D-optimal mixture design optimization of a natural date syrup jelly with an orange albedo powder–lemon juice suspension: quality attributes and processing-induced citrus pectin modifications

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This study optimized a natural, additive-free jelly formulated from date ( Phoenix dactylifera L.) syrup (DS), orange (Citrus sinensis) albedo powder (OAP), and lemon ( Citrus limon ) juice (LJ) using a constrained D-optimal mixture design. The study also investigated whether the mild acid–heat process involved in jelly manufacture could induce partial and potentially favorable structural modifications of citrus pectin. To balance nutritional quality (sugars, vitamin C) and texture (hardness), the optimal formulation was 75% DS (75 °Brix), 2% OAP, and 23% LJ (w/w), yielding a hardness of 10 N, pH 3.46, total sugar content of 79.01 g/100 g (mainly sucrose), 40 mg/100 g vitamin C, total phenolic content of 220.95 mg GAE/100 g, and a specific antioxidant activity of 34.5%. The optimized jelly was rich in K, Mg, and Ca (715, 339, and 206 mg/100 g, respectively). Structural characterization of the optimized jelly pectic material using FTIR, 1 H NMR, and GC-MS monosaccharide profiling provided evidence of processing-induced remodeling, including partial de-esterification, increased structural heterogeneity, and changes in HG- and RG-I-related features, suggesting that gelation was driven mainly by HG-rich domains. Neutral sugar analysis mainly reflected co-extracted DS sugars, while OAP-associated neutral sugars (galactose, arabinose, mannose) supported a contribution of OAP-derived RG-I domains. Overall, beyond yielding an additive-free jelly with desirable nutritional and structural properties, the results suggest that the jelly-making process may generate, in situ, a food matrix containing partially modified pectic material of potential biofunctional relevance, while highlighting orange albedo powder as a multifunctional citrus co-product.