Photometric extinction measurements to study dissolution kinetic of skim milk powder
Introduction
Skim milk powder (SMP) is often used for recombination or reconstitution purposes, particularly in countries where the fresh milk production is not sufficient to meet the demand (Anema & Li, 2003a). Besides, the functional properties of SMP make them a valuable ingredient in many other formulated foods such as confectionery and bakery products or soups and sauces (Oldfield, Taylor, & Singh, 2005; Sharma, Jana, & Chavan, 2012). Since industrial processing of SMP often involves rehydration in aqueous formulations, a rapid and complete rehydration is essential for practical use and an important quality attribute of the powder (Gaiani, Scher, Schuck, Desobry, & Banon, 2009; Schuck et al., 2007).
In general, rehydration is the process of adding a dry powder to water. The process of rehydration includes the stages wetting, submerging, dispersing and dissolution. These stages may occur concurrently during rehydration (Crowley, Kelly, Schuck, Jeantet, & O'Mahony, 2016; Hogekamp & Schubert, 2003). For milk powders a complete rehydration results in a polydisperse system containing colloidally dispersed proteins, emulsified fat droplets and dissolved lactose and salts. Since only the two last-mentioned substances exhibit true molecular solubility, the term milk powder solubility should be considered more as ‘suspension stability’, i.e., the ability of the proteins to redisperse in a stable suspension, rather than as molecular solubility (Knipschildt, 1969).
Powders for which the aforementioned rehydration stages are completed within a few seconds have so called instant properties (Hogekamp & Schubert, 2003; Schubert, 1990). For SMP it has been shown that a particle size of 150–250 μm (Gaiani et al., 2011; Lascelles & Baldwin, 1979; Pisecky, 1986, 2002; Schubert, 1990), a surface fat content of less than 0.03% (Westergaard, 2006) and a particle density of at least 1.2 g cm−3 (Pisecky, 2002) are required to reach the instant quality criteria, i.e., wettability time of <30 s, dispersibility index of minimum 90% and insolubility index of <0.2 mL (Pisecky, 2002).
Methods to determine the instant characteristics and the insolubility of milk powders are well described by IDF standards (IDF/ISO, 2005, 2014). However, these methods give only static values without information about the kinetics of the rehydration process. To monitor milk powder rehydration dynamically, several methods have been used as reviewed by Crowley et al. (2016). These methods primarily include time dependent measurement of particle size (Fang, Selomulya, Ainsworth, Palmer, & Chen, 2011; Mimouni, Deeth, Whittaker, Gidley, & Bhandari, 2009; Richard et al., 2013), turbidity (Freudig, Hogekamp, & Schubert, 1999; Gaiani, Banon, Scher, Schuck, & Hardy, 2005), viscosity (Ennis, O'Sullivan, & Mulvihill, 1998; Gaiani et al., 2006) and conductivity (Marabi et al., 2008). In most of these studies rehydration behaviour of milk protein powders, like micellar casein concentrate, milk protein concentrate or whey protein concentrate has been investigated. Especially casein dominant powders with a protein content of higher than 70% exhibit only poor rehydration characteristics (Gazi & Huppertz, 2015; Mimouni, Deeth, Whittaker, Gidley, & Bhandari, 2010; Schokker et al., 2011). It has been shown that this is related to a cross-linking of casein micelles at the particle surface mainly through hydrophobic interactions and dissociated caseins, resulting in a barrier to water transport and a delayed hydration of powder particles (Anema, Pinder, Hunter, & Hemar, 2006; Fyfe et al., 2011; Havea, 2006; McKenna, 2000; Mimouni et al., 2010).
Concerning insolubility of SMP, it has been found that an increase in powder storage temperature in the range of 50–100 °C led to a faster increase of the insolubility index with time (Kudo, Hols, & van Mil, 1990; Nielsen, Fee, & Chen, 1996). Furthermore, Straatsma, Van Houwelingen, Steenbergen, and De Jong (1999) showed that during spray drying the insoluble material increased with increasing particle diameter. Baldwin and Truong (2007) reported that the maximum reaction rate of the development of SMP insolubility was in the intermediate moisture range at approximately 30% moisture, while in the dry state insoluble material increased very slowly. Thus, higher temperatures can be used in the second drying stage without negative effects on insolubility index (Baldwin & Truong, 2007).
Although SMP rehydration has been studied in terms of instant properties (Gaiani et al., 2011; Hogekamp & Schubert, 2003; Schubert, 1990) and insolubility index, there are few studies considering dissolution kinetic. Anema and Li (2003a) examined the re-equilibration of minerals during reconstitution of skim milk. They stated that most of the observed changes occurred within the first 3 h of rehydration followed by only small changes over the next 30 h. Besides, Marabi et al. (2008) used conductivity measurements to study SMP dissolution kinetic and reported that dissolution was completed within 10 s whereas dissolution time increased with increasing fat content. A decrease in turbidity during SMP reconstitution was observed by Martin, Williams, and Dunstan (2007). The reason of the turbidity decrease was attributed to the reversal of micelle alterations induced by evaporation and drying. Further studies also revealed that turbidity can be used as an indicator of changes in the mineral and micelle system (Liu, Dunstan, & Martin, 2012; Liu, Weeks, Dunstan, & Martin, 2013).
However, since only few SMP were investigated with respect to turbidity changes during rehydration, it is still not clear whether there are differences in the dissolution kinetic of SMP with different processing histories. Also, there is no defined dissolution time for SMP which may be helpful for practical purposes. Thus, the aim of this study was to investigate the solubility kinetic of different commercial SMP by means of photometric extinction measurements to find out whether this technique can be a useful tool to detect quality differences in the solubility behaviour of SMP. Another objective was to derive a SMP dissolution time from the measured extinction profiles.
Section snippets
Skim milk powders
Twenty-six commercial spray dried skim milk powders were collected from different dairy companies in Germany. The powders had different processing histories including standardisation (lactose, permeate, none), pre-heating (low, medium or high heat), evaporation (43–52% concentrate dry matter) and spray drying conditions (inlet temperature 160–220 °C, outlet temperature 77–94 °C). According to their volume-weighted mean particle sizes (d4,3), the SMP could be classified into non agglomerated
Insolubility index
The solubility behaviour of SMP is often characterised by means of the insolubility index (ISI), which expresses the volume of insoluble material remaining in the milk solution after mixing and centrifugation. The examined SMP exhibited ISI of <0.05 mL for 12 SMP samples, 0.05 mL for 10 of the used SMP samples, 0.1 mL for 2 SMP samples, 0.2 mL for 1 SMP and 0.4 mL for 1 SMP (data not shown). Thus, most of the commercial SMP contained a very low amount of insoluble material. The two SMP that had
Conclusions
In this study we have shown that extinction measurements can be used to characterise dissolution kinetic of commercial SMP. After powder addition to water, extinction increased quickly due to disintegration of particulate material and release of casein micelles. The subsequent extinction decrease was modelled as a two-step process using a biexponential function, and dissolution times for the fast and slow period were estimated. The fast period represented the major part of the re-equilibration
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This IGF Project AiF 19360 BG of the FEI was supported via AiF within the programme for promoting the Industrial Collective Research (IGF) of the German Ministry of Economic Affairs and Energy (BMWi), based on a resolution of the German Parliament.
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