Polyphenol Release from Wheat Bran Using Ethanol-Based Organosolv Treatment and Acid/Alkaline Catalysis: Process Modeling Based on Severity and Response Surface Optimization

Abstract

Wheat bran (WB) is globally a major food industry waste, with a high prospect as a bioresource in the production of precious polyphenolic phytochemicals. In this framework, the current investigation had as objectives (i) to use ethanol organosolv treatment and study the effect of acid and alkali catalysts on releasing bound polyphenols, (ii) establish linear and quadratic models of polyphenol recovery based on severity and response surface, and (iii) examine the polyphenolic composition of the extracts generated. Using sulfuric acid and sodium hydroxide as the acid and the alkali catalyst, respectively, it was found that the correlation of combined severity factor with total polyphenol yield was significant in the acid catalysis, but a highly significant correlation in the alkali-catalyzed process was established with modified severity factor, which takes into consideration catalyst concentration, instead of pH. Optimization of the process with response surface confirmed that polyphenol release from WB was linked to treatment time, but also catalyst concentration. Under optimized conditions, the acid- and alkali-catalyzed processes afforded total polyphenol yields of 10.93 ± 0.62 and 19.76 ± 0.76 mg ferulic acid equivalents g−1 dry mass, respectively. Examination of the polyphenolic composition revealed that the alkali-catalyzed process had a striking effect on releasing ferulic acid, but the acid catalysis was insufficient in this regard. The outcome concerning the antioxidant properties was contradictory with respect to the antiradical activity and ferric-reducing power of the extracts, a fact most probably attributed to extract constituents other than ferulic acid. The process modeling proposed herein may be valuable in assessing both process effectiveness and severity, with a perspective of establishing WB treatments that would provide maximum polyphenol recovery with minimum harshness and cost.

Hydrothermal Treatment of Wheat Bran under Mild Acidic or Alkaline Conditions for Enhanced Polyphenol Recovery and Antioxidant Activity

“…As can be seen in Figure 3 , alkaline hydrolysis resulted in an extract enriched in ferulic acid (FA), which, based on earlier studies, was the anticipated outcome [ 29 ]. However, p -coumaric acid ( p -CA), which may usually accompany FA, was not detected.…”

“…The Y TP values attained with 10% citric acid and sodium carbonate were 23.76 and 23.60 mg FAE g −1 DM, respectively. These yields may be considered exceptionally high, in the light of recent studies that reported levels of 19.76 ± 0.76 mg FAE g −1 DM, achieved with hydroethanolic solutions and sodium hydroxide catalysis [ 29 ]. In general, it has been demonstrated that the conventional extraction of WB with commonly used solvents could afford only low Y TP levels, i.e., 0.84 mg FAE g −1 DM for aqueous ethanol extraction [ 30 ], 4.66 mg GAE g −1 DM for 50% acetone extraction [ 31 ], and 5.90 mg caffeic acid equivalents g −1 DM for deep eutectic solvent extraction [ 32 ].…”

“…Raising CSF’ from 7.67 to 8.37 did afford a significantly higher Y TP . In a recent study, where sulfuric acid was used as the acid catalyst, the maximum Y TP found was 10.96 mg FAE g −1 DM, achieved at an CSF’ of 7.93 [ 29 ]. Based on this finding, it could be argued that the treatment carried out with 10% citric acid was far more effective, with lower severity.…”

“…This finding was in accordance with previous examinations on WB polyphenol extraction with hydroethanolic mixtures and pressurized liquid extraction, which demonstrated that severity was linked to total polyphenol yield with a linear and statistically significant correlation [ 36 ]. Additionally, more recent examinations on ethanol organosolv WB treatment showed that Y TP displayed statistically significant linear correlation with CSF’ only when the process was acid-catalyzed, whereas for the alkali-catalyzed process, such a relationship was significant when the modified severity factor (MSF) was considered [ 29 ]. In that study, maximum Y TP was achieved with alkaline catalysis, at CSF’ of 7.63.…”

“…Thus, an alkaline hydrolysis methodology was employed to serve as a reference procedure [ 18 ]. The protocol implemented was based on a previous one [ 17 ], with modifications dictated by recent studies, pertaining to methanol incorporation into the reaction mixture and the adjustment of sodium hydroxide concentration and temperature [ 29 ]. The exact mass of 1.0 g of WB was mixed with 10 mL 60% methanol containing 2 M NaOH, and the mixture was stirred for 4 h at 40 °C.…”