Resumen de Browning reactions during storage of low-moisture Australian sultanas: evidence for arginine-mediated Maillard reactions
Ian Gould, Mary Millikan, Damian Frank
Browning during storage of low-moisture dried Vitis vinifera L. cv. Sultana (Thompson Seedless) grapes was examined in a multifactorial treatment and storage trial. Grapevines were subjected to two different levels of sun exposure, harvested fruit was dipped and subjected to different drying treatments to obtain a range of initial moisture contents (aw = 0.419-0.558). The storage effects of temperature (10°C and 30°C), and the presence of oxygen on colour change (CIE L*a*b* tristimulus values, hue-angle (hab*)) and chroma (Cab*) over a fourteen-month period were observed. The most significant changes in colour were measured for samples stored at 30°C, both aerobically and anaerobically, although the largest changes occurred in the presence of oxygen. Initial aw had a strong effect on colour changes; higher aw non-sunfinished samples underwent more significant browning compared to lower aw sunfinished controls regardless of their oxygen status. Changes in the concentration of the free-arginine and free-proline, the most abundant free amino acids in sultanas, were monitored throughout the storage period. Free arginine decreased significantly at 30°C in both the absence and presence of oxygen, whereas free proline increased (at both 10°C and 30°C), implying that free proline did not play a role in browning reactions at those temperatures. In addition to the decreases in free arginine, the concentration of 5-hydroxymethyl furfural (5-HMF), a marker of Maillard browning reactions, increased significantly in samples stored at 30°C. Significant differences in the concentrations of 5-HMF under the two oxygen conditions indicated sultana Maillard reactions, and possibly other non-enzymatic browning processes, were oxygen sensitive.
