1. Check the shelf life: check whether the old yogurt has expired or is close to the shelf life. Don't eat any more after it expires.

2. Look at the characteristics: If you can't see the shelf life of old yogurt, you can judge whether it has deteriorated by observing the characteristics of old yogurt. Deteriorated old yogurt will turn into water sample, yellow or green in color, accompanied by foam.

3. Taste: Good old yogurt tastes delicate and sweet, while deteriorated old yogurt will have a strong sour taste or alcoholic fermentation taste, and may even have a musty taste.

4. Outer packaging: If the outer packaging of old yogurt expands, it means that yogurt has deteriorated and should be banned from eating.

The above are some ways to judge whether the old yogurt has gone bad, and I hope it will help you.