Gamma radiation sterilization
Data in Table (1) and Fig. (2) showed that γ-radiation at all tested doses (6 kGy, 12 kGy, or 24 kGy) is an effective technique in the sterilization of paper sheets contaminated with Gram-positive bacteria like Bacillus cereus and Staphylococcus aureus, Gram-negative bacteria like Escherichia Coli and Salmonella typhi, and fungi like Candida albicans .
Dry heating sterilization
The effectiveness of dry heating in the sterilization of paper samples depending on the type of tested pathogenic species, temperature degree, exposure period. The dry heating sterilization using temperature at 100, 150, and 200 ⁰C for 15 mint markedly reduced the number of Bacillus cereus by 99.0%, 99.9%, and 100%, Staphylococcus aureus by 99.0%, 99.6% and 100%, E. Coli by 97.2%,97.7%, 100%, Salmonella typhi 98.3%,98.6 and 100%and Candida albicans by 97.1%, 97.6%, and 100%, respectively compared to the corresponding control (Table 2 and Fig. 3).
Data represented in Table (2) and Fig. (4) showed that temperature at 100 ⁰C for 30 mint inhibitedthe growth of Bacillus cereus by 100% and the number of Staphylococcus aureus, E. Coli, Salmonella typhi, and Candida albicans by99.9% for each mentioned pathogen compared to the corresponding controls. Moreover, 150 ⁰C or 200 ⁰C for 30 mint can destroy all tested pathogenic microorganisms contaminated paper. Besides, dry sterilization using 100, 150, or 200 ⁰C for one hour is an effective method for killing all tested pathogens.
Effect of gamma radiation on paper structure
The scanning electron microscope (SEM) technique was used to study the structure and morphology of the sterilized paper sheets. Each SEM micrograph in Fig. 5 showed the changes in the paper structure after gamma sterilization. Control samples of paper sheets have a high-density of intertwined cellulose fibers, different shapes and sizes, and calcium carbonate agglomerates (Fig. 5a). On the other hand, gamma radiation at 6 kGy caused flatness of the cellulose micro-fibrils, providing a larger surface area (Fig. 5b). In addition, a high-density structure of intertwined cellulose fibers and calcium carbonate agglomerates was observed. However, the irradiated paper sheets with 12 kGY showed a decline in binding joints resulted in the reduction of inter-fiber forces (Fig. 5c). Furthermore, the high dose of γ-radiation (24 kGy) resulted in a severe degree of horniﬁcation (Fig. 5d).
Effect of dry heat sterilization on paper structure
In Fig. 6, SEM images of the dry heating paper sheets for one hour were examined and compared to unheated ones (Fig. 6a). The results showed that the treated sheet with 100°C attained a slight reduction in bonding between cellulose microfibers (Fig. 6b). However, the deformation of cellulose microfiber was detected in the microstructure images of the dry heating paper sheets at 150 °C or 200 °C (Figs. 6c and 6d). Only the 200 °C dry heated sheets gained a severe heterogeneous microstructure as well as a yellow color.