Summary 

This study tested how well a natural coating called chitosan could help preserve the quality of ‘Somerset Seedless’ grapes during cold storage. Grapes were treated with chitosan mixed with acetic acid and compared to two reference treatments (acetic acid alone or plain water) over seven weeks of storage in a commercial cooler. Chitosan reduced mold and decay while keeping the grapes looking fresh, without affecting internal quality like sugar or acidity. However, due to the high cost of ingredients and labor, this method may not be practical for small-scale vineyards. For most growers, grapes should be sold within three weeks of harvest. This research offers a promising, though currently costly, option for extending fruit shelf-life. 

Introduction 

‘Somerset Seedless’ is a cold-hardy table grape suited for northern climates. While it has good flavor and texture, there’s little research on how to store it after harvest. Common storage problems include mold, berry splitting, and stem browning. Chitosan, a natural substance derived from shellfish or fungi, has been used on other fruits to reduce spoilage. Both sources are flavor-neutral and non-allergenic. This study tested whether chitosan could help preserve ‘Somerset Seedless’ grapes during storage. 

Study design

Grapes were harvested at peak ripeness and divided into three groups: 

  • Chitosan (1.4%) coating, prepared by dissolving chitosan powder in 1% acetic acid* 
  • Acetic acid (1%) – control 
  • Distilled water – control 

Each treatment was applied by dipping grape clusters for 15 seconds, then air drying them before placing them in cold storage (1–4°C, high humidity) for seven weeks. Grapes were checked regularly for appearance, weight loss, sugar, acidity, and antioxidant levels. 

* Chitosan requires an acidic environment to dissolve and therefore is not soluble in water at neutral pH. Acetic acid is commonly used because it lowers the pH enough to break down the amino groups in chitosan, making it soluble in water. 

Findings

  • Chitosan-treated grapes had the least mold, decay, and berry splitting after three or more weeks in storage. 
  • Acetic acid helped control mold but caused more berry damage, increasing the percentage of berries splitting open and falling off the stems after a month or more of storage. 
  • Water-treated grapes showed the most spoilage and would not meet USDA standards after three weeks. 

  • All grapes lost some weight over time, but there were no major differences between treatments. 
  • Sugar (°Brix), pH, and acidity stayed stable across all groups. 
  • Chitosan didn’t affect the grapes’ internal quality.  

  • Antioxidant levels declined slightly over time, which is expected in postharvest fruit storage. 
  • No treatment significantly changed antioxidant or flavonoid content. 
  • Chitosan preserved quality without reducing nutritional value.

Conclusions

Chitosan is a promising postharvest treatment for cold-hardy grapes like ‘Somerset Seedless.’ It helps reduce mold and decay while maintaining appearance and internal quality. Acetic acid alone is less effective and may cause damage. This is the first study to test chitosan on this grape variety, and it offers a potential solution for large-scale growers looking to extend shelf-life and reduce losses. 

However, the economic feasibility of this treatment is limited. Chitosan is relatively expensive—ranging from $1.60 to $2.36 per gram—and preparing enough solution to treat even small batches of grapes can cost $3–$5 per pound of fruit. Additionally, labor costs are high, as clusters must be dipped and hung individually to dry. These factors make the method impractical for small-scale vineyards. 

For small growers, the best strategy remains selling grapes within three weeks of harvest, especially if only rinsed in water. Future research could explore more cost-effective formulations or automation methods to make chitosan treatments more accessible. 

Funding acknowledgement 

The work was supported by the Montana Department of Agriculture [AM22SCBPMT1127].