Alkali-based pre-treatment may prevent ochratoxin A in grapes

In traditional raisin production process in Turkey, one of the major raisin producers in the world, grapes undergo an alkali treatment prior to drying. This procedure involves dipping grapes in a potassium carbonate solution for 10-15 seconds to remove the wax layer on the surface to accelerate the drying process. In this study, we aimed to investigate the effect of alkali treatment on ochratoxin A (OTA), an important risk factor for grapes and grape-derived products. First, the stability of the OTA was examined under variable pH (4-12), ion type (Na+, K+), ionic strength (0.1-10%) and temperature (4-60 °C). Then,Aspergillus ochraceus contaminated grapes were treated with an alkaline solution and a subsequent OTA analysis was performed through a high performance liquid chromatography-fluorescence detection. OTA started to decompose at pH values exceeding 10.0. The degradation followed a first order kinetics. Although both sodium carbonate and potassium carbonate solutions were at the same alkali pH and they both triggered the OTA degradation at increasing concentrations, potassium carbonate was more effective even at low concentrations. Elevated temperatures were also found to effect degradation. The degradation rate constant at 40 °C was approximately 5 times greater than the one at 25 °C. Furthermore, OTA levels dropped up to 50% in contaminated grapes after the alkali treatment. Our results confirmed that the amide bond of OTA rapidly hydrolysed to a non-toxic ochratoxin-alpha and phenylalanine under strong alkaline conditions. The results suggest that the alkaline pre-treatment unintentionally offers a great advantage to reduce OTA levels in raisins and it could be a viable approach for other dried fruits through the application of good manufacturing practice.