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As one type of the oldest fermented food, yogurt is very popular around the world. Different from other traditional dairy products (cottage cheese, sour cream and etc.), yogurt wins its popularity due to the health benefits it can offer. Primarily, yogurt comes from milk and is nutritionally rich in protein, calcium, riboflavin, vitamin B6 and vitamin B16. Additionally, Lactose-sensitive individuals may tolerate yogurt better than other dairy products due to the conversion of lactose to glucose and galactose, and the fermentation of lactose to lactic acid carried out by the bacteria in the yogurt. Most importantly, it generally possesses a certain amount of probiotics, which is necessary in our digestive tracts.

Fig1. Delicious yogurt

In spite of these health benefits, yogurt is easy to go bad and spoiled yogurt has more harm than good. Last semester, one of our teammates was absent for class due to eating some spoiled yogurt, although it was still within the expiration date. Generally, yogurt within expiration date is safe for eating if it is always kept cold, which was neglected by this teammate who consequently suffered from diarrhea. As you can see, yogurt may cause food poisoning even though it has not expired. This is one of the reasons why we came up with an idea of developing a yogurt guarder.

We searched the Internet for relevant information and found varieties of news about yogurt spoilage within shelf-life. Bacteria contamination in yogurt cannot be recognized by our naked eyes. Thus, many people may suffer from that.

To understand why yogurt may not be safe for eating even before the expiration date, we need to know how yogurt is produced. First, raw milk is treated with pasteurization to kill most microorganisms inside without destroying its nutritional components. Second, Lactobacillus is inoculated for fermentation. Then, yogurt is made. Unlike sterilization, pasteurization is unable to kill all microorganisms in the food. Instead, it intends to reduce the number of pathogens without significantly affecting nutrient. So yogurt must be kept in cold for a limited time period; otherwise pathogenic bacteria that survive from pasteurization can quickly grow and cause yogurt spoilage.

We did a simple experiment to test the appearance and contents of yogurt under different conditions. We bought fresh yogurt from the same batch and kept some of them in fridge and some at 35℃ for half an hour. Then, we took pictures of them. The yogurt kept at both conditions looked the same. However, our subsequent experiments revealed that yogurt kept at 35℃ had a higher number of coliform bacteria than that from the fridge and the pathogens significantly exceeded the qualify standard. This means the pathogen contents of yogurt may significantly increase when we carrying yogurt from a supermarket to home, since yogurt will be exposed to ambient temperature for a while, especially in summer. (Visit Result for more details)

Fig2. Yogurt treated with different temperature. (A) Fresh yogurt kept in 4℃ for 0.5hour (B) Fresh yogurt kept in 35℃ for half an hour (C) Yogurt kept in 4℃ until one-day past expiration

Needless to say, no one would like to eat yogurt if he or she knows it may contain such a high amount of coliform. However, the current methods to test pathogenic bacteria generally contain multiple cumbersome steps, are time consuming and require special equipment. They obviously cannot be used in our daily life. Therefore, a handy detecting approach for pathogenic bacteria in yogurt is urgently needed.

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