Structural modification of bacterial cellulose fibrils under ultrasonic irradiation

Highlights

• Short US treatment leads to a decrease in BC fibril dimensions.

• Increase in stability, viscosity and thixotropy of BC suspensions.

• Improvement of the physical properties of BC suspensions.

Abstract

Ιn the present study we investigated ultrasounds as a pretreatment process for bacterial cellulose (BC) aqueous suspensions. BC suspensions (0.1–1% wt) subjected to an ultrasonic treatment for different time intervals. Untreated BC presented an extensively entangled fibril network. When a sonication time of 1 min was applied BC fibrils appeared less bundled and dropped in width from 110 nm to 60 nm. For a longer treatment (3–5 min) the width of the fibrils increased again to 100 nm attributed to an entanglement of their structure. The water holding capacity (WHC) and ζ-potnential of the suspensions was proportional to the sonication time. Their viscosity and stability were also affected; an increase could be seen at short treatments, while a decrease was obvious at longer ones. Concluding, a long ultrasonic irradiation led to similar BC characteristics as the untreated, but a short treatment may be a pre-handling method for improving BC properties.

Introduction

Cellulose is a linear biopolymer of glucose that mainly exists in plants as a structural component of cell walls. Cellulose consists of an amorphous and a crystalline portion. While crystalline cellulose consists of long chains bound together by strong hydrogen bonds, amorphous cellulose is made up of shorter and weaker chains (Türünç & Meier, 2012).

BC and plant derived cellulose have the same chemical structure, but BC is obtained from bacterial species, such as Komagataeibacter sucrofermentans, which have the ability to synthesize pellicles of cellulose, when placed in a culture medium (Martinez-Sanz, Lopez-Rubio, & Lagaron, 2011; Okiyama, Shirae, Kano, & Yamanaka, 1992). This pellicle consists of a bundle of fibrils of about 4 μm wide, which are composed of random nanofibrils less than 100 nm wide (Okiyama, Motoki, & Yamanaka, 1993).

BC has unique physicochemical properties such as higher water holding capacity, higher crystallinity and higher purity as it does not associate with lignin and hemicelluloses, in contrast to plant derived cellulose (Iguchi, Yamanaka, & Budhiono, 2000; Martínez-Sanz et al., 2013; Salas, Nypelö, Rodriguez-Abreu, Carrillo, & Rojas, 2014). Thanks to these properties, BC has been receiving increased attention and has been used in various areas such as biomedicine, cosmetics, paper industry and many others (Iguchi et al., 2000). Although not extensively used in food yet, BC has great potential as a food ingredient, changing the rheological profile of a food, as it serves as thickening, stabilizing or gelling agent. Recently, BC has been shown to act as a stabilizer in emulsions (Kalashnikova, Bizot, Cathala, & Capron, 2011; Paximada, Koutinas, Scholten, & Mandala, 2016; Paximada, Tsouko, Kopsahelis, Koutinas, & Mandala, 2016).

One of the reasons why BC is not systematically used in the food industry is its low ability to be dispersed into water (Agoda-Tandjawa et al., 2010; Lowys, Desbrières, & Rinaudo, 2001). In the food industry the thickeners have to be well-dispersed in order to be more acceptable by the consumers (McClements, 2005), while BC suspensions present pronounced particle aggregation due to Van der Waals attractions and hydrogen bonds (Kuijk et al., 2013).

A number of technological approaches have been developed to enhance the physical properties of the colloidal suspensions of polymer fibrils. The most commonly used method is to submit polymer to controlled acid hydrolysis conditions (Hirai, Inui, Horii, & Tsuji, 2009; Martinez-Sanz et al., 2011; Olsson et al., 2010). However, this is of high energy and cost process that causes intense degradation of the polymer and hence the industry would have had benefit from cheaper alternative methods.

Chemically less aggressive concepts could be the mechanical treatment of cellulose, such as a high pressure homogenization which is used to treat microfibrillated cellulose (MFC) resulting in changes in the microstructure of the cellulose (Agoda-Tandjawa et al., 2010; Saito, Nishiyama, Putaux, Vignon, & Isogai, 2006).

What is more, high-intensity ultrasound (16–100 kHz, 10–1000 W cm⁻²) has immense potential for structural and functional properties of cellulose modification. By this method, the energy of ultrasound is transferred to the polymer chains through a process called cavitation, which is the formation, growth and violent collapse of cavities in the water. Therefore, the effect of ultrasound is related to cavitation, heating, dynamic agitation, shear stresses, and turbulence (Vilkhu, Mawson, Simons, & Bates, 2008). Recently, structural and functional changes in ultrasound irradiated plant cellulose, have been reported by (Dehnad, Emam-Djomeh, Mirzaei, Jafari, & Dadashi, 2014; Liu and Yang, 2008, Wang and Cheng, 2009). These authors reported that the controlled depolymerization of plant cellulose can be achieved by employing suitable ultrasonication settings.

However, to the best of our knowledge, the literatures about structural modification of bacterial cellulose under high-intensity ultrasound were limited, and the effects of ultrasound irradiation on the physical properties of bacterial cellulose nanofibrils (BCN) aqueous suspensions of cellulose have not been reported to-date.