramic membrane (Vaillant, Millan, O'brien, Dornier, & Decloux, 1999). To reduce viscosity and facilitate microfiltration, the juice was first liquefied with enzymes. The clarified juice was then stored at −20°C until needed.
Juice was analysed for total soluble solids (TSS) content, titratable acidity and density, using standard AOAC
Preliminary tests with water
The first set of experiments was carried out with tap water at around 30°C to evaluate the evaporation performance of the pilot plant. The evaporation flux fluctuated between 0.72 and 0.81 kg h−1 m−2, whereas the water temperature fluctuated between 28°C and 31°C, and the brine concentration fluctuated between 5.1 and 5.6 M. The relative pressures inside both the concentrate loop and the brine rig remained constant, at around 0.1 bar, which corresponded to the pressure drop on both sides of the
Conclusions
Osmotic evaporation readily concentrates a clarified juice up to 60 g TSS/100 g, a value that is higher than is obtained with other ‘low temperature’ concentration techniques. When juice concentration reaches 40 g TSS/100 g, evaporation flux is only 12% less than the flow rate initially registered with water. Continuous OE with constant extraction of concentrate and feeding with extemporarily processed, fresh, clarified juice is more hygienic and performs well. The same procedure can be done at
Acknowledgements
The authors wish to thank Victor Amu R. for his valuable technical help, Passicol S.A. (Chinchina, Colombia), Colciencias (Colombia) and the French Embassy in Santa Fe de Bogota for providing this project with financial assistance.
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