Pressure-induced pH changes in aqueous solutions – On-line measurement and semi-empirical modelling approach

Investigating the microbial inactivation effect of low temperature high pressure carbon dioxide and its application in frozen prawn (Penaeus vannamei)

During the pandemic of coronavirus disease 2019, the fact that frozen foods can carry the relevant virus raises concerns about the microbial safety of cold-chain foods. As a non-thermal processing technology, high pressure carbon dioxide (HPCD) is a potential method to reduce microbial load on cold-chain foods. In this study, we explored the microbial inactivation of low temperature (5-10 °C) HPCD (LT-HPCD) and evaluated its effect on the quality of prawn during freeze-chilled and frozen storage. LT-HPCD treatment at 6.5 MPa and 10 °C for 15 min could effectively inactivate E. coli (99.45%) and S. aureus (94.6%) suspended in 0.85% NaCl, SARS-CoV-2 Spike pseudovirus (>99%) and human coronavirus 229E (hCoV-229E) (>1-log virus tilter reduction) suspended in DMEM medium. The inactivation effect of LT-HPCD was weakened but still significant when the microorganisms were inoculated on the surface of food or package. LT-HPCD treatment at 6.5 MPa and 10 °C for 15 min achieved about 60% inactivation of total aerobic count while could maintain frozen state and quality of prawn. Moreover, LT-HPCD treated prawn exhibited significant slower microbial proliferation and no occurrence of melanosis compared with the untreated samples during chilled storage. A comprehensive quality investigation indicated that LT-HPCD treatment could maintain the color, texture and sensory of prawn during chilled or frozen storage. Consequently, LT-HPCD could improve the microbial safety of frozen prawn while maintaining its original quality, and could be a potential method for food industry to improve the microbial safety of cold-chain foods.

Free of salt high-pressure deliming of animal hides

The wastewater pollution of tanneries is of high concern. The investigation of technologies to minimize the consumption of chemicals in the leather production process can reduce the environmental burden. We focus on the reduction of ammonium salts in the leather production process. Salt-free deliming of animal hides with compressed carbon dioxide as deliming agent is performed for the first time in a technical scale 20-L drum. As a result, CO₂-deliming at 30 bar and 30 °C is two times faster than conventional deliming. In addition, the deliming efficiency is slightly improved. The initial calcium (Ca) content of the hides of 8 g/kg reaches the lowest value of 2 g/kg after a process time of 3 h. However, a process time of 60 min is sufficient to reach an elimination of 50 wt% of the initial lime. The resulting Ca-content of 4 g/kg after 60 min CO₂-deliming at 30 bar is comparable with the Ca-content of conventional delimed hide. We clarify that the ampholytic character of the collagen itself enables a buffering of the pH-value at pH-7. The stable pH-value supports the selection of specific bating enzymes that decompose non-collagen proteins. No buffering salts contaminate the wastewater. The high-pressure CO₂-deliming process has high potential to reduce wastewater emissions, save costs for chemicals, and process time in industrial beamhouse applications.

Novel silica surface charge density mediated control of the optical properties of embedded optically active materials and its application for fiber optic pH sensing at elevated temperatures

Silica and silica incorporated nanocomposite materials have been extensively studied for a wide range of applications. Here we demonstrate an intriguing optical effect of silica that, depending on the solution pH, amplifies or attenuates the optical absorption of a variety of embedded optically active materials with very distinct properties, such as plasmonic Au nanoparticles, non-plasmonic Pt nanoparticles, and the organic dye rhodamine B (not a pH indicator), coated on an optical fiber. Interestingly, the observed optical response to varying pH appears to follow the surface charge density of the silica matrix for all the three different optically active materials. To the best of our knowledge, this optical effect has not been previously reported and it appears universal in that it is likely that any optically active material can be incorporated into the silica matrix to respond to solution pH or surface charge density variations. A direct application of this effect is for optical pH sensing which has very attractive features that can enable minimally invasive, remote, real time and continuous distributed pH monitoring. Particularly, as demonstrated here, using highly stable metal nanoparticles embedded in an inorganic silica matrix can significantly improve the capability of pH sensing in extremely harsh environments which is of increasing importance for applications in unconventional oil and gas resource recovery, carbon sequestration, water quality monitoring, etc. Our approach opens a pathway towards possible future development of robust optical pH sensors for the most demanding environmental conditions. The newly discovered optical effect of silica also offers the potential for control of the optical properties of optically active materials for a range of other potential applications such as electrochromic devices.

Carbon dioxide induced silk protein gelation for biomedical applications

We present a novel method to fabricate silk fibroin hydrogels using high pressure carbon dioxide (CO(2)) as a volatile acid without the need for chemical cross-linking agents or surfactants. The simple and efficient recovery of CO(2) post processing results in a remarkably clean production method offering tremendous benefit toward materials processing for biomedical applications. Further, with this novel technique we reveal that silk protein gelation can be considerably expedited under high pressure CO(2) with the formation of extensive β-sheet structures and stable hydrogels at processing times less than 2 h. We report a significant influence of the high pressure CO(2) processing environment on silk hydrogel physical properties such as porosity, sample homogeneity, swelling behavior and compressive properties. Microstructural analysis revealed improved porosity and homogeneous composition among high pressure CO(2) specimens in comparison to the less porous and heterogeneous structures of the citric acid control gels. The swelling ratios of silk hydrogels prepared under high pressure CO(2) were significantly reduced compared to the citric acid control gels, which we attribute to enhanced physical cross-linking. Mechanical properties were found to increase significantly for the silk hydrogels prepared under high pressure CO(2), with a 2- and 3-fold increase in the compressive modulus of the 2 and 4 wt % silk hydrogels over the control gels, respectively. We adopted a semiempirical theoretical model to elucidate the mechanism of silk protein gelation demonstrated here. Mechanistically, the rate of silk protein gelation is believed to be a function of the kinetics of solution acidification from absorbed CO(2) and potentially accelerated by high pressure effects. The attractive features of the method described here include the acceleration of stable silk hydrogel formation, free of residual mineral acids or chemical cross-linkers, reducing processing complexity, and avoiding adverse biological responses, while providing direct manipulation of hydrogel physical properties for tailoring toward specific biomedical applications.