FACTORS INFLUENCING TEXTURE AND NODULATION IN STIRRED YOGHURT

Stirred yoghurt is a cultured dairy product produced using mixed lactic acid bacteria cultures. It is characterised by a semi-viscous consistency, where texture and flow behaviour are key quality attributes. Consumers expect stirred yoghurt to be smooth, creamy, and free from visual defects. As a result, controlling rheological properties during manufacturing is essential.

One of the most commOne of the most common quality defects in stirred yoghurt is protein nodulation, which appears as visible grainy or particulate structures. Understanding how processing conditions and formulation influence both texture and nodulation is critical for consistent product quality.

Impact of Heat Treatment on Stirred Yoghurt Properties

Heat treament of yoghurt milk plays a central role in determining texture, viscosity, and stability. Milk subjected to pasteurisation temperatures (80 - 95 °C), high pasteurisation (85 °C for 30 min or 92 °C for 7 min), or UHT processing (130 - 145 °C for 5 sec) showed similarly high levels of whey protein denaturation (approximately 85 - 99 %). In contrast, milk treated at lower temperatures for extended periods (65 °C for 4 hours) exhibited significantly lower protein denaturation (around 55 %).

Yoghurts produced from pasteurised and UHT-treated milk demonstrated comparable hardness and viscosity, while yoghurts made from low-temperature, long-time heated milk showed noticeably weaker texture and lower viscosity. Water-holding capacity and syneresis levels were also similar between yoghurts produced from pasteurised and UHT milk, indicating that both heat treatments can produce stable stirred yoghurt when properly controlled.

Effect of Added Ingredients on Texture

The inclusion of non-dairy ingredients such as gelatine, inulin, and sugar significantly improved yoghurt texture. These formulations resulted in increased hardness, viscosity, and water-holding capacity, with no visible syneresis. Gelatine played a particularly important role by strengthening the protein network, binding water, and stabilising the yoghurt structure.

Causes of Nodulation in Stirred Yoghurt
Nodulation in stirred yoghurt is influenced by a combination of processing conditions and formulation choices.

Heat treatment intensity was a major factor. Severe pasteurisation treatments at temperatures below 100 °C resulted in more nodules than milder heat treatments. Interestingly, yoghurts made from UHT-treated milk showed considerably less nodulation than those made from pasteurised milk, even when whey protein denaturation levels were similar. This indicates that whey protein denaturation alone is not the primary driver of nodulation.

Sugar addition also had a strong effect. Increasing sucrose levels from 0 to 6.5 % led to a corresponding increase in nodulation. A synergistic effect was observed when high sugar levels were combined with severe heat treatment, producing the highest incidence of nodules. Glucose behaved similarly to sucrose, while lactose and fructose did not promote nodulation to the same extent.

Influence of Starter Cultures and Fermentation Conditions

Starter culture selection significantly affected nodulation. Yoghurts produced with ABT 10 cultures showed substantially fewer nodules compared to those made with ABT5 and ABT6 cultures, even under conditions known to promote nodulation. The amount of exopolysaccharide (EPS) produced by these cultures did not correlate with nodulation levels, suggesting that EPS alone does not explain the phenomenon.

Fermentation  stability was also important. Disturbances during gel formation - such as temperature changes during fermentation - increased nodulation. In contrast, refrigerated storage and pH adjustments during heat treatment did not significantly influence nodule formation.

Role of Ultrasonication

Ultrasonication of yoghurt milk reduced nodulation levels. This effect was attributed to the disruption of starter culture bacterial clumps, which helps prevent localised acid build-up. Such acid concentration zones can act as nucleation points for nodule formation, so reducing bacterial aggregation improves product uniformity.

Conclusion

The extent of nodulation in stirred yoghurt is primarily influenced by heat treatment conditions, type and level of added sugars, and starter culture behaviour. Secondary factors, such as fermentation stability and bacterial clumping, also play important roles. By optimising these parameters, yoghurt manufacturers can significantly reduce nodulation and consistently produce smooth, high-quality stirred yoghurt that meets consumer expectations.

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