I use heated skim milk, 2% starter culture and 430C incubation temp. After 7hr fermentation in rheometer by bob and cup method with 1Hz frequency and 0.1% strain , G' is shown around 20 - 30 Pa. I see in the paper it is around 150 - 250 Pa. What mistake I have made in this procedure?
Dear sir
When yogurts have reached the desired pH (e.g., ~4.6),yogurts are partially cooled (~20°C). Yogurt products are often blast chilled to
Thank you Mohamed Shenana for your answer. Actually problem is during fermentation. In the whole fermentation period it shows very small G' like 30 or 40 Pa. But what is the main cause to not improve the G' during acidification?
Greetings,
Fermentation with skim milk only may not give you correct amount of casein to produce desired viscosity.
My suggestion would be addition of nonfat dry milk in variable amounts (try 2, 3, 4%, etc) until desired G' is achieved
Steps:
1) Heat skim milk to 20C and add NFDM (or add to cold milk and hydrate overnight)
2) Continue to heat milk to 92C (HTST or vat) and hold for 3-5 minutes (critical for complete denaturing of casein)
3) Cool to fermentation temp (37-43C)
4) Inoculate with DVS culture (0.013-0.02%)
5) Ferment to pH 4.6 (Reaching isoelectric point is critical for complete cross-linking of casein)
*Different culture will produce EPS/CPS and increase viscosity
Sorry to answer your question with a question: did the paper you referred to use starter culture or GDL? The experience in our lab with starter culture is much like you, very low G', which we believe is due to syneresis caused by uneven fermentation. As soon as you have any syneresis, you have slipping and low G'. Try running your experiment with GDL first and see if you get better results. The other thing that you need to look out for is vibration: I believe that your rheometer is seated on a marble bench to stop vibration. Vibration can break your gel and give you low G'. I don't work with yoghurt much, but these are some thoughts.
Thank you Siew Kim Lee. I agree with your valuable answer. I will try according to your suggestions....
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