Any cotton imported into Australia is treated to ensure that the consignment is free of live insects, soil and other debris (faeces, animal materials etc) and to verify that any quarantine risk material present will be dealt with during processing. The quarantine treatments used can be either or chemical (fumigation). Following previous studies AQIS have agreed to suspend the treatment of samples with gamma irradiation due to the resultant damage to the physical fibre properties. There are two quarantine treatments currently prescribed by AQIS which involves chemical (fumigation) with ethylene oxide. The two treatments involve fumigation under an initial minimum vacuum of 50 kilopascals at 1200 g/m³ for 5 hours at 50oC or at 1500 g/m³ for 24 hours at 21C.

This study has shown that the fumigation of cotton lint samples with ethylene oxide for 24 hours at 21C and 5 hours at 50C, as per AQIS requirement had no effect on the physical properties (such as length, strength and Micronaire) of the fibre. The study however found that fumigation with ethylene oxide did result in a permanent change in the colour value and subsequently the colour grade of the cotton. In most cases the reflectance value (Rd) decreased while the yellowness (+b) was unaffected, which in essence means that the fibre has become darker. This was most apparent for the Upland USDA cotton Grades 11, 21 and 31 as well as the ELS USDA cotton grades Pima 1A-3B. These changes in the reflectance values will result in the HVI instrument wrongly classifying the cotton one grade higher (i.e. worse than), for example the Grade 11 cotton will be graded 21 and the 21 will be graded 31.This will lead to the instrument failing to calibrate and it will also impossible to qualify the instrument. It is however interesting to note that the Australian cotton which is generally whiter than the US cotton seemed unaffected by the fumigation treatments, with only a slight change to the +b value, with the cotton becoming slightly yellower.

Although the results vary considerably and there are no definite trends, in general this change in the Rd value is related to a decrease in residual ethylene oxide. We surmise that the cause of this could be associated with the ethylene oxide damaging the surface wax layer causing it to become pitted and less smooth resulting in the surface of the fibre reflecting light more diffusely. As the Australian cotton is whiter, with lower +b values it seems that the damage by the ethylene oxide is not as apparent as noted with the US cottons. This hypothesis will be tested in future work.

We recommend that further work needs to be carried out on cotton to further refine our understanding of the effects of the fumigation process.