Fermentation increased anthocyanin release from grape marc by up to 63.3%, study finds

Researchers reported in a study published Sept. 10 in the journal Waste Management that lactic acid fermentation can change the phenolic makeup of grape marc, the skins, seeds, and other solid material left after winemaking, in ways that may improve the release and bioaccessibility of some compounds during digestion.

The work focused on phenolic compounds, a broad group of plant molecules that includes anthocyanins and flavonoids and is often linked to antioxidant activity. Grape marc is produced in large volumes by wineries and is usually treated as a low-value by-product or waste stream, even though it still contains a significant amount of these compounds after pressing.

According to the study, researchers fermented grape marc with lactic acid bacteria under controlled conditions and then used a simulated in vitro digestion model to test what happened to the transformed compounds during gastric and intestinal phases. The goal was to see not only how fermentation changed the original phenolic profile, but also whether those changes affected the compounds’ stability and potential availability for absorption after digestion.

The researchers found that fermentation led to clear shifts in the chemical profile of the grape marc. Complex polyphenols were broken down into simpler forms, a process the authors described as biotransformation. The study said this step appeared to increase the amount of some compounds in forms that are easier to access during digestion than their native state in unfermented material.

One of the reported effects was a rise in anthocyanin release of up to 63.3% compared with solvent extraction, according to the monitoring summary of the paper. The study also found improved bioaccessibility of flavonoids after the in vitro digestion stage. At the same time, the authors said the stability of phenolic compounds during digestion was not uniform. Some metabolites remained more stable and accessible than others as the samples moved through the simulated digestive phases.

That variation is important because the presence of a compound in a raw ingredient does not necessarily mean the body can use it after consumption. Many plant compounds degrade, bind to other molecules, or change structure as they pass through the digestive system. By testing the fermented grape marc after simulated digestion, the researchers aimed to measure a more practical outcome than simple chemical concentration alone.

The study adds to a growing body of work that looks at fermentation as a tool for upgrading food and beverage by-products. Lactic acid fermentation is already widely used in food processing, and its role here was to act on compounds already present in winery residue rather than to create an entirely new ingredient from scratch. The paper suggests that this treatment could raise the nutraceutical value of grape marc by changing phenolics into forms with better potential availability.

The authors also framed the findings as part of broader waste management and circular economy efforts in the wine industry. Grape marc is one of the largest solid residues generated during winemaking. Finding higher-value uses for it could reduce disposal pressures while opening a market for ingredients derived from production leftovers. The paper said the results support the valorization of grape marc as a functional food ingredient and point to a more sustainable way to handle winery waste.

For the beverage sector, the results may draw interest because they suggest a possible route for wineries and other drink makers to recover more value from grape by-products instead of treating them mainly as waste. If the findings are confirmed in further research, fermented grape marc could become a source of functional ingredients for beverage formulations, nutrition products, or related applications. That would offer a potential link between waste reduction and product development, although the present study did not test commercial beverage use directly.

The work also comes at a time when producers are facing stronger pressure to show environmental gains from manufacturing and to make fuller use of agricultural raw materials. In wine production, grape marc has long been reused in limited ways, including composting, animal feed, distillation, and extraction of specific compounds. The new study suggests fermentation may provide another processing path, especially when the aim is to improve how phenolic compounds behave after digestion rather than only how much of them can be extracted in a laboratory solvent system.

The researchers did not present the findings as proof of health effects in people. The digestion stage was simulated in vitro, which means the experiment reproduced stomach and intestinal conditions in a controlled laboratory model rather than in human subjects. That makes the results useful for screening and comparison, but it also means the outcome cannot be treated as direct evidence of clinical benefit. Human digestion, metabolism, gut microbiota, dose, and food matrix effects can all change how compounds behave in real consumption settings.

Even so, the study offers a clearer view of what happens to grape marc phenolics after fermentation and during digestion, two steps that are often studied separately. By linking those stages, the authors were able to show that fermentation does more than alter composition on paper. It can also affect whether the resulting compounds remain available through digestion, which is a key issue for any future food or beverage ingredient based on winery residue.