Effects of Gamma Radiation on Shiga Toxin-producing Escherichia coli Inoculated in Spinach (Tetragonia expansa) and on Sensory Characteristics

Introduction: Vegetables, including spinach, have been associated with many foodborne outbreaks. Gamma radiation can be used to reduce the population of pathogenic bacteria on food or to increase the shelf life. However, depending on the dose used, it can cause undesirable changes in sensorial attributes of food.

Purpose: To evaluate the efficacy of irradiation on the reduction of Shiga toxin-producing E. coli, and the effect on the sensory characteristics of minimally-processed spinach.

Methods : Samples of spinach (Tetragonia expansa) were inoculated with a cocktail of three strains of Shiga toxin-producing E. coli O157:H7 (10⁷–10⁸ CFU/ml) and exposed to doses of 0, 0.2, 0.4, 0.6, 0.8 and 1.0 kGy. Surviving bacterial populations were evaluated using a non-selective medium (Tryptic Soy Agar) with an overlay of Sorbitol MacConkey agar plus cefixime and potassium tellurite. Samples of non-spiked spinach exposed to 1.0 and 1.5 kGy and control sample, kept under refrigeration (4 °C) for up to 12 days were evaluated for appearance, flavor, smell and texture.

Results : The D₁₀ values ​​determined for Shiga toxin-producing E. coli was 0.17 kGy. All samples were accepted by the sensory panel with no statistically significant differences (P < 0.05) among the control and test samples.

Significance: The results showed that gamma radiation can be used for improving the quality of minimally-processed spinach without impairing its sensory characteristics.