Development of a microbial bioassay system for detection of boric acid using Paecilomyces variotii.

Boric acid is a water soluble chemical preservative that has been used as food preservative by some local manufacturers. This chemical is used to preserve food products such as noodle and fish ball in order to inhibit the growth of microorganism, so that the preserved food can stay fresh and longer. However, its usage is prohibited by government of Malaysia as boric acid is considered harmful to human health if consumed in a considerably large quantity. Therefore, the detection method for boric acid is important. To date, no study has been performed to detect boric acid by using microorganism as sensing element. Hence, this study was aimed to develop a simple, fast and environmental friendly bioassay system incorporated with Paecilomyces variotii as bioreceptor for detection of boric acid in food. This detection system was based on the measurement of the changes of β-glucosidase produced by the microorganisms in response to the presence of boric acid. The changes of β-glucosidase concentration were assayed spectrophotometerically and correlated to the concentration of boric acid. In this system, P. variotii was grown in cellobiose medium for two days before its mycelia were entrapped in calcium alginate in bead form. In order to optimize the best condition for β-glucosidase production, the important factors such as initial pH, temperature, amount of cell loading, concentration of sodium alginate and calcium chloride were determined. The system was found to show optimum β-glucosidase production when 2% (w/v) sodium alginate and 0.25 Molar calcium chloride were used. Maximum enzyme production was also obtained with initial pH 7 and temperature 45 °C, using 6% (w/v) mycelia after three hours of incubation. By using these optimum operating conditions, a lower detection limit of 0.037% (w/v) was obtained from a linear range of 0% to 0.215% (w/v). The reproducibility of the system was acceptable with an observed relative standard deviation of 4.96% (n=10) and 4.81% (n=10) in the presence of 0.2% (w/v) boric acid and absence of boric acid, respectively. The bioassay system was then applied to determine boric acid in fish ball and the results of recovery ranging from 61% – 86% were recorded for boric acid spiked at different concentrations of boric acid from 0.05% to 0.20% (w/v). The developed microbial bioassay system not only represents a simple, inexpensive and environmental friendly alternative for determination of boric acid, but also offers a new idea and promising approach to detect boric acid.

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