Non-enzymatic Browning Kinetics of Tamarind Seed Coat Proanthocyanidins System Affected by Food Formulation Factors

This study was executed to investigate the non-enzymatic browning kinetics and key phenolic composition of tamarind seed coat proanthocyanidins extract (TSPA) with different food formulation factors in order to elucidate the effects of food formulation factors and thermal storage on the key proanthocyanidin compounds, and to infer the possible mechanism of non-enzymatic browning. TSPA, prepared with 55% (V/V) ethanol, had total phenolic content of (397.1 ± 3.1) mg/g, mainly consisting of total proanthocyanidins (tannins) at (218.3 ± 3.9) mg/g. The ultra high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) results indicated that the main proanthocyanidin components of TSPA were procyanidins B2 (PCB2), epicatechin (EC), protocatechuic acid (PCA), and caffeic acid hexoside (CAH). The results of zero-order browning kinetics showed that under different conditions of TSPA concentration, ethanol concentration, pH, and food additives (metal ions except Cu²⁺, thickening gums, and protectants), the browning rate constant (k₀) of TSPA was 0.0207–0.1581 d^-1 (R² = 0.969–0.999) during storage at 55 ℃ for 6 days. Addition of 1.0 mg/mL ginseng flower extract significantly (P < 0.05) reduced the browning degree of TSPA (at 1 mg/mL in 5% (V/V) ethanol). At pH 3.0–3.8 browning was reduced. Addition of 5 mmol/L metal ions significantly reduced the content of phenolic compounds in TSPA; Ca²⁺ and Cu²⁺ reduced the contents of PCB2 and EC by 96%–98%, decreased PCA content by 33% and 38%, respectively, and Na⁺ and Zn²⁺ reduced phenolic content by 10%–25%. The results of principal component analysis indicated that (-)-epicatechin-3,5-digallate (ECdG), PCA, PCB2 and EC were key for browning. Possible responses related to changes in the phenolic composition of TSPA were roughly deduced. This study may provide a theoretical basis for understanding the non-enzymatic browning mechanism of TSPA and controlling it.