Breaking it Down: How Researchers and Industry are Improving Hemp Retting

September 14, 2022

As the focus on creating an economically-viable hemp fiber market grows in the United States – for use in construction, plastics, textiles, and other industries – stakeholders are increasingly investigating new ways to improve an important part of the hemp fiber production process: retting.

Retting is a process that has been used for centuries in agriculture; for hemp as well as jute, flax, kenaf, and other crops. It involves allowing microbes to break down the chemical bonds that hold together bast fibers from the plant’s hurd, or woody core. “This is necessary for industrial decortication of bast fiber plants, including hemp,” Luke Moe, Associate Professor in the Department of Plant and Soil Sciences at the University of Kentucky, told Hemp Benchmarks. “Microbes break down the ‘glue’ that keeps the fibers attached, which consists of layers of the polysaccharides pectin and hemicellulose.”

What are the Types of Retting?

Traditionally, there are two types of hemp retting. Field or dew retting, as the names suggest, involves leaving the harvested hemp plants in the field for several weeks and allowing them to partially decompose. As the USDA’s Economic Research Service (ERS) notes, hemp farmers must “monitor the process closely to ensure that the bast fibers separate from the inner core without much deterioration in quality.” While moisture is needed for the required microbial breakdown to occur, cultivators must also ensure the retted hemp is dry enough for baling.

The other traditional method is water retting. In this approach, the harvested hemp stalks are immersed in water – rivers and ponds, as well as water tanks or vats – and must also be monitored frequently. According to the ERS, water retting produces more uniform and high-quality fiber, “but the process is very labor- and capital-intensive.” It can also create environmental issues with the waste water.

Still an ‘Art Form’

Morgan Tweet is Chief Operating Officer at IND HEMP, a Montana-based industrial hemp company that processes both hemp fiber and grain. She told Hemp Benchmarks that hemp retting is still “a little bit of an art form.” The process is not very well known in the U.S., she added, and many hemp producers and other stakeholders are still not well-versed in retting. However, Tweet emphasized its importance: “Retting plays a big part in the quality of the product that you’re trying to achieve. For us, specifically, it makes an impact on our yields and capabilities, as well as the finished quality both on the hurd and the fiber side.”

Audrey Law is a biology instructor at Bluegrass Community and Technical College in Kentucky. She is coordinator of the school’s biotechnology program and also co-authored an academic paper on hemp retting with the University of Kentucky’s Luke Moe. The challenge with hemp retting, she told Hemp Benchmarks, is that hemp fiber is made of cellulose and “we don’t want that to degrade – so it becomes a fine line between retting enough to separate the fibers from the plant, and not over-retting to weaken those fibers so they’re less valuable after they’re finished processing.”

How to Make Retting More Consistent and Efficient?

Law believes a lot of improvements still need to be made if the retting process is to become more economically efficient. “The process needs to be more consistent. The more experiences, the better you will be at this.” At the moment, there are concerns about additional cost inputs – “Anything the farmer has to do [for] extras, like spraying an additive over a field,” she noted – given the slim margins faced by hemp farmers. Law explained that researchers are looking at different varieties of hemp, including those found at similar latitudes in Europe, to determine which varieties might be best suited for advancing the retting process.

She also shared that farmers experienced with hemp retting already have a simple field test for their plants, “where you grab the stalks and rip them apart in your hands. It’s pretty easy to determine under-retted [hemp]. The fibers stick together, which is bad for machinery. The trickier part is knowing when it is overdone.” 

Law added that laboratory testing on ways to advance hemp retting have so far yielded only mixed results. “We took some stalks and tried to ret them in the greenhouse under different conditions; there has been some previous research done on adding enzymes,” she said. “But it was not developed on the field scale; not cost-effective or easily done by the farmer.” According to Law, the retting experiments she was involved with also looked at the potential optimal levels of moisture needed to speed up the microbial process.

Understanding Mother Nature and the Biochemical Process

Morgan Tweet at IND Hemp said her company has been experimenting with ways of shortening field retting to several weeks instead of several months’ time. “It’s still a big question mark for us … as far as controlling it,” she added. “It’s all mother nature. Not only is it water, but it’s heat and it’s time. It’s three variables that we have very little control over.”

Since hemp field retting is affected by factors such as climate, day length, and regional moisture levels, researchers are looking at ways to assist the retting process, to make it more uniform and cost efficient. “Once we know more about the biochemical process and which microbes are important in this process, we can start to explore things like microbial inoculants or enzyme applications,” said Luke Moe. 

“Standardizing retting is a key goal for researchers and producers, but we have plenty of work to do on this front. Future research should focus on generating a better understanding of the biochemical processes, and even developing methods for in-field testing of the completeness of retting and fiber quality. Of course this is all contingent on finding appropriate plant varieties and optimizing the agronomic variables. In short, there is plenty of research to be done – and this makes it fun for those of us who are involved in these projects.”

Hemp Industries Association Midwest Hemp Council National Hemp Association