Effect of potato starch addition on the acid gelation of milk
Introduction
Milk can be gradually acidified to form acid gels using bacterial cultures, which ferment lactose to lactic acid, or using glucono-δ-lactone (GDL), where the hydrolysis of GDL to gluconic acid results in a reduction in the pH (Lucey & Singh, 1997). The properties of these acid gels can be altered by a number of different methods. For example, different properties of acid gels, such as the pH of gelation, the gelation time and the storage modulus, can be manipulated using heat treatment of the milk before acidification (Lucey, 2004; Lucey & Singh, 1997). The rheological properties of acid gels or yoghurt can be further modified by fortifying the milk with dairy-based ingredients, non-dairy ingredients or a combination of both prior to heat treatment and acidification.
The addition of whey-protein-based solids prior to heating and acidification has been investigated in a number of studies (Graveland-Bikker & Anema, 2003; Lucey, Munro, & Singh, 1999). Lucey et al. (1999) reported that the addition of whey protein concentrates to milk followed by heat treatment at 80 °C caused a further increase in the pH of gelation, a reduction in the gelation time and an increase in the storage modulus of the skim milk acid gel. The effects of the whey proteins were found to be dependent on the type of whey proteins and the addition levels (Bikker, Anema, Li, & Hill, 2000; Graveland-Bikker & Anema, 2003).
Non-dairy ingredients, especially polysaccharides such as locust bean gum, xanthan gum, guar gum, pectin and starches, can also be used in yoghurt in conjunction with dairy ingredients or on their own to modify the rheological properties (Decourcelle, Lubbers, Vallet, Rondeau, & Guichard, 2004; Keogh & O’Kennedy, 1998; Williams, Glagovskaia, & Augustin, 2003; Williams, Glagovskaia, & Augustin, 2004). The viscosity of stirred yoghurt increased when 1% (w/w) modified waxy corn starch was added to yoghurt milk but the yoghurt developed a grainy texture (Williams, Glagovskaia, & Augustin (2003), Williams, Glagovskaia, & Augustin (2004)). Keogh and O’Kennedy (1998) used an array of different polysaccharides in stirred yoghurt and showed that the yoghurts made with wheat starch had the highest shear consistency. Sandoval-Castilla, Lobato-Calleros, Aguirre-Mandujano, and Vernon-Carter (2004) examined the use of tapioca starch as a fat replacer along with other whey-protein-based fat replacers. They reported that yoghurt with tapioca starch showed higher firmness than full-fat yoghurt. The microstructure of the yoghurt with tapioca starch showed some solubilized starch molecules integrated into the casein micelle network as well as starch gel fragments forming independent structures.
In addition to modifications to the rheological properties, added polysaccharides were also found to cause a decrease in the concentration of aroma compounds in the headspace of yoghurt (Decourcelle et al., 2004). In the case of starch, it was suggested that molecular interactions between the helical chains of starch and the aroma compounds could cause the decrease in the concentration of aroma compounds in the headspace of the yoghurt. Heinemann, Zinsli, Renggli, Escher, and Conde-Petit (2005) also found that linear amylose in particular was able to form inclusion complexes with a wide variety of flavour compounds.
Changes in the native structures of both milk proteins and starch occur upon heat treatment above their relevant critical temperatures. The heat treatment of milk at temperatures above about 70 °C results in denaturation of the whey proteins (Anema & McKenna, 1996; Dannenberg & Kessler, 1988). These denatured whey proteins can undergo a complex series of aggregation reactions with other denatured whey proteins and with the casein micelles (Anema & Li 2003a; Corredig & Dalgleish, 1996; Mulvihill & Donovan, 1987). In contrast, starches ‘gelatinize’ when heated in the presence of water, with the critical temperature dependent on the starch type. Starch gelatinization encompasses disruption of the granular structure, swelling and hydration, and solubilization of starch molecules (Appelqvist & Debet, 1997). Such developments in a mixed system of milk and starch during heat treatment may lead to different characteristics in the final acid gel compared with acid gels made from milk alone.
The current study was conducted to examine the effect of the addition of potato starch to skim milk at its natural pH on the rheological properties of acid gels. The starch was added prior to heat treatment and acidification. As well as the rheological and physical properties of final acid milk gels with added starch, which previous studies have shown (Keogh & O’Kennedy, 1998; Sandoval-Castilla et al., 2004; Williams, Glagovskaia, & Augustin (2003), Williams, Glagovskaia, & Augustin (2004)), this study provides insight into the rheological changes during the acid gelation of skim milk samples with added potato starch to provide more comprehensive understanding of the acid gelation of milk in the presence of starch. The acid gelation processes and the properties of the final gels with different levels of added starch were compared and their microstructures were studied.
Section snippets
Materials
Native potato starch (amylose:amylopectin, 24:76) was obtained from Penford New Zealand Limited (Auckland, New Zealand) and was used as supplied. The potato starch contains 0.05% protein, 0.3% ash and a maximum of 20% moisture. Low-heat skim milk powder was obtained from the Edendale site, Fonterra Co-operative Group Limited, New Zealand. GDL was obtained from Sigma-Aldrich (St. Louis, MO, USA).
Sample preparation
Reconstituted skim milk samples were prepared by adding low-heat skim milk powder to purified water
Change in pH with time after GDL addition
The addition of starch to milk had only a small effect on the change in pH with time after GDL addition. As shown in Fig. 1a, at a starch concentration of 1.5%, which was the highest level used in this work, there was a slightly faster rate of pH decrease than for the samples with no starch. In both cases, the initial pH of the sample was 6.5 and dropped to approximately pH 4.1 after 360 min of acidification with 2% GDL. As the differences in pH change were small, no attempts to correct for this
Conclusions
This study demonstrated the changes in acid gelation curves with the addition of potato starch prior to heat treatment, which have not been shown previously. It was found that starch addition resulted in a reduction in the gelation time and an increase in the gelation pH. The addition of starch increased the final G′ values of acid milk gels and the magnitude of the increase was dependent on the level of starch added to the milk. The CSLM micrographs revealed that the starch granules appeared
Acknowledgements
The financial support of the New Zealand Foundation for Research, Science and Technology (DRIX0201 and FCGL0467) is gratefully acknowledged.
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