´ INGRID AGUILO-AGUAYO, ROBERT SOLIVA-FORTUNY, AND OLGA MART´N-BELLOSO I
ABSTRACT: The influence of high-intensity pulsed electric field (HIPEF) parameters, namely, pulse frequency, pulse width, and polarity on tomato juice lipoxygenase (LOX) and hydroperoxide lyase (HPL) activities was studied using a response surface methodology. Samples were subjected tosquare-shaped pulses of 35 kV/cm for 1000 μs, with pulse width ranging from 1 to 7 μs at frequencies from 50 to 250 Hz, either in monopolar or bipolar mode. Tomato LOX was more resistant to HIPEF than HPL within the range of assayed conditions. An increase in frequency or pulse width resulted in a decrease of both residual LOX (RALOX ) and HPL (RAHPL ) activities. The lowest RALOX (81%) was observed whentomato juice was treated at 250 Hz for 7 μs in bipolar mode. Moreover, the same conditions led to the highest HPL activity reduction (RAHPL = 10%). A validation of the predictive models determined that 2nd-order expressions were accurate enough to fit the experimental results. Keywords: high-intensity pulsed electric fields, hydroperoxide lyase, lipoxygenase, tomato juice
lavor is one of the mostimportant quality attributes of tomato juices. A number of volatile products can be derived from polyunsaturated fatty acids through the sequential action of lipoxygenase (LOX) and hydroperoxide lyase (HPL). According to the commonly accepted scheme, fatty acids are first oxidized by the action of LOX to their hydroperoxy (HPO) derivatives, which are then cleaved by HPL to form 6-carbon aldehydes(Riley and others 1996). In tomato fruit, LOX has high specificity to form 9-hydroperoxides (9-HPOs) but there are apparently little or no 9-HPLs to act on 9HPOs (Gaillard and Matthew 1977). A much smaller portion of 13HPOs formed in tomato appears to be metabolized further by the action of the 13-HPLs to give rise to C6-aldehydes such as hexanal, cishexanal, and trans-2-hexanal (Smith and others1997). In addition, recent studies have identified a specific isoform of tomato LOX involved in the formation of 13-HPLs (Chen and others 2004). The cited aldehydes lead to fresh ripe tomato aroma due to their low flavor threshold, but at high concentrations rancid flavors resulted (Kazeniac and Hall 1970). Moreover, LOX has been claimed to catalyze the oxidation of carotenoids in tomato by a freeradical mechanism that requires the presence of polyunsatured fatty acids (Cabible and Nicolas 1991). The oxidation products of the enzyme activity (peroxyl radicals) react with carotenoids, thus leading to color degradation (Whitaker 1991). Traditionally, inactivation of enzymes has been achieved by thermal processing. Anthon and Barrett (2003) reported that the inactivation for LOX and HPL oftomato juice would be rapidly inactivated at the cold break target temperature of 60 ◦ C. However, heating can adversely affect the sensory and nutritional qualities of juices (Dunn 2001). As an alternative, high-intensity pulsed electric
field (HIPEF) processing has been successfully applied to inactivate enzymes without significant adverse effects on quality attributes of some juices. Forinstance, it has been demonstrated that HIPEF treatments (35 kV/cm for 1000 μs applying 5.5-μs bipolar pulses at 220 Hz) reached almost complete peroxidase (POD) inactivation in tomato juice while increasing its relative content of lycopene up to 122.5% (Odriozola-Serrano and others 2007; Aguil´ -Aguayo and o others 2008a). Reports concerning the effects of HIPEF on LOX inactivation differ widely,which may be attributed to differences between the properties of the substratum to be treated, processing conditions, and even the technical characteristics of the HIPEF equipment used (Elez-Mart´nez and others 2005). HIPEF processing inactiı vated 54% of LOX in tomato juice when applying bipolar treatment at 40 kV/cm, pulse duration time of 2 μs, pulse repetition rate of 1000 pps and total...