Phenolic compounds, xylose, and glucose production from vine shoots through sequential microwave-assisted processing

Vine shoots (VS) are an abundant lignocellulosic biomass whose valorization is crucial for advancing the circular bioeconomy in the grape and winery sectors. This study evaluated the effects of three process configurations for VS valorization using microwaves for extraction and pretreatment to obtain phenolic compounds and fermentable sugars. The three process configurations were: an acid pretreatment with 0.8% H₂SO₄ at 160°C for 5 min, and two sequential processes consisting of an extraction with 69% ethanol at 80°C for 4 min followed by acid pretreatment, and an extraction with 58% ethanol at 155°C for 3.5 min followed by the same pretreatment. The phenolic profiles of the extracts and pretreatment liquors were acquired by high-performance liquid chromatography (HPLC) at 280 nm. Moreover, the hemicellulosic sugars were also determined in the pretreatment liquors. Additionally, the pretreated solids were compositionally characterized and subjected to enzymatic hydrolysis for 72 h to obtain glucose.

The standalone pretreatment degraded VS stilbenoids, suggesting that the extraction should be performed as the primary operation in the valorization cascade. The sequential process involving extraction at 155 °C with 58% ethanol, followed by acid pretreatment, enabled obtaining an extract with a total phenolic content of 21.7 mg gallic acid equivalents/g VS (including the stilbenoids trans-resveratrol and trans-ε-viniferin) and showed the highest recovery of hemicellulosic sugars in the liquid fraction, i.e., 95.5 g/100 g of hemicellulosic sugars in raw biomass, mainly xylose. This previous extraction also facilitated subsequent enzymatic hydrolysis, yielding 16.0 g/L of glucose after 72 h (17.2 g of glucose/100 g VS), indicating that enzymatic digestibility increased from 41.2% to 56.7%.

In conclusion, integrating microwave-assisted extraction before pretreatment offers a more efficient route for VS valorization than standalone pretreatment.