A single-celled organism that thrives in warm and moist foods is called yeast or Saccharomyces cerevisiae. They come under fungi and are unicellular eukaryotes.
The bread remained flat and did not raise cause there was no sugar present in the dough.
The yeast converts the sugar and starch molecules present in the food particles into alcohol and carbon dioxide.
In the above-stated case the absence of sugar did not produce the carbon dioxide that forms the air bubbles.
Therefore, the bread remained flat.
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