Summary

Light green or tan faba beans attract the highest prices in whole-bean export markets and unlike other grain crops, bean hull colour continues to darken after harvest. This project investigated hull browning in storage and confirmed that it is influenced by both storage conditions and variety. Significant genetic differences were found between commercial faba bean varieties in how quickly hull colour darkens, particularly at higher storage temperatures. Exposure to heat, oxygen, humidity and light accelerated browning, while cooler, darker and low-oxygen conditions slowed browning. While a simple, low-cost post-harvest treatment to reduce browning was not identified, the results point to clear storage management principles and highlight that selecting hull colour stability as a trait in future breeding programs could help extend market opportunities.

Background

Hull colour is a key quality trait in faba beans and an indicator of freshness in high-value export markets, particularly in the Middle East. Beans with darker brown hulls are often downgraded as old crop, significantly reducing their value. Currently, the premium marketing window for Australian faba beans is limited to about nine months after harvest. Growers and traders rely heavily on cool storage to preserve colour, significantly increasing storage cost and complexity. A better understanding of why hull browning occurs, and whether there are varietal differences in browning, is critical to improve grain value and access to markets.

Research Aims

The core objectives of the project were to:

• Identify the biochemical processes driving hull browning in stored faba beans
• Assess whether low-cost post-harvest treatments could slow hull colour change
• Determine whether commercial faba bean varieties differ in their susceptibility to storage browning.

In The Field

A series of controlled storage experiments was conducted using commercial faba bean samples of PBA Samira sourced from Australian Grain Export receivals following the 2023 harvest. A broader set of 20 varieties grown at Kapunda, Roseworthy and Narrabri was also tested. Beans were stored for up to 12 months under a range of temperatures and conditions designed to test the effects of heat, oxygen, humidity and light. A protocol was developed to measure colour change of faba bean hulls with a digital camera and light box. Accelerated storage at higher temperatures was used to reveal differences in browning rates. Seed coat samples were also analysed using advanced laboratory techniques to investigate which compounds may be involved in colour change.

Results

The project clearly demonstrated that hull browning during storage is not uniform across varieties. Some commercial varieties showed rapid browning, while others remained significantly lighter after extended storage, even under high temperatures. PBA Samira and PBA Marne consistently showed faster and more severe browning, while varieties such as Ascot and PBA Warda were more stable. Importantly, these varietal differences were consistent across growing environments for PBA Warda, PBA Samira and PBA Marne which suggests a genetic basis rather than a seasonal effect. However, Ascot hull colour was more influenced by the environment.

Storage conditions had a strong influence on the rate of hull browning. Temperatures above 30°C, exposure to oxygen, high humidity and sunlight all accelerated colour change. Beans stored under cooler, darker and low-oxygen conditions maintained lighter hull colour for longer. Treatments that restricted oxygen exposure, including vacuum or alternative gas environments, were particularly effective, while an oxygen gas treatment markedly accelerated browning.

The results suggest that much of the browning process is due to oxidation of compounds in the seed coat. While no single compound was clearly identified in this project, future work to identify the compounds involved could help researchers to develop effective strategies to reduce hull browning.