Grainy defect with images in natural yoghurt

Thanks for your post. Yes protein denaturation / destabilisation is probably the root cause behind this problem. Difficult to work with because it happens so infrequently. Long set stirred yoghurt is usually where I see this. Stirring at too high a pH will often give something similar. There are lots of work "arounds" that work to a degree.

Graininess or the appearance of insoluble nodules in stirred yoghurt occurs occasionally. However it is unusual to see this defect in a branded product sold by major retailers. It is fairly easy to detect this fault and defective batches are usually quarantined until it has been decided what to do with them. It is not acceptable for product with this fault to be released for sale.

A few days ago I bought two 450 g cartons of natural yoghurt in a major UK retailer in Cookstown. According to the label the yoghurt contained 4.2% fat and 3.6% protein and no additives apart from the starter. The sell by date was the 29th May.

On opening and looking at the lid it was apparent there was a problem. The yoghurt was grainy. This can be seen clearly in the yoghurt in the spoon. The body was also weak / runny, the yoghurt tasted slightly watery and syneresis, or wheying of, was also evident. It was just about edible. However the condition of the product raised a number of issues and it was not eaten.

Product faults like this demonstrate the need for companies to have access to dairy scientists with an expert knowledge of milk production, milk composition and the effects of the processes involved in yoghurt manufacture on product stability.

If you were a newly-appointed production manager how would you investigate a problem like this?