Microgel particle formation in yogurt as influenced by sonication during fermentation

• Short term sonication during mechanically sensitive fermentation.
• Image analysis of particle number, size and shape.
• Highest particle formation due to sonication at pH 5.4–5.1.
• Negative impact on the gel network structure and grainy texture of yogurt.

Textural defects such as lumpiness and graininess which cannot be explained by an altered yoghurt composition or processing parameters can arise due to external oscillations. The effect of sonication applied during fermentation/acidification of skim milk (3.4% protein, 0.1% fat) to yoghurt was investigated with regards to the appearance of visually detectable particles in the stirred milk gel. The structure formation during acidification is mechanically sensitive so that additional particles were observed on application of sonication. These were characterized by image analysis and static light scattering, and sensorially assessed. Since the gel structure arises due to a change in pH, the critical pH range for the generation of these particles was determined by short term sonication (45 kHz, 5 min). It was found to be between pH 5.4 and 5.1. This is discussed in terms of a kinematic and molecular mechanism at the isoelectric point of the milk proteins.