Final answer:
Acid production in starch gelatin agar refers to the change in pH due to bacterial fermentation of carbohydrates, which can be indicated by color changes in the presence of pH indicators like phenol red in mannitol salt agar or by the absence of blue-violet color in starch agar when iodine is added.
Step-by-step explanation:
The process of acid production in the presence of starch gelatin agar carbohydrates can be observed using biochemical media that change color in response to acid generation from carbohydrate fermentation. This concept applies specifically in the context of microbial testing, where certain bacteria are known to ferment carbohydrates, leading to the production of acidic byproducts. For example, Staphylococcus aureus can be isolated using mannitol salt agar, which contains the pH indicator phenol red. This indicator turns yellow in the event of acid production due to the fermentation of mannitol by the bacteria. Starch agar, containing iodine, reacts with starch, producing a characteristic blue-violet color due to the amylose-iodine complex formed if starch is not broken down. However, if a microorganism can hydrolyze starch, the area around the bacterial growth will not turn blue-violet upon addition of iodine, indicating starch degradation and possibly acid production.