A New Review Finds Grape Stem Browning Starts Before Fruit Shows Damage

A review published Wednesday in OENO One says one of the most visible signs of grape decline after harvest may begin in the stem long before the fruit itself looks damaged.

The paper, by Yaser Khandani and Mahmoud Koushesh Saba, examines rachis browning, the darkening and drying of the green stem structure that holds grape berries together. The disorder is especially important in table grapes because consumers often read a green rachis as a sign of freshness and a brown one as a sign of age or poor handling. The authors describe it as a major postharvest problem that can cut shelf life, reduce market value and increase waste across the supply chain.

The review brings together current research on the biological and storage-related causes of browning and on the measures growers, packers and exporters use to slow it down. According to the paper, the problem is driven by a mix of oxidative stress, enzymatic oxidation, cell degradation and microbial activity. Water loss is central. Once grapes are cut from the vine, the rachis loses moisture faster than the berries because it is made of more exposed, herbaceous tissue. The review says the rachis can respire at a rate 11 to 28 times higher than the berries, making it more vulnerable to dehydration and faster aging.

That process sets off other changes. As tissue dries and weakens, enzymes including polyphenol oxidase and peroxidase help turn phenolic compounds into brown pigments. Chlorophyll also breaks down, removing the green color that signals freshness in the market. Reactive oxygen species, which are associated with stress in plant tissue, can damage membranes and accelerate senescence. The review also points to a role for hormones. Even though grapes are considered non-climacteric fruit, the rachis appears to produce more ethylene than the berries, and ethylene is linked to faster browning and tissue decline. Other hormonal imbalances may push the stem further toward discoloration.

The practical effect reaches beyond appearance. Studies cited in the review link rachis browning to more berry drop, greater sensitivity to handling damage and a decline in quality during storage. Browned or brittle stems can make clusters less stable in transport and more exposed to pathogens and dehydration. The review notes that grapes with visible rachis browning are often discounted, moved into lower-value channels or thrown away even when the berries are still firm and sweet.

That matters in a market where visual standards are strict and trade routes are long. The paper notes that table grapes account for about 36% of total grape production worldwide, with major markets in China, India and the European Union. Exporters including Peru, Chile, Italy, the United States, South Africa and China depend on fruit arriving with premium appearance. The authors argue that preserving rachis quality is not just a cosmetic issue but part of protecting returns in a highly competitive trade.

The review says cold storage remains the main line of defense, but it is not enough on its own. Storage near 0°C, or more broadly 0–2°C, combined with relative humidity of 90–95%, can slow respiration and reduce water loss. Yet cold rooms do not fully stop browning, and they do not eliminate fungal threats such as Botrytis cinerea. The paper says pre-cooling soon after harvest is critical because it removes field heat and helps maintain firmness and moisture balance before longer storage or shipping begins.

Atmosphere management is another common tool. Controlled-atmosphere storage and modified-atmosphere packaging can lower oxygen and increase carbon dioxide around the fruit, slowing respiration and reducing the activity of browning-related enzymes. The review notes that mild carbon dioxide enrichment in the 15–20% range has shown benefits, though overly high levels can trigger off-flavors. Some studies have also tested elevated oxygen environments of 70–80% to limit microbial growth and preserve firmness, showing that the best atmosphere may depend on variety and storage conditions.

Sulfur dioxide fumigation remains widely used in commercial handling, mainly because it helps control decay, but the review places it among conventional tools rather than a complete answer to stem browning. Packaging design has also become a larger part of the strategy. Polyethylene and polypropylene liners help retain moisture, while microperforated films aim to balance humidity and gas exchange so clusters do not dry out or collect excessive condensation.

The review gives special attention to newer approaches that are being explored as the industry looks for more sustainable options. These include edible coatings, plant-derived compounds, ultraviolet C light, ultrasound, cold plasma and nanotechnology. Some coatings made with materials such as pectin or polyvinyl alcohol can act as a barrier to moisture loss while also carrying antioxidant or anti-browning compounds. Ethylene inhibitors such as 1-methylcyclopropene, or 1-MCP, have also shown an ability to delay stem aging in some trials.

The paper also points back to vineyard decisions. Heat stress, irregular irrigation and nutrient imbalances involving nitrogen, potassium and calcium can leave rachis tissue more prone to damage after harvest. Mechanical injuries during picking, packing and transport add another layer of risk because damaged tissue is more exposed to oxidation. The authors say better postharvest results will likely depend on combining field management with storage, packaging and handling changes rather than relying on a single treatment.

Although the review is centered on table grapes, its findings may also matter to the broader beverage business because grape quality decisions begin before fruit reaches any market channel. Better control of stem dehydration and postharvest decline can help companies across grape supply chains decide how to manage fruit, storage and transport, and may have implications for juice and wine operations when grapes face delays or extended handling. The authors also point to research gaps around cultivar-specific responses, treatment combinations and the commercial scaling of newer technologies, areas that could shape how growers and packers manage grape quality in the years ahead.