Study Finds Heat Treatment Helps Restore Wine Made From White Rot Grapes

A new study published July 1 in the journal LWT found that red wine made from grapes infected with Coniella vitis, the fungus that causes grape white rot, showed weaker taste and more unwanted aromas, but that two pre-fermentation treatments, thermal maceration and microwave maceration, helped recover part of the wine’s quality.

The research was carried out by scientists from the Shandong Academy of Grape and other Chinese institutions using Moldova grapes grown in Shandong, China. The team examined how white rot affects wine chemistry and aroma and whether infected fruit could still be used more effectively instead of being discarded. White rot is a common vineyard disease in warm and rainy regions and has been linked to annual yield losses of at least 16%.

The authors worked with grapes inoculated with C. vitis and then blended infected fruit with healthy fruit so that 20% of the lot was diseased. That infection level matters because earlier work has suggested that white rot can begin to affect wine quality around that range, although detection depends on the taster and the style of wine.

The study compared four wines in its main trial: a control made from healthy grapes macerated at room temperature, a wine made from infected grapes also macerated at room temperature, a wine from infected grapes treated with heat at 60°C for 6 hours before fermentation, and a wine from infected grapes treated with microwaves at 1024 watts for 3 minutes before fermentation.

The clearest result was that infection changed both taste and aroma. Wine made from infected grapes without any corrective treatment had lower sourness, astringency and saltiness than the control. It also had a flatter overall taste profile. On the aroma side, infection reduced total volatile organic compounds by 20% and pushed the wine toward herbaceous, earthy and chemical notes that are generally considered faults in young red wines.

The researchers found that infection also altered the wine’s phenolic makeup. Total phenols increased in the untreated infected wine, but the composition shifted in ways that mattered for structure and flavor. Some non-anthocyanin flavonoids rose while others fell, suggesting that fungal infection changed how grape compounds were broken down or transformed before and during fermentation. The study also reported increases in compounds linked to plant defense, including resveratrol-related substances.

Color changed less than flavor did. The authors measured standard CIELab color values and found only small differences among treatments. Although heat and microwave maceration shifted some parameters toward less red hue and slightly more brick-red tones, the overall visual difference remained below one CIELab unit, which means most drinkers would be unlikely to notice it by eye.

Where the treatments made a stronger difference was in taste recovery and aroma rebuilding. Compared with untreated infected wine, both thermal and microwave maceration increased several monomeric phenols and improved most taste signals measured by electronic tongue analysis. The gains were especially clear in aftertaste measures tied to astringency and bitterness, while the wines kept an overall balance rather than becoming harsh.

Microwave treatment had the broader effect on taste intensity. According to the paper, it raised nearly all measured taste values in the infected wines. Thermal treatment also improved taste but stood out more clearly for its effect on off-aromas.

The aroma data showed why. Both treatments increased higher alcohols, ethyl esters, aldehydes, fatty acids and total volatile compounds compared with untreated infected wine. Ethyl esters were especially important because they are closely tied to fruity notes in wine. In both treated wines, ethyl ester levels were more than 50% higher than in the untreated infected sample.

Several compounds emerged as key markers of improvement: ethyl butyrate, ethyl lactate and ethyl octanoate all increased after treatment and are associated with fruitier aromas. Statistical modeling also pointed to nonanal and methionol as important compounds in explaining differences among wines.

That distinction mattered because not all changes were positive. In the untreated infected wine, methionol rose above its odor threshold. This compound is associated with cabbage, cooked potato and garlic notes. Nonanal also increased with infection and contributed green or pungent aromas. The study found that thermal maceration was able to suppress the rise of both methionol and nonanal caused by infection, while microwave treatment did not show the same corrective effect on those specific off-notes.

That gave heat treatment an advantage in one important area. While both methods improved aroma compared with untreated infected fruit, thermal maceration produced a profile closer to the control wine made from healthy grapes. Microwave treatment generated a wider range of fruity esters, but it was less effective at reducing some herbaceous or vegetable-like notes linked to disease.

The researchers used principal component analysis and orthogonal partial least squares discriminant analysis to compare all volatile compounds across samples. Those tools showed that wines from infected grapes treated with heat clustered more closely with wines from healthy grapes than did wines treated with microwaves. In practical terms, that suggests thermal maceration may better restore an aroma profile associated with sound fruit.

The paper also adds detail to a broader issue facing growers and wineries: what to do with diseased fruit when infection is present but not total. Fungal diseases reduce yield and quality, but they also create waste when fruit cannot be sold fresh or processed into wine. The authors said their findings offer new evidence that some infected grapes may still be used rationally if winemaking techniques are adjusted early in production.

The work does not suggest that diseased fruit becomes equal to healthy fruit or that every winery should process infected lots without caution. The experiments were done under controlled microvinification conditions with one grape variety, one pathogen strain and one infection level. Still, the results point to a practical direction for wineries dealing with white rot pressure: targeted pre-fermentation maceration may help recover fruit character and reduce some sensory damage.

For producers, the most relevant takeaway may be that the two methods solved different parts of the problem. Microwave maceration strengthened taste expression and boosted fruity esters. Thermal maceration also improved aroma intensity but did more to limit unwanted green and sulfur-like notes caused by infection. In this study, both approaches outperformed standard room-temperature maceration when infected grapes were used.

The authors said future research should look more closely at how C. vitis damages grape quality at the metabolic level and whether yeast strains selected for high ester production could further improve wines made from compromised fruit.