Kinetic study of lipid distribution after washing with lipases: Microscopy analysis
Authors: Obendorf S. Kay , Mejldal Rie , Varanasi Arindam , Thellersen Marianne 
Published: November 2006

Effect of lipase on removal of lard from a cotton fabric by detergency was studied in detail by radioisotope and X‐ray microanalysis over 60‐min washing. Osmium tetroxide was used to label the soil for microscopy. Backscattred electron images were used to study residual soil present on and within the cotton fibers and in the interfiber spaces of the yarn bundle. Lard‐soiled samples had large deposits on the fabric surfaces; oil was present in the crenulation, secondary walls, and the lumen of the fibers. Relative concentrations of oil were determined for selected morphological locations within the fiber structure and at the fiber surface using X‐ray microanalysis. Specimens soiled with lard and tagged with osmium tetroxide immediately from the wash bath at 0, 5, 10, 20, and 60 min provide snapshots of the washing process. Within the yarn structure, domains with high oil concentration developed during washing. These domains were not adsorbed onto the fiber surfaces, but were contained within the twisted yarn structure. The inclusion of lipase “accelerates” the washing process with the dislodgment of the soil from the fiber surface being more complete for the yarn specimens washed with lipase for 20 and 60 min. The three main stages of the soil‐removal process—transport of water and detergent to the soil on the substrate, separation of soil and substrate, and transport of dislodged soil into the wash bath—were observed. Lipase functions in detergency by hydrolysis of triglycerides forming mono‐ and diglycerides and fatty acids that are more soluble in water, can undergo soap formation, enhance mesomorphic phase formation, and increase emulsification. The quantification of the mass of the residual oil is well‐defined using the radioisotope methodology. The effectiveness of lipase on removal of lard stains is observed at all points in the washing cycle (5, 10, 20, 60 min). Our data fit a classical kinetic description of the soil‐removal process described by an exponential decay, with the presence of lipase greatly increasing the rate of oil removal during the early period of washing (0 to 20 min). With the use of lipase in the washing bath, the amount of soil removal after 10 min is about the same as observed after washing 60 min with detergent only. Washing removes soil located on the fibers and yarns on the fabric surfaces early in the process and more completely. Soil that is located within the yarn structure is removed by washing but this requires more time, and lard removal from these interfiber capillaries is greatly enhanced by use of lipase. Washing does remove soil from within the cotton fiber, i.e. secondary wall and lumen, and this is also more complete with the use of lipase. After washing with a detergent for 60 min, the highest concentration of residual soil is in the lumen, although the concentration has been reduced by a factor of two. It appears that the lumen is the last region of the cotton fabric structure to be cleaned by laundering. The use of lipase in the wash bath increased the rate of soil removal for all locations with the cotton fabric and fiber structures.