Bernardi, Amita (2002) The use of isothermal membrane distillation in tomato processing. PhD thesis, Victoria University of Technology.

Abstract

This thesis is an investigation of tomato paste preparation by the use of isothermal membrane distillation (IMD). Traditional vacuum evaporation processes used for tomato paste production operate at high temperatures for long periods and can cause thermal damage to heat sensitive components and loss of volatile flavour and aroma compounds. In addition to these changes, other undesirable browning reactions such as the formation of hydroxymethylfurfural (HMF) can occur. IMD is a low temperature membrane process used for the concentration of heat sensitive pharmaceutical products. Its use in the food commenced with the concentration of grape juice for winemaking. Although IMD can concentrate grape juice up to 70% total solids level, its use is limited to clarified juices due to membrane materials currently available. Tomato juice contains a considerable amount of suspended solids hence clarification of the juice is necessary for subsequent concentration processes. Primary clarification involved the use of centrifiigation to fractionate the tomato juice into solids and semm components. The tomato serum was unsuitable for concentration by IMD since it still contained low levels of suspended solids hence required secondary clarification. Ultrafiltration (UF) or microfiltration (MF) were two membrane processes used for secondary clarification of tomato serum. The resultant UF or MF permeates were subsequently processed by IMD or by linked reverse osmosis (RO)-IMD processes. The linked processes were chosen because RO had the capability of preconcentrating the IMD feed which reduced the duration of the IMD process. The IMD concentrate was then recombined with the tomato solids obtained from centrifugation to produce two different tomato pastes of 25% and 30% total solids levels. The recombined pastes were then compared with commercial concentrates by the use of taste panels. Since tomato pastes are often used on pizza bases, the pastes were also assessed on pizza bases. Physiochemical properties of colour, consistency, ascorbic acid and hydroxymethylfurfural of the recombined and commercial pastes were also examined and compared. The microbiological status of these pastes was also assessed to confirm that they were safe for consumption.

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