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 (10oC and 30oC), 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 30oC, 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 30oC in both the absence and presence of oxygen, whereas free proline increased (at both 10oC and 30oC), 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 30oC. 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.

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Sultana grapevines (Vitis Vinifera L. cv. Sultana syn. Thompson Seedless) were subjected to four shading regimes: 50% shading (1), 25% shading (2), fully exposed‐top of canopy (3) and beneath canopy (4) and harvested early (21 February) and late (13 March) in the 1996/1997 sultana season. Grapes from each of the eight field‐treatment combinations represented a range of maturities (14.4 to 23.50oBrix). Grape samples from each of the treatments were dipped and dried to 18% moisture, with half of each of the sultana samples further reduced in moisture by sunfinishing on plastic sheets in direct sun. These field treatments resulted in sixteen unique dried sultana bulk samples with a range of initial chemico‐physical properties; aw (0.481–0.691), skin‐polyphenoloxidase (PPO) activity (4.40–9.05 μmol O2/g.minute) free arginine in skin tissues (1.0–5.10 mg/g) and protein (16.40–27.18 mg/g). Sultanas were stored at 10oC and 30oC in either the presence or absence of oxygen for 10 months, and changes in CIE L*a*b* tristimulus values, hue‐angle (hab*) and chroma (Cab*) were monitored. Significant changes in sultana colour occurred in samples stored at 30oC, especially in higher aw non‐sunfinished sultanas. Although browning was more intense in the presence of oxygen, significant browning also occurred in the absence of oxygen. Lower concentrations of 5‐hydroxy methylfurfural, a key marker of Maillard browning in samples stored at 30oC in the presence of oxygen, indicated that the non‐enzymatic reactions were sensitive to oxygen. Changes in the concentration of trans‐caftaric acid, the main substrate of grape PPO, were also measured during sultana drying. Storage browning (changes in L*, b*, hab*, Cab*)in dried sultanas could be predicted by regression models using pre‐storage aw, free‐skin arginine or Kjeldahl protein after 10 months' storage between 10oC and 30oC. Non‐enzymatic and Maillard‐type reactions (sensitive to both oxygen and aw), made an important contribution to sultana storage browning. We provide only weak evidence that either shaded (immature) or green fruit was more susceptible to storage browning.

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