Characterisation and antioxidant activity of glycated casein hydrolysate with xylose: Impacts of zinc sulphate and cupric chloride

Abstract

Impacts of zinc sulphate and cupric chloride on characteristics and antioxidant capacities of glycated products (GPs) derived from casein hydrolysate (CH) and xylose were studied. The browning degree of xylose–CH GPs was the largest at zinc sulphate addition of 15 mg L⁻¹, while its browning degree increased with cupric chloride extended from 0 to 15 mg L⁻¹. Loss of free amino groups, fluorescence intensity and the 2,2-diphenyl-1-picryl-hydrazyl (DPPH) radical scavenging capacity of xylose–CH GPs increased with the increase of cupric chloride addition, but loss of amino groups, fluorescence intensity and DPPH radical scavenging capacity were largest at the zinc sulphate addition of 15 mg L⁻¹. The addition of zinc sulphate and cupric chloride promoted production of higher and smaller molecular substances in xylose–CH GPs and CH. Therefore, zinc sulphate and cupric chloride promoted glycated reaction of xylose and CH and enhanced antioxidant activity of xylose–CH GPs.

Introduction

Casein is the most abundant protein in milk and consists mainly of αs₁-casein, αs₂-casein, β-casein and κ-casein, providing essential nutrients for organisms (Brantl, Teschemacher, Henschen, & Lottspeich, 1979). Casein hydrolysate (CH) can be obtained by using various proteases to hydrolyse casein, and is more easily absorbed than the maternal protein (Martinez-Maqueda, Miralles, Cruz-Huerta, & Recio, 2013).

In addition to its high nutritional values, casein hydrolysate also exhibits higher antioxidant activity (Shazly et al., 2017). For instance, casein is employed to produce hydrolysates by applying 2% (w/w) amino peptidase at pH 8.5 and temperature 55 °C, and its hydrolysate shows the strongest 2,2-diphenyl-1-picryl-hydrazyl (DPPH) radical scavenging activity at 6 h (Rahulan, Dhar, Nampoothiri, & Pandey, 2012). Bovine casein hydrolysate had the highest antioxidant activity after consecutive pepsin and trypsin treatments (Irshad, Kanekanian, Peters, & Masud, 2015). Therefore, casein hydrolysate has a broad market prospect as a natural antioxidant.

Glycation reaction is a very complex reaction between reducing sugar and free amino groups of proteins, peptides and amino acids (Yue et al., 2013). This reaction can produce a large number of glycated products (GPs), including aroma compounds, UV-absorbing intermediates and brown compounds (Hong, Meng, & Lu, 2015). Glycation reactions can occur spontaneously during food production and storage, being considered an effective way to improve antioxidant activities or functionality in food proteins and peptides. Previous studies have proved that temperature (Lan et al., 2010), reaction time (Sun & Zhuang, 2012), pH (Wang & Ismail, 2012), type and concentration of reactants affect glycation reaction (Lertittikul, Benjakul, & Tanaka, 2007). For instance, histidine–glucose glycated products heated at 120 °C, had the strongest antioxidant activity (Yilmaz & Toledo, 2005). Emulsifying activity of casein–glucose glycated products reached the highest level after the thermal treatment of 132.7 min (Gu, Abbas, & Zhang, 2009). Furthermore, glycated products between porcine plasma protein and glucose show the highest browning at pH 12 (Lertittikul et al., 2007). Accordingly, a high galactose concentration (45 g L⁻¹) can enhance antioxidant activities of bovine casein hydrolysate (Wang et al., 2015). After 4 h of heat treatment, fluorescence intensities of the glycated products of casein and xylose reached maximum, indicating that precursors of brown pigments were formed (Chen, Zhao, Shi, Abdul, & Jiang, 2019). Solubility of glycosylated products can be increased by 31% at the ratio of lysine and glucose for 1 to 3 (Hrynets, Ndagijimana, & Betti, 2013). Thus, reaction conditions may remarkably affect glycation reaction.

Additionally, metal salts can also influence the glycation degree (O'Brien & Morrissey, 1997). In some studies, FeCl₃ and FeSO₄ can promote glycation (Xue, Wang, & Yu, 2011), whereas CaCl₂ and MgCl₂ could slow glycation rate (Kwak & Lim, 2004). The influence of metal salts on glycation depends potentially on the types and concentrations of metal salts. However, it is not expounded whether zinc sulphate and copper chloride could enhance glycation reaction. Therefore, in this study, we will discuss the effect of zinc sulphate (6 levels: 0, 7.5, 15, 37.5, 75, or 100 mg L⁻¹) and copper chloride (5 levels: 0, 1.5, 3, 7.5 or 15 mg L⁻¹) on the glycated products of casein hydrolysate and xylose by measuring browning intensity, free amino groups, fluorescence intensity, molecular size distribution and DPPH radical scavenging activity.