From fields to bioreactors: hemp water retting

words Editorial Team

Italian Hemp Water Retting: monitoring water biological parameters, the new Bioreactor by Prototipo Studio is ready for operations

Hemp water retting: process definition

Hemp water retting is the set of biological mechanisms capable of decomposing the pectins and glues that bind hemp fibers to the stem and bark tissues. The process is described as natural, responsible and positive.

Hemp water retting in water: how does it work?

The fibers are placed in a tank where the water is monitored: temperature, acidity and oxygen level stay under control. The fibers come into contact with the water for a number of days that can vary, but hardly exceed six days. Bacteria, that naturally form in the context of fibrotic stems and water, digest the pectins without damaging the cellulose structure of the fibers.

Retting of hemp in water: empirical tests, protocols, variables, outcome

Both empirically and scientifically, procedures and protocols have been confirmed: PH and oxygen level, as well as water temperature, days the fibers should remain submerged – all of this data have been tested and properly filed. There are variables: the current season, the outside temperature and the sourced water characteristics. The result must be the same: the digested pectins appear as a viscous, grainy mush, which can be easily separated from the cellulose fiber.

Prototipo Studio and the bioreactor for water-retting

Prototipo Studio, an incubator for hemp-related projects supported by Lampoon Publishing House, supports water-retting research. In April 2023, a new bioreactor entered a final construction phase. It is completed in 2024 and it can macerate 100 kilograms of hemp per week. The Bioreactor will further reduce water and energy consumption, already well considered.

Field retting of hemp (dew-retting): the difference between flax and hemp

Field retting (also known as dew-retting) takes advantage of natural moisture in the soil and air during autumn. It is used mainly in northern Europe, where flax processing is widespread and where one wants to apply the proven process to hemp also. One difference that precedes logic: flax has thinner stems resembling grass blades, while hemp stems have a diameter of one to three centimeters.Other issues: taking advantage of the natural moisture in the field encourages fungi, and bacteria – damaging the material. Still: periodic tipping of the stems must be provided to ensure homogeneous action of microorganisms and complete retting of the stems – a time-consuming and resource-intensive operation and complicated for farm machinery.
With rising temperatures and weather irregularities in accordance with the seasons, retting in dew and rainwater is likely to appear even less reliable than it has been so far. The focus is therefore on retting in controlled tanks called bioreactors.

Industrial steeping methods for hemp: chemical intervention: caustic soda

Another method reported to be in use today is hemp retting using chemical solvents. Fibers can be freed using soda, quicklime, acids and alkalis. Caustic soda retting is extensively used in Russia and China (to date the largest hemp yarn producers).
Caustic soda acts on lignin and pectin, almost burning both, leaving the fiber stable in cellulose but poor in organic matter. It results in a fiber similar to flax – without the softness proper to fiber macerated in water. Caustic soda management in the circular economy presents disposal and reuse issues – whose control is being improved.

Historical background and traditions: steeping hemp in water in canals

In addition to dew-retting, another traditional technique for macerating hemp that was once used in Italy involved soaking hemp stems in water inside natural basins and irrigation channels. Arranged in bundles tied together – as if to form rafts – the hemp was left to macerate for three or four weeks, depending on weather conditions. Once collected from the water – which had become stagnant and swampy – the hemp bundles were again dried in the sun for two to three days before being scavenged, that is, freed from the woody inner part of the stems.

At this point, the remaining cortical fiber was ready to be beaten and spun or woven. Nowadays, this method is not considered (it has been abandoned since the 1960s). Water consumption would be excessive; stagnant mills would be unhealthy.