Effect of carbon dots in combination with aqueous chitosan solution on shelf life and stability of soy milk

Highlights

• Aqueous chitosan solution (CH) can be used as thickener in soy milk.

• Banana can be used to synthesize carbon dots (CDs).

• CDs can effectively inhibit growth of E. coli, S. aureus and B. subtilis.

• CDs + CH can effectively extend shelf life of soy milk without changing its taste.

Abstract

Use of carbon dots (CDs) in combination with aqueous chitosan solution to extend shelf life and improve stability of soy milk was investigated. Soy milk samples with chitosan solution (0.00%, 0.08%, 0.12%, 0.16% and 0.20%) and banana-based CDs (4%, 6% and 8%) were prepared and stored at room temperature (25–30 °C) for shelf life evaluation. Soy milk with 0.16% chitosan solution exhibited improved stability as evident by increased viscosity, stability coefficient, zeta potential and decreased centrifugation rate compared with soy milk without chitosan. The suitable amount of carbon dots could effectively inhibit the growth of Escherichia coli, Staphylococcus aureus and Bacillus subtilis. Soy milk with 0.16% chitosan and 8% CDs exhibited longer shelf life and significantly lower total bacterial count after storage at room temperature for up to 4 days. Electronic nose-based flavor characteristics of all treated soy milk samples were not far from that of the control sample.

Introduction

Soybean is an important plant-based protein source as it contains all nine essential amino acids at the levels as high as those from egg, milk and meat (Nguyen et al., 2016). Soy-based foods have also received increasing attention after the Food and Drug Administration (FDA) has confirmed the relation between soy protein and reduced risk of coronary heart disease (D'Adamo and Sahin, 2014).

Soy milk is a popular beverage prepared from soaked and ground soybean. Soy milk has a long history as a meaningful part of the East Asian cuisine and has recently achieved more acceptances in the Western countries. Since soy milk contains most of the soybean nutrients, including proteins, polysaccharides, lipids, minerals, vitamins and isoflavones (Subrota et al., 2013), it is acknowledged to provide a number of health benefits, including the ability to prevent the onset of certain types of cancers and the remission of hypertension (Rivas et al., 2002).

Despite its benefits, the shelf life of fresh soy milk is very short. This limits the consumption of fresh soy milk only to those consumers residing close to a production site. Processed soy milk is an obvious alternative to alleviate the above-mentioned limitation. Thermal treatment is the most typical process applied to enhance the microbiological safety and storage life of soy milk; such a treatment also helps enhance the nutritional value of soy milk by inactivating anti-nutritional factors and increasing protein digestibility (Riaz, 2005). Thermal treatment nevertheless adversely affects some quality characteristics (e.g., those related to heat-sensitive vitamins and bioactive compounds) of soy milk and causes distinct off-flavor (Amigo-Benavent et al., 2007). For this reason, it is always desirable to develop and/or investigate a milder process to improve the shelf life of soy milk. Using an appropriate antimicrobial agent is a possible means to inhibit proliferation of microorganisms, while maximizing the retention of soy nutrients during processing (Yuan et al., 2013).

Chitosan is a natural biopolymer obtained by deacetylation of chitin, which is the main constituent of crustacean shells and skeletons. Chitosan has widely been used in food processing due to its biocompatibility and non-toxicity (Borges et al., 2006). The antimicrobial activity of chitosan has also widely been demonstrated (Carrión-Granda et al., 2016; Mellegård et al., 2011; No et al., 2003). Zheng et al. (2017), for example, reported that chitosan at 5 g/L significantly inhibited gray (Botrytis cinerea) and blue mold (Penicillium expansum) in kiwifruit stored at 25 and 4 °C and exhibited beneficial effects on the fruit quality. A number of studies have also used chitosan in combination with other active substances to improve the shelf life of fresh food. Combination of chitosan and peony extracts were, for instance, noted to help inhibit the growth of native fungal microflora on strawberries, thus improving the quality and shelf life of the fruit (Pagliarulo et al., 2016). Combination of chitosan and ClO₂ was noted to reduce the counts of total aerobic bacteria and yeasts and molds, thus extending the post-harvest life of fresh-cut bamboo shoot; the antimicrobial effect of the combined treatment was superior to that of ClO₂ or chitosan treatment alone (Yang et al., 2015). The ability of chitosan as a thickener in food production has also been reported (Eduardo et al., 2014). Chitosan has nevertheless been used mostly as an antibacterial agent in films and coatings (Ma et al., 2016; Yu et al., 2018); fewer studies focused on both antibacterial and stability enhancement properties of chitosan. In the present study, chitosan was added directly to soy milk rather than being used as a coating and both its antibacterial and thickening properties were studied.

Carbon dots (CDs) have recently received attention as a novel nanomaterial because of their non-toxicity, excellent biocompatibility, cell membrane permeability, extraordinary opto-electrical properties, good photo-stability, and inexpensive nature with ease of availability (De and Karak, 2013; Fang et al., 2016). CDs have widely been used in photocatalysis, biological imaging, among other fields; increasing attention has also been paid to their antibacterial property. Some investigators have indeed modified CDs via doping and noted that the modified CDs exhibited high potential from microbial bacteriostatic and bactericidal points of view (Chen et al., 2017; Yang et al., 2016). Singh et al. (2017) recently proposed a newer approach to prepare curcumin quantum dots, which was noted to exhibit 90 times higher antimicrobial activity than the native curcumin. Again, most prior studies mainly focused on the bacteriostasis of CDs; very few studies exist on the use of CDs to prolong the shelf life of food. Application of CDs to prolong the shelf life of beverages has not been reported.

The purpose of the present investigation was to assess the application of chitosan as a thickening and antimicrobial agents in soy milk. Banana was selected as the starting material to synthesize CDs, which were then used in combination with aqueous chitosan solution as a natural bacteriostatic agent to extend the shelf life of soy milk. The preservation effect of soy milk was studied through various indices in order to explore a new technique for the preservation of soy milk.