Production and Characterization of Xanthan Gum from Low-Quality Dates of Different Cultivars as a Fermentation Substrate

Approximately 5–33% of the dates growing in Saudi Arabia are downgraded to low-quality fruit that either goes to waste or is used to make animal feed, which is considered a potential feedstock abundant in sugar. By a thorough comparative study, this study evaluated the up-cycling of low-quality Saudi dates (Wannana, Shagra, Sabbaka, Barhi, Saqai, Khalas and Sukkari) as fermentation substrates for xanthan gum production by Xanthomonas campestris. Pure glucose, pure sucrose, and a commercial standard were used as a baseline. The sole carbon source was date juice (≈12.5–17 °Brix) in a batch aerobic fermentation conducted at the standard conditions of temperature (30 °C), speed 180 rpm, and time (120 h). The xanthan gum was quantified and tested for its properties like functional groups (FTIR), color, thermal behavior (TGA and DSC), and rheology in the form of both steady- and dynamic-shear rheology. Xanthan gum was produced in the range 5.60–7.77 g L−1 by the date-based substrates with Barhi (7.77 g L−1) and Saqai (7.58 g L−1) surpassing those of the glucose (6.68 g L−1) and sucrose (6.23 g L−1) controls. FTIR spectra of date-derived gums were almost identical to that of the commercial standard, indicating that their functional groups and the primary structure were very similar. In addition, the date-derived powders were darker and yellower (L* 61.09, 71.00; whiteness index 54.92, 63.34) than the commercial gum (L* 84.71; whiteness index 78.19). This is likely attributed to the presence of residue date pigmentation and products of Maillard and caramelization. Thermogravimetric analyses revealed that the breakdown of materials occurred in two stages, of which the char residue of the date-derived gums was much higher for those degraded at 500 °C (48.93, 54.05%) versus that of the commercial reference (33.49%), which is to be interpreted as a better ability of the former to resist thermal degradation. All solutions acted as pseudoplastic, shear-thinning liquids (flow behavior index n < 1); the consistency coefficient (K) rose with concentration and fell with temperature. The activation energy varied between 9.98 kJ mol−1 (commercial) and 29.39 kJ mol−1 (Saqai). Overall, low-quality Saudi dates can be considered a technically and economically viable, sustainable, and low-cost carbon substrate suitable for upcycling to produce xanthan gum, which is safe for use as a food additive.