Examination of the pH levels in several milk samples
There were small changes in pH after OH. Ohmic heated skimmed and double-toned milk with a minimum variance of ±0.05 revealed a pH of 6.67. A minor reduction in pH (6.7±0) from their respective raw states was shown, suggesting slight acidification during the OH process, although no statistically significant difference was found (
p>0.
05). Ohmic heated toned milk (6.63±0.05), ohmic heated standardized milk (6.53±0.05) and ohmic heated full cream milk (6.53±0.05) all exhibited a consistent pH level, slightly lower than their raw counterparts, but not significantly different from one another (
p>0.
05).
Minor acidification may occur throughout the OH process because certain chemicals break down or release acidic byproducts. Temperature and milk concentration affected calcium and phosphate distribution between the soluble and colloidal phases. Raising temperatures caused them to swiftly enter the colloidal phase and lower pH
(Anema, 2009;
Janahar et al., 2021). These findings suggest that the OH treatment did not significantly change the pH of the milk sample (
p>0.
05), as shown in Fig 1.
One important conclusion was that heating milk below the critical temperature of 100°C had no discernible effect on its pH. Because of its reversibility, it is likely that the structural alterations or chemical reactions that take place in the milk when it is heated below 100°C would not result in a significant enough change in acidity over time
(Ma and Barbano, 2003). Due to this, the pH levels of the various milk samples before and after OH were consistent in this study, suggesting that the heating technique had no appreciable effect on the milk’s acidity or alkalinity. This is the reason the pH was disregarded for further investigation and analysis.
Examination of the descriptive statistics for several milk sample parameters
The OH caused distinct modifications in raw milk (Table 2) and ohmic heated milk (Table 3) at 90°C. Ohmic heated milk showed a lower average viscosity (2.04 mPa.s) than raw milk (2.13 mPa.s). The average viscosity decreased after OH, suggesting that the milk’s shear pressure resistance had changed. After thermal treatment, the standard deviation of viscosity increased from 0.13 mPa.s for raw milk to 0.18 mPa.s for heated milk, indicating a larger range. Heating milk to 90°C ohmically increased viscosity variability from 0.39 mPa.s in raw milk to 0.82 mPa.s in heated milk. During viscosity measurements, raw milk samples have an average shear rate of 99.06 1/s. After OH, the shear rate averaged 99.06 1/s. This indicates that the heat treatment did not substantially affect the milk’s resistance to flow
(Cooper et al., 2010). Raw milk’s average torque of 30.52 N-m reflected viscosity test requirements. Post-OH torque averaged 30.488 N-m, quite stable. This suggests that the OH process did not significantly modify the rotating force needed to measure viscosity.
The steady shear rate and minute torque fluctuations suggest that OH has complicated effects on interior flow behavior. The concentrate’s viscosity changed after OH because heating the milk broke down whey proteins and increased their voluminosity
(Anema et al., 2014). These minor fluctuations show OH’s distinct rheological changes at high temperatures, revealing how temperature influences milk’s properties.
Analysis of the correlation between the different milk samples
According to the correlation matrix (Fig 2), a strong positive correlation (0.66) between viscosity and fat content indicates that greater fat content is linked to higher viscosity. Other researchers have discovered similar findings, stating that there was a positive correlation between an increase in temperature and viscosity
(Sutariya et al., 2017). OH was used in this research to raise the temperature to 90°C. As a result, the whey protein denaturation process occurred, resulting in a link between heating and viscosity. Fat had a positive connection with both shear stress (0.20) and torque (0.50), suggesting that increased fat content also tended to increase both parameters. When speed is examined, there is a very significant correlation (1.00) between it and shear rate, which supports the basic link between these dynamic characteristics. The link between rotational speed and shear rate during the viscosity measurements is highlighted by this. Shear stress (0.93) and torque (0.82) showed a high positive association with speed, indicating that as rotational speed rose, so did shear stress and torque. Shear stress (0.21) and torque (0.08) exhibited a positive connection with viscosity, meaning that an increase in viscosity is associated with an increase in shear stress and torque. This is consistent with the knowledge that greater viscosity equals greater resistance to shear forces
(McCarthy and Singh, 2009). Interestingly, temperature showed minor correlations with the other factors, indicating that the relationships found were not significantly affected by temperature fluctuations within the experimental range. In another study, researchers used a water bath to study whether the viscosity of coconut milk altered at different temperatures (up to 90°C). They found that the water bath significantly affected the viscosity of the milk samples with varying fat contents
(Simuang et al., 2004). However, OH did not correlate with altering the various milk sample parameters in this investigation.
Analysis of the eigenvalues of the correlation matrix of the milk sample
Fig 3 shows that the first principal component (PC1), which accounted for 53.72% of the total variance, had the greatest eigenvalue of 3.76072. This implies that PC1 captured most of the main causes of variance in the sample. PC2 came in second with an eigenvalue of 1.87, making up 26.77% of the variance overall. PC1 and PC2 together accounted for almost 80.49% of the total variance, demonstrating their significant contribution to the understanding of the variation in the milk viscosity parameters. The eigenvalues drop further after PC3, with PC7 having the lowest eigenvalue of 0.0002.
These outcomes highlight the extent to which PCA captured the important patterns in the data. Regarding how OH affects milk viscosity, PC1 and PC2’s dominance suggests that these elements represent the main variables that were impacted by the heating process. The total variability was less substantially contributed by the succeeding components, indicating that the first two primary components essentially describe the observed effects of OH on milk viscosity.
Analysis of the principal components loading plot of the various milk samples
With a value of 0.51 for PC1, speed was the largest contribution, followed by torque (0.46), shear rate (0.51) and shear stress (0.50) (Fig 4). These positive coefficients imply that these parameters and PC1 have a favorable relationship. Although it had a negative coefficient (-0.14) and suggested an inverse link, viscosity also contributed to PC1. With positive coefficients, fat (0.66) and viscosity (0.66) were the main factors for PC2. Shear stress (0.03) and temperature (-0.20) also had an impact, but not as much (Fig 4).
It was clear from looking at PC1, which represented the leading causes of variation, that torque, speed, shear rate and shear stress all positively impacted this principal component. This implies that changes in fat content and OH caused torque, shear rate, rotational speed and shear stress to all rise, ultimately increasing PC1. The total solids (protein, lactose and minerals) in milk samples increase as their fat content rises, leading to an overall increase in viscosity
(Liu et al., 2023). These variations in dynamic parameters demonstrated the influence on the fluid’s flow and deformation properties and indicated the rheological reaction of milk to OH. Temperature has a little positive coefficient (0.01), indicating that PC1 only contributes slightly to the variation that is explained. This suggests that, although it was limited, temperature was positively correlated with the total response of the parameters in the context of PC1. This implies that the response of the observed parameters represented by PC1 may rise slightly concurrently with temperature.
The positive coefficients in PC2, where fat and viscosity were significant contributors, suggested that OH contributed to both fat content and viscosity. This emphasizes how the heating process alters the composition. The precipitation of protein upon heating to 90°C may have caused these modifications
(Kumbar and Nedomova, 2015). temperature had a negative coefficient (-0.20). This negative correlation implies an inverse link between temperature and the parameters represented by PC2. In this case, the responsiveness of the parameters denoted by PC2 may decrease as the temperature rises. The effects of OH on milk viscosity are explained scientifically by the eigenvectors. Changes in milk composition were indicated by the contributions of fat and viscosity to PC2, while the positive coefficients in PC1 indicated changes in dynamic characteristics.
Analysis of the principal components of the biplots of the various milk samples
The distinctive patterns of various milk kinds to the experimental settings were reflected in the varied pattern shown in Fig 5. Variations in the amounts of specific parameters were responsible for the split along PC1, which significantly adds to the dataset’s total variance. For example, the reactions of raw skim milk, raw double-toned milk and raw full-cream milk clustered together in the negative range of PC1. Similar properties like viscosity, shear stress and fat content also might have an impact on this alignment.
On the other hand, ohmic heated full-cream milk, ohmic heated skim milk, ohmic heated double-toned milk and ohmic heated standardized milk were all included in the positive range of PC1. The unique clustering of these ohmic heated samples suggests that the procedure had caused comparable modifications to their characteristics and may be associated with the effects of OH on protein denaturation, lipid emulsification, or other structural changes.
The separation along PC2, where ohmic heated toned milk was in the negative range and raw standardized milk and raw toned milk were positioned in the positive range, points to the possibility of other variables impacting the responses. These might involve variations in shear rate, temperature sensitivity and other parameters.
