APPLE JUICE PASTEURIZATION USING ULTRAFILTRATION AND PULSED ELECTRIC FIELDS E. ORTEGA-RIVAS, E. ZA Â RATE-RODRI Â GUEZ and G. V. BARBOSA-CA Â NOVAS* Postgraduate Programme in Food Science and Technology, University of Chihuahua, Chihuahua, Chih, Me Âxico *Biological Systems Engineering Department, Washington State University, Pullman, USA A pple juice was treated by means of non-thermal processing methods as a way of verifying whether a microbial stable and quality-acceptable product could be obtained. Ultra®ltration and high electric voltage pulsed electric ®elds were the techniques used. For ultra®ltration, the factors under study were membrane pore (10,000 and 50,000 daltons), trans-membrane pressure (103, 120.5, 138 and 155 kPa) and recovery percentage (0, 25, 50 and 75% for the 10,000 daltons membrane, as well as 0, 10, 20, 30, 40, 50 and 60% for the 50,000 daltons membrane). In terms of the pulsed electric ®elds experiments, electric ®eld strength (50, 58 and 66 kVcm ±1 ) and number of pulses (2, 4, 8 and 16) were the factors investigated. The responses to these factors were evaluated for microbial determinations, namely: aerobic plate count, yeasts and moulds, aciduric bacteria, and quality attributes such as pH, acidity, soluble solids and colour. Appropriate statistical comparisons were made between types of membrane and techniques. Multiple linear regression models were ®tted by means of a step- wise technique. Both membrane treatments and pulsed electric ®elds technique showed that microbial inactivation is possible and ef®cient. Except for colour, quality attributes were kept unaltered. Signi®cant relative colour changes were observed for both techniques such as browning for the ultra®ltered juice and fading for pulsed electric ®eld treatment. Keywords: non-thermal pasteurization; membrane separation; high voltage pulsed electric ®elds INTRODUCTION Ultra®ltration is essentially a sieving or ®ltering process dependent largely on the size and shape of the compounds present for its separation ability. By selecting a proper membrane, sterile permeates are produced since micro- organisms are unable to penetrate such a membrane, while valuablenutrient and ¯avour componentsmay be recovered. This technique has good ¯exibility in terms of the availability of various membrane con®gurations and processing modes. The separating capability of ultra®ltra- tion can be identi®ed in terms of the rejection coef®cient of a membrane against a speci®c molecular weight. The rated molecular weight cut-off de®nes the smallest particle or molecule which can be retained by a membrane. There are available membranes in a number of increments in the molecular weight cut-off range from 1000 up to 100,000 daltons. The importance for processing of some liquid foods lies in the fact that these membrane characteristics will give speci®city in terms of permeating soluble sugars and ¯avour components while retaining suspended solids and large molecules. Since micro-organisms, spores and other parti- culates responsible for spoilage are retained, diverse liquid foods treated by ultra®ltration are said to be `cold sterilized’ 1 . Conventional nomenclature of ultra®ltration membranes includes two pre®x letters (for example UM, PM and XM) which refer to different polymers, and two last digits which indicate the nominal molecular weight cut-off. For instance, a commercial range of ultra®ltration mem- branes would cover UM-05 to XM-50, which would mean the retention of macromolecules from about 5,000 daltons (UM-05) to, approximately, 50,000 daltons (XM-50). The ¯ow rate through ultra®ltration membranes can be deter- mined by: Q w KA t D P 1 Q w is the ¯ow across the ultra®ltration membrane, K is a membrane permeability constant, A is the membrane area, t is the membrane thickness and D P is the pressure drop through the membrane. High voltage pulsed electric ®eld (PEF) treatment is a promising non-thermal processing method that may radi- cally change liquid food preservation technology. Treating liquid foods with high voltage pulsed electric ®elds may inactivate micro-organisms and enzymes with only a small increase in temperature, simultaneously providing consu- mers with safe, nutritious, and fresh-like quality foods. PEF treatment is conducted at ambient temperature for a short time (in microseconds), and energy lost due to heating of foods is minimized. Study of PEF inactivationdemonstrated that, for achieving seven log reduction in survivability of S. cerevisiae in apple juice, PEF utilized less than 10% of the electric energy for heat treatment 2 . Most of the earlier studies with PEF treatment were carried out using only small sample amounts 3±6 . To continue the development of 193 0960±3085/98/$10.00+0.00  Institution of Chemical Engineers Trans IChemE, Vol 76, Part C, December 1998