Pronunciation: [tˈɜːn sˈa͡ʊ͡ə] (IPA) 
The common phrase "turn sour" is pronounced /tərn saʊr/. The first syllable "turn" is pronounced as "tərn," with a short "u" sound and an unstressed "e." The second part "sour" is pronounced as "saʊr," with a diphthong "au" sound and an emphasized "s." The proper spelling of this phrase is "turn sour," without any alterations or alternate spellings, as it is a fixed phrase in the English language.
To "turn sour" refers to a situation or state of affairs that takes a negative, unfavorable, or disappointing turn. It describes when something that was previously pleasant, successful, or harmonious begins to deteriorate, lose its appeal, or become unpleasant. The phrase can be used to depict the transformation of various things, such as relationships, events, experiences, or even food and drinks.
When applied to interpersonal relationships, "turning sour" suggests that a once amicable or loving connection becomes strained, bitter, or hostile. It might indicate a shift in attitudes, feelings, or behaviors, resulting in conflicts, disagreements, or a breakdown in communication. Similarly, in the context of events or experiences, "turning sour" implies a negative shift, possibly leading to disappointment, frustration, or discontentment. This could refer to an initially promising opportunity that doesn't pan out as expected or an enjoyable occasion that becomes marred by unforeseen circumstances.
Moreover, the term can also be used to describe the undesirable change in taste, smell, or quality of food or drinks. For instance, when a meal or beverage is left to spoil or ferment, it can "turn sour," resulting in a disagreeable or acidic flavor.
In essence, "turn sour" captures the essence of something going from a positive or favorable state to a negative, disappointing, or unpleasant state, whether in terms of relationships, circumstances, experiences, or even culinary aspects.
The word "sour" is derived from the Old English word "sūr" which means "acidic or tart". It can be traced back to the Proto-Germanic word "sūraz", and ultimately to the Proto-Indo-European root "*sūs", meaning "to cause fermentation, sour".
When referring to something turning sour, it implies a taste or flavor going from sweet or pleasant to acidic or bitter. The concept of sourness in this sense is commonly associated with the fermentation of certain foods or the oxidization of substances, resulting in a change of taste. The phrase "turn sour" simply describes this transformation from a pleasant state to an unpleasant one.
