Yogurt made from milk heated at different pH values.

Area of Science:

• Food Science
• Dairy Science
• Rheology

Background:

• Yogurt production involves heat treatment to denature whey proteins, influencing gel properties.
• The pH of milk during heating affects the association of denatured whey proteins with casein micelles or their solubility.
• Previous studies on acid gel properties used model systems, not commercial starter cultures.

Purpose of the Study:

• To investigate the impact of milk heating pH on the rheological, light backscattering, and microstructural properties of yogurt gels.
• To compare the effects of different heating pH values using commercial starter cultures for yogurt fermentation.
• To determine the optimal milk heating pH for desirable yogurt gel characteristics.

Main Methods:

• Milk samples were adjusted to pH 6.2, 6.7, and 7.2, heated at 85°C for 30 min, and some were readjusted to pH 6.7 post-heating.
• Yogurt gels were prepared using commercial starter cultures and incubated to pH 4.6.
• Gel properties were assessed using dynamic oscillatory rheology (storage modulus G', loss tangent LT), light backscattering (R, R'), and fluorescence microscopy.

Main Results:

• Yogurt gels made from milk heated at pH 6.7 exhibited the highest storage modulus (G'), indicating greater stiffness.
• The loss tangent (LT) parameter showed a maximum at pH ~4.8 for yogurts heated at pH 6.7 or 7.2, but not for pH 6.2.
• Higher heating pH values (6.7 and 7.2) resulted in higher backscatter ratios (R) and two peaks in the R' profile, unlike pH 6.2.

Conclusions:

• Heating milk at its natural pH (around 6.7) optimizes the balance between casein-bound and soluble denatured whey proteins.
• This optimal balance leads to enhanced yogurt gel stiffness and desirable rheological properties.
• The findings suggest that controlling milk pH during heating is crucial for producing high-quality yogurt with specific textural attributes.