Influence of molecular weight and degree of substitution of carboxymethylcellulose on the stability of acidified milk drinks
Introduction
Acidified milk drinks can be described as an acidified protein liquid system with stability and viscosity similar to natural milk. Such drinks usually comprise a large range of products, from those usually prepared from fermented milk with stabilizers and sugar to those prepared by direct acidification with fruit juices and/or acids. The pH of these products ranges from 3.6 to 4.6 (Nakamura, Yoshida, Maeda, & Corredig, 2006). At neutral pH, caseins exist in the form of micelles, which are stabilized by steric repulsion due to the extended conformation of κ-casein present mainly on the surface of micelles (de Kruif, 1998, Tuinier and de Kruif, 2002). During acidification, at a pH close to the isoelectric point (pH 4.6) the casein micelles aggregate mainly because of the collapse of the extended conformation of κ-caseins (Holt, 1982). On account of the instability of casein in the abovementioned pH range, stabilizer needs to be added to avoid the flocculation of milk proteins and subsequent macroscopic whey separation. High methoxyl pectin (Boulenguer and Laurent, 2003, Liu et al., 2006, Parker et al., 1997) and soybean soluble polysaccharides (SSPS) (Asai et al., 1994, Nakamura et al., 2003, Nakamura et al., 2006) are often used to achieve this, and much attention has been paid to pectin. In addition, propylene glycol alginate (PGA) and carboxymethylcellulose (CMC) are also mentioned to be able to use as stabilizers (Keiichi, 2006, Koji et al., 2004, Mann, 2004, Masaki et al., 2004, Murray, 2000, Nishiyama, 1978, Ogasawara et al., 2003, Syrbe et al., 1998, Young and Bluestein, 2002).
As one of the most important derivatives of cellulose, CMC is a typical anionic polysaccharide and has been widely used as a stabilizer in food. CMC chains are linear β(1 → 4)-linked glucopyranose residues. The average degree of substitution (DS) of CMC is defined as the average number of carboxymethyl groups per repeating unit and is usually in the range 0.4–1.5. CMC is generally found under sodium salt form, a water-soluble product for DS > 0.5. A maximum degree of substitution of 1.5 is permitted, but more typically DS is in the range 0.6–0.95 for food applications (Coffey et al., 2006, Murray, 2000).
CMC is commonly chosen as a stabilizing agent for its low cost in acidified milk drinks instead of pectin in Asia, especially in China (Chen, Zheng, Chen, & Rao, 1996). The application and the stabilization mechanism of pectin and SSPS in acidified milk drinks have been extensively studied in recent years (Liu et al., 2006, Nakamura et al., 2006). However, the stabilizing effects of CMC on this kind of drinks are less reported. The stability of casein micelles at low pH could be improved by CMC. In a previous work (Du et al., 2007), we found that electrosorption of CMC onto casein micelles took place below pH 5.2 and the adsorbed CMC layer on the surface of casein could prevent flocculation of casein micelles by steric repulsion. In addition, the non-adsorbed CMC increased the viscosity of serum and slowed down the sedimentation of casein particles. The adsorbed CMC layer caused a repulsive interaction between the casein micelles at low pH in the same way as κ-caseins do at neutral pH. This phenomenon is related to the interaction between protein (mainly casein micelles) and CMC.
The stability of acidified milk drinks depends largely on the interactions between casein and polysaccharides, which can be influenced by the concentrations of protein and polysaccharides (Tromp et al., 2004, Tuinier et al., 2002), pH (Nakamura et al., 2003), molecular properties of polysaccharides (Laurent and Boulenguer, 2003, Maroziene and de Kruif, 2000, Pereyra et al., 1997), ionic environment (Ambjerg Pedersen & Jorgensen, 1991), milk protein composition and processing (Boulenguer and Laurent, 2003, Glahn, 1982, Sedlmeyer et al., 2004;), and thermal history of the sample (Horne, 1998, Lucey et al., 1999, Zaleska et al., 2000) etc.
Although the interactions between casein micelles and CMC and the stability of the acidified milk drinks might be primarily dependent on pH and concentration of CMC as previously reported (Du et al., 2007), the molecular weight and substitution pattern of carboxymethyl groups on CMC should be emphasized because in the practical processing of acidified milk drinks the properties including stability of the drinks can be obtained by the adjustment on the molecular parameters of CMC. In the present work, we aim to investigate the influence of Mw and DS of CMC on the interaction between CMC and casein micelles and thus on the stability of acidified milk drinks.
Section snippets
Materials
A series of CMC with different Mw (250,000 Da and 700,000 Da) and different DS (0.7, 0.9 and 1.2) were purchased from the Acros organics (Morris Plains, New Jersey). Skim milk powders were obtained from Fonterra Co-operative Group (Wellington, New Zealand). Citric acid monohydrate was obtained from Shanghai Chemical Reagent Co., Ltd. (Shanghai, China).
Preparation of samples for dynamic light scattering (DLS) and ζ-potential experiments
The sample was made by dispersing 80 g/kg reconstituted skim milk in simulated milk ultra filtrate (SMUF) (Jenness & Koops, 1962) (1:100). SMUF
Influence of Mw and DS of CMC on the diameter of casein micelles during acidification
The particle size evolution of casein micelles in the system prepared by diluting reconstituted skim milk in SMUF (1:100) was followed by DLS. The ratio of 800 mg/kg reconstituted skim milk and 400 mg/kg CMC was selected based on our previous work (Du et al., 2007), at which CMC could sufficiently form thick adsorbed layer on casein micelles and maintained their stability. Fig. 1 illustrated the effect of Mw of CMC (DS = 0.9) on the particle diameter of casein micelles during acidification with
Conclusions
Both Mw and DS of CMC influenced the interaction between CMC and casein micelles and thus the stability of acidified milk drinks. At pH 6.7, there was no interaction between caseins and CMC due to charge repulsion and mixtures of casein and CMC were stable at low CMC concentrations. Above a certain CMC concentration, depletion flocculation occurred leading to phase separation. Below pH 5.2 CMC adsorbed onto casein micelles. In the case of low CMC concentrations, CMC/casein micelles mixture was
Acknowledgements
H. Zhang thanks for the support from Danisco-China Co., Ltd. and Shanghai Leading Academic Discipline Project (No. B202).
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