Rheological methods to analyse the thermal aggregation of calcium enriched milks
Introduction
The need for increasing population calcium intake has led to the development of a variety of calcium-enriched dairy products (Canabady-Rochelle et al., 2007, Janve and Singhal, 2018, Koutina et al., 2015). However, when the delicate mineral equilibria in milk are modified, different changes can occur. Particularly, the addition of calcium salts reduces the pH and influences the level of colloidal calcium phosphate, the proportion of caseins in the micellar and serum phases, the activity of Ca2+ and the ionic strength of the milk. Also, the hydration of casein micelles (CM) and the zeta potential are reduced (Famelart et al., 1999, Koutina et al., 2015, Philippe et al., 2003, Philippe et al., 2005). As a consequence, these changes can affect the heat stability of milk (Omoarukhe et al., 2010, On-Nom et al., 2012, Sievanen et al., 2008, Singh, 2004).
On the one hand, heat processing of calcium enriched milk is challenging from a technological point of view (Koutina, Christensen, Bakman, Andersen, & Skibsted, 2016). For instance, the sediment deposition or film formation in heat exchangers (recognised as fouling) are increased by the addition of calcium in milk (Bansal and Chen, 2006, Boumpa et al., 2008). Fouling reduces the process efficiency, can compromise the product by contamination, and needs a regular and more intensive cleaning (Hooper et al., 2006, Sadeghinezhad et al., 2013). In this case, calcium addition has a negative impact on heat processing of enriched milk.
On the other hand, the heat treatment of calcium enriched milk was proposed as a novel process that leads to milk coagulation and to formation of the so called “calcium-milk coagulum” (Koutina et al., 2016, Ramasubramanian et al., 2014). For instance, a fine gel network can be obtained by combining calcium addition to milk, pH adjustment and heat treatment (Koutina et al., 2016) or strong calcium-induced milk gels can be obtained using heat treatment at 70 °C (Ramasubramanian et al., 2014). In this case, calcium addition to milk can be advantageously used for making novel dairy products.
Therefore, there are many aspects that justify the study of the heat stability of milk as affected by calcium addition. The literature relating milk calcium supplementation, physicochemical changes and heat stability is vast and a wide variety of techniques has been used to fulfil this purpose (Dumpler and Kulozik, 2015, Jeurnink and de Kruif, 1993, Kaushik et al., 2015, Koutina and Skibsted, 2015, Koutina et al., 2016, Lin et al., 2018, Omoarukhe et al., 2010, On-Nom et al., 2012, Sievanen et al., 2008, Singh, 2004, Singh et al., 2007). However, the characterisation of calcium enriched milk during heating through rheological properties is still scarce and necessary. The aim of the present study was to analyse the rheological properties of calcium enriched milks during heating through the classical shear rate rheometry. Particularly, we intended to get additional information about the extent to which the stability of milks changes due to calcium enrichment and the microstructures obtained when the thermal aggregation begins after CM destabilisation and before gel formation.
Section snippets
Preparation of milk samples
Skim milk powder (SanCor Cooperativas Unidas Ltda., Sunchales, Argentina: 4%, w/w, moisture; 1.5%, w/w, fat; 35%, w/w, protein; 8.5%, w/w, ash; 31 mmol kg−1 calcium) obtained with a low heat treatment that reduces the level of denatured whey proteins was used. Milk samples were prepared at a level of 10% (w/w), following the manufacturer's recommendations. The required amount of powder was gradually added to purified water at 25 °C while stirring at moderate speed. Samples were sealed and
Thermal aggregation of CM according to the Brownian aggregation theory
The heating of milk at high temperatures leads to coagulation, which is a consequence of the loss of CM stability. In this sense, the surface properties of CM rather than the interior ones are likely to be important (Singh, 2004). As a result, the CM surface characteristics provide electrostatic and steric stabilisation (Dalgleish and Corredig, 2012, de Kruif, 1999, Horne, 2006). Heat treatment markedly modifies the serum phase environment around the CM. pH decreases, calcium phosphate is
Results and discussion
Fig. 1 shows experimental results obtained through Test A (temperature sweep). Two well differentiated temperature ranges were observed: (a) from 25 to 60 °C, where the viscosity slowly decreases as temperature increases; and (b) above 60 °C, where the viscosity sharply increases but at different temperatures depending on the amount of CaCl2 added.
It is widely recognised that equilibria in milk are affected by temperature. For instance, between 4 and 40 °C, among the expected changes that
Conclusions
In this work, we explored some rheological properties of calcium enriched milks during heating through shear rate rheometry. When the viscosity of milks was measured as function of temperature, it was observed that between 25 and 60 °C, the viscosity of all milks slowly decreases. This decrease of viscosity was analysed by an Arrhenius-type equation and, from the values of obtained, it was observed that the viscosity change with heating is similar for all cases studied. Also, the viscosity
Acknowledgements
This study was conducted with the financial support of Universidad Nacional del Litoral (project CAI+D: 504 201501 00051 LI) (Santa Fe, Argentina), Consejo Nacional de Investigaciones Científicas y Técnicas (project CONICET: 11220150100606) (Argentina), and Agencia Nacional de Promoción Científica y Tecnológica (projects ANPCyT: PICT 2015-365 and 2016-249) (Argentina).
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2020, LWTCitation Excerpt :Under these conditions, the viscosity of milk samples changes as temperature increases to reach a critical temperature (Tc) when viscosity suddenly diverges (Meza et al., 2019). To obtain representative critical temperatures, the experimental data of viscosity versus temperature were analyzed following the procedure reported by Meza et al. (2019). Briefly, a linear regression of each linear segment (before and after the beginning of the aggregation process) was obtained.
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2020, LWTCitation Excerpt :In other words, the higher viscosity of calcium chloride fortified sample (130 mgL−1) can be related to its higher water solubility compared to calcium phosphate and citrate salts, while the lower solubility of tricalcium citrate and tricalcium phosphate in water caused the lower viscosity of the related ice creams. Meza, Zorrilla, and Olivares (2019) studied the rheological methods to analyse the thermal aggregation of calcium enriched milks. They reported increasing of milk viscosity by increasing calcium chloride concentration in a constant temperature and related this behaviour to the modification of serum phase viscosity or casein micelles structure even at relatively low temperatures, also among different factors affecting the milk viscosity, they mentioned to the reduction of calcium in serum milk phase as a result of decreasing calcium phosphate solubility due to temperature increase.