When the milk is left open at room temperature, its taste changes and becomes sour due to the breakdown of lactose into lactic acid because of the lactic acid bacteria which also lower the of the milk due to the formation of lactic acid.
Final answer:
When the milk is left open at room temperature during summers, its taste changes and becomes sour due to the breakdown of lactose into lactic acid because of the lactic acid bacteria which also lower the of the milk due to the formation of lactic acid.
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