Respiratory-Deficient Mutants, also referred to as RD mutants, are defective yeast cells. All yeasts used in brewing have the ability to ferment or respire specific carbon sources. Alcohol is only produced when sugars are fermented; however, respiratory ability, too, appears to be important for yeast’s ability to produce acceptable beers.

RD yeast mutants are also called petite (French for small) mutants because of their small colony size when they are propagated on agar medium. These mutants show sublethal damage to their mitochondrial DNA. They occur more frequently than nuclear mutants because of their proximity to the highly oxidative electron chain. Importantly, they lose their ability to respire and use carbon sources such as ethanol or glycerol. But they can still ferment, albeit slowly.

There are two forms of RD mutants, each differing in the nature of their DNA damage: rho^- mutants contain incomplete mitochondrial DNA, whereas rho⁰ mutants do not contain mitochondrial DNA at all. Only rho^- mutants are observed in brewing and they occur spontaneously in concentrations of up to 4% depending on the yeast strain. Increased mutation rates can develop if brewers yeast is stressed or not handled properly during harvest and storage. Mutations also accumulate during serial repitching. A culture containing high levels of RD mutants—of perhaps 10%—can affect fermentation performance as well as produce noticeable off-flavors, particularly the clove-like 4-vinyl guaiacol. RD mutants are easily detected in brewery lab tests, and high concentrations will spur a brewery to reculture the yeast strain to arrest the development of further yeast health issues.