Introduction

Pectin is not only a vital component of cell walls that provides strength to plant stems; emerging evidence suggests that pectin and other cementing materials are critical for determining the yield and quality of plant stem fibres [23].

Removal of pectin is the key to the retting process

Pectin, an acidic polysaccharide found in the cell wall of plants, contains both soluble and insoluble components. The soluble components foster microbial proliferation and aid in the establishment of the microbial community necessary to dissolve the remaining pectin components of the stem during the initial stage of water retting. Hence, the water-soluble fraction was found only in the nonretted stems (0.1% of the stem dry weight), and this fraction was absent in the total pectin content extracted from the midretted and retted fibre samples. Once the pectin-rich middle lamella is removed from around and between the fibre bundles, the bast fibres begin to separate from the core of the epidermis and one another.

The pectinolytic bacteria present in an aquatic environment are responsible for the breakdown of pectic materials and the subsequent release of fibres; as a result, the water retting procedure consistently yields excellent quality fibres in contrast to other retting techniques [24]. Previously, flax was also retted by enzyme mixtures comprising polygalacturonase from various fungal sources to investigate the effects on the qualities of the fibre. These findings demonstrated that retting, which causes pectin to break down around and between fibre bundles, can be caused only by the homogalacturonan areas of the flax stem wall, which are unmethylated or have a low degree of esterification [25]. Ruan et al. (2015) [9] defin

ed the term ‘degumming rate’ as the rate of change in the pectin content in flax stems during water retting. The only factor that contributed to the weight loss of the flax stems during water retting was the dissolution of pectin and other noncellulosic components. In our study, the amount of pectin in the jute stems changed significantly after 7 days of retting, accounting for approximately 78% of the total pectin degradation, and after 10 days of retting, the degumming rate exceeded 93%. The separation of the fibre bundles from the core was also significantly enhanced by the disintegration of pectin-rich tissues around the bast fibre bundles.

Ultrastructural changes in jute stems during water retting

The gradual decomposition of pectin-rich parenchyma and the separation of fibre cells from the other bast tissues were observed via light microscopic examination of LS and TS sections of retted, mid-retted, and non-retted bast fibres and stems. Jute stems that were not retted showed intact structural arrangement. By releasing degradative enzymes into the environment over time, microbial colonization alters the structural integrity of bast fibre cells, causing the retting process to proceed. Morphological analyses using atomic force microscopy (AFM) of the flax cell walls during retting, revealed that the distance between fibres gradually increased with the disappearance of the middle lamellae, demonstrating the progressive action of enzymes on the digestion of the middle lamellae [26]. The progressive stages of microbial colonization and ultrastructural alterations of hemp stems during water retting were reported in a study by Fernando et al. (2019) [8]. Another study on the influence of chelating agents and mechanical pretreatment of flax fibre showed that during enzymatic retting, fibre bundles are separated from the core and cuticle and further dissociated into elementary fibres [27]. A study highlights the efficacy of a microbial food supplement in enhancing the growth of retting microbial populations, consequently resulting in improved bundle strength and finer fibrils, ultimately leading to higher fiber quality [28]. In our study, the preliminary stage of microbial water retting in jute stems lasted for nearly seven days. At this stage, microbial colonization occurred, and the fibre bundles began to separate from the phloem cells and the epidermal layer, while maintaining their structural integrity, with undamaged phloem present between the bundles. The middle lamellae connecting the fibres contain less methylated pectin and exhibit increased resistance to pectin-degrading enzymes, which could be the cause [29]. At the later stage of retting, the fibres finally separate. Fibre bundles become isolated from the remaining underlying tissues and from one another after 10 days of microbial water retting. The pectin-rich parenchyma is completely disintegrated by pectin-degrading enzymes produced by the bacterial consortia.

Microbial colonization and bacterial-fungal interactions during retting

A recent study investigated the microbial dynamics during the retting process of jute using a metagenomic approach across three phases: pre-retting, aerobic retting, and anaerobic retting and identified dominant pectinolytic microflora in retting process [30]. The clear evidence provided in our study by the advanced microscopy images established the colonization of rod-shaped Bacilli and their interaction with retting water fungi. During the progression of retting, bacterial colonization on the inner and outer surfaces of the bark differed considerably. The first colonization point was when the outer surface was in direct contact with the retting water. At approximately day 3, rod-shaped Bacilli were visible on the surface, in contrast to the samples from day 0. Bacterial ingress to the inner bark takes some time, and SEM micrographs of mid-retted (day 5- day 7) bark samples revealed the establishment of microbial colonies on the inner surfaces and distorted exterior surfaces covered in microbial mats and fungal spores. Micrographs of the retted fibre samples showed almost complete dissolution of the pectineus matter. Figures 3 and 4 also show the presence of bacterial biofilms and fungal spores on the bark. This unequivocal evidence suggests that bacterial colonization begins on the outer surfaces, after which the bacteria enter the inner bark, peaking within 5–7 days, and by 10 days, complete removal of pectineus material occurs under ideal conditions. This allows the possibility of designing suitable techniques and quickly washing and recovering intact fibres and thus may save up to 5–7 days of retting time.

Conclusions

Retting is a complex procedure with a notable influence on the ultimate quality of the jute fibre. Inadequate separation of fibers and weakening are consequences of both underretted and overretted fibers, respectively.Using a combination of microscopic and biochemical techniques we examined the structural and biochemical changes that occur during the water retting of jute fibres.

The study showed that the bacteria-mediated sequential breakdown of pectin around fibre bundles is the principal incident during water retting of bast fibres. It was observed that the pectin acts as a glue, binding the fibres together and preventing their separation. During the retting process, the pectin undergoes enzymatic degradation, leading to breakdown of the glue and separation of the fibres. The study also revealed that the retting process leads to a significant decrease in the pectin content in the jute stem. This change in composition may have important implications for the processing and properties of the fibres, and microbial breakdown of pectin is of paramount importance.

In conclusion, this study provides new insights into the retting process of bast fibres, shedding light on the structural and biochemical changes that occur during this critical step in the production of natural fibres. The findings of this study could be useful for improving the efficiency and quality of the retting process and for develo** new techniques for the production of natural fibres.