Coffee grown in the United States represents less than 1% of the world's supply, and most of it comes from the state of Hawai'i. Kona coffee, grown on the western side of the island of Hawai'i, is the most recognized and the highest value Hawaiian coffee. The majority of this coffee is sun-dried after harvest and washing.

Sun-dried coffee should reach 12-13% moisture within 4 to 6 days. Sun-drying will reduce both the moisture content and the water activity (aw). Reducing aw below 0.75, especially in the first week of drying, is important for preventing or limiting mold growth. The purpose of this study was to 1) compare drying rates of Kona coffee bean batches using aw and moisture content % measurements, 2) evaluate factors affecting the drying time of sun-dried Kona coffee, and 3) provide recommendations for post-harvest processing of sun-dried Kona coffee to optimize quality and safety.

Ten farms in the Kona coffee region of Hawai'i were visited in the fall of 2017 to record data on the drying rate of coffee bean batches and to record observations on the post-harvest handling and storage of coffee beans and the environmental conditions that may affect the quality and microbial contamination of drying coffee. The coffee drying surfaces, physical enclosures, fan use, and elevation varied among farms. Daily measurements of coffee moisture level, water activity, depth of bean layer and temperature were recorded along with air temperature, relative humidity and cloud cover during drying for 30 batches.

Most sun-dried batches reached 13% moisture in 6 to 10 days. Initial moisture content (31.6 4.3%), and drying yard characteristics varied greatly among farms. Coffee batches reached 0.75 aw within 6 days on average, but some batches required more than 10 days. Moisture content and aw measurements were weakly correlated and water activity level increased at times during drying for some batches. Allowing airflow around drying beans and maintaining a bean layer depth of less than 5 cm appeared to improve drying rates.

Drying coffee parchments within 7 days post-harvest can inhibit growth of molds that may impact quality or molds that could produce mycotoxin. Controlling the drying conditions, including raking the layered beans, and monitoring moisture content can accelerate, or improve the consistency of, batch drying time.