Dry beans with coloured markings on a white background – cranberry and pinto beans – are prone to darkening after harvest, which can cause their price to be discounted. So Peter Pauls at the University of Guelph (UofG) is leading a research effort to develop nondarkening varieties for Ontario.
Post-harvest darkening is an issue all along the value chain, right up to the consumer. “Pinto and cranberry consumers are quite knowledgeable about our products and demand a bright and natural colour for their product use. The visual stimulation of the cranberry and pinto colours is a big factor in consumer purchases,” notes Brad Chandler, commercial business manager for the Hensall District Co-operative (HDC), a farmer-owned, diversified agri-business that includes dry bean processing and marketing.
Darkening is influenced in part by how the beans are handled and stored; it tends to happen faster if the beans are exposed to light, heat and high humidity.
“Let’s say I spill some cranberry beans on the ground when I’m harvesting. If they’re left out in the sun for a day, they will be brown,” says Mike Donnelly-Vanderloo, who chairs the Ontario Bean Growers’ research committee. “In a very short time, cranberry beans can go from a nice creamy white with these beautiful reddish-maroon stripes, a very colourful bean, to a tan to medium brown bean where you can hardly distinguish the stripes. Pintos have the same problem; when they darken, you don’t see the natural colour patterns of the bean.”
With proper handling and storage, the beans are much slower to darken. “Typically, bean moisture and storage conditions are key factors to modifying colour loss, and we take great efforts to ensure that our beans are at optimum moisture and in proper warehouse storage,” Chandler says.
However, it may be a year or more before the beans are consumed, so pinto and cranberry beans that never darken would be an advantage.
Chandler notes, “Having [nondarkening varieties] will ensure our consumers receive a consistent, bright-coloured product, regardless of the harvest date. HDC would expect to see retail and food service sales strengthen as consumers gain an awareness of the lack of colour deterioration.”
From Donnelly-Vanderloo’s viewpoint, the key advantage of nondarkening beans for growers is risk reduction. He observes that Ontario crop insurance rates for cranberry bean are higher than those for other dry beans. For instance, the maximum coverage available for cranberry bean is 80 per cent and costs $33.24 per acre. For white beans, the maximum coverage is 85 per cent and costs only $16.91 per acre. “Part of the reason for the higher insurance for cranberry beans is because they can deteriorate in the field quite quickly, especially if harvest is delayed. They really weather and part of that has to do with this darkening,” he says.
Donnelly-Vanderloo is especially interested in the possibility of nondarkening cranberry beans because of the importance of this bean to Ontario growers. In 2014, cranberry bean (also called Romano bean) was the second most commonly grown dry bean in the province.
Breeding brighter beans
To breed nondarkening cranberry and pinto beans, Pauls’ research team began by screening hundreds of bean lines. “We ordered bean germplasm from around the world to look for ones that didn’t darken,” notes Pauls, who is a professor and chair of the UofG department of plant agriculture.
“You can accelerate the darkening by exposing the beans to ultraviolet light, sort of like a sun tanning light. If you do that overnight, the beans will be brown the next morning, so it is an easy screen to sort through material,” he explains.
“We found one accession from the Fort Collins germplasm bank that didn’t darken at all. It was a very pale cranberry bean, with very pale pink markings on a white background, and that white background didn’t change at all even when exposed to the UV light.”
Unfortunately, the line they found wasn’t really a commercial bean and had some undesirable traits. So the researchers began crossing it into elite pinto and cranberry bean lines to bring the nondarkening trait into those lines.
The pinto breeding work has proceeded relatively quickly. Pauls says, “The normal breeding timeline, from the time you make a cross to the time it has a commercial impact, is typically about eight to 10 years. It has been six years since we made the initial cross, and we now have a nondarkening pinto line in the provincial bean trials for yield.”
This nondarkening pinto line has beans that look like regular pintos, plus the plant is an upright bush. He adds, “Pintos are notorious for being viny and growing along the ground, but this line has a nice plant architecture that’s suited to direct combining.”
Development of nondarkening cranberry bean lines is going a little slower. The researchers now have nondarkening lines, but so far the red stripe on the seed is not quite as intense as it is on a typical cranberry bean.
Donnelly-Vanderloo emphasizes Pauls’ research team is using traditional plant breeding, so these are non-GMO beans, which is particularly important in European markets. “A lot of Ontario beans head off to Europe, with the cranberry beans going to Spain, Portugal and Italy.”
Assessing nutrition and more
In addition to the breeding work, Pauls’ team is looking into several consumer-related aspects of nondarkening beans. One component involves evaluating the beans’ taste and cooking characteristics. Pauls explains consumers care about the darkness of pinto and cranberry beans because they associate darkness with older beans, and they associate older beans with harder-to-cook traits, like longer soaking and cooking times. So the researchers are starting cooking and taste trials to see if nondarkening beans differ from darkening beans in these characteristics.
Another component of this research is investigating how the nondarkening trait relates to the health benefits of the beans. Studies by other researchers have indicated that darker coloured beans appear to have higher levels of antioxidants, which are beneficial for human health. So Pauls’ group is wondering if post-harvest darkening might be related to higher antioxidant levels.
To answer that question, their nondarkening and darkening lines are being included in some broader studies to evaluate the health effects of beans, led by Krista Power, a research scientist with Agriculture and Agri-Food Canada (AAFC) in Guelph. Currently, Power and her research associate Jennifer Monk are examining the effects of beans on gut health, using mice feeding trials. A recent trial assessed the effects of beans on colitis, a type of inflammatory bowel disease. “Those tests show a protective effect of beans against that severe intestinal inflammation, and this beneficial effect occurs whether the beans are darkening or nondarkening,” Pauls says.
That’s good news, and Pauls is hopeful that the ongoing health studies will continue to show that nondarkening beans are just as healthy as regular beans.
His team is also digging deeper into the genetic and biochemical aspects of the nondarkening trait. For instance, with the genetic work, the researchers are already fairly certain that a single gene controls whether or not a bean will darken after harvest. They are now conducting a number of studies, with both the cranberry bean and the pinto bean, to find out more about that gene, such as determining exactly where it is located along the plant’s DNA, and developing DNA markers to help breeders select for the nondarkening trait more efficiently.
The research on nondarkening pinto and cranberry beans involves many players. Mohammad Erfatpour, one of Pauls’ PhD students, is playing a major role in the pinto work, examining the genetics of the nondarkening trait as well as conducting the cooking and taste tests in collaboration with Lisa Duizer, who is an associate professor in the food science department at UofG. Masters student Dana McRobert is working on the genetics of the nondarkening cranberry beans. Pauls’ colleague Gale Bozzo is working with PhD student Jose Freixas Coutin to investigate which genes are turned on and which are turned off in nondarkening. Mahbuba Siddiqua, a post-doctoral researcher in Pauls’ lab, is looking into other aspects of gene expression. And AAFC research scientist Rong Cao and his graduate student Peter Chen are conducting biochemical analyses of the compounds involved in the darkening reaction.
All this work may seem like a lot of effort going into a single trait, but this research is helping to shed light on many factors that are very important for high quality dry bean varieties such as seed coat appearance, nutritional value and cooking characteristics.
“People make a lot of decisions about what to purchase based on what a product looks like. In beans, appearance is important because it defines a bean’s market class. Very small differences in the genetics can define those classes. But at this point we don’t understand a single one of those traits in enough detail to be able to [identify the particular genes involved]. For plant breeding, we would like to understand the traits at that level,” Pauls says. “This research gives us tools in terms of making the breeding easier. And it also helps us to try to anticipate advantages or disadvantages of the traits we are working with.”