If you eat the bread immediately before blowing into an ignition interlock or any other breathalyzer your breath will contain alcohol. This is because alcohol is basically the waste product of yeast, a common ingredient in bread.
When you chew the bread the small amounts of alcohol in the bread are released into your mouth. This alcohol is in such small quantities that it won’t significantly affect the concentration of alcohol in your blood or get you drunk. But if that small amount of alcohol is blown directly into a breathalyzer machine it will produce erroneously high BAC readings.
This video shows Luke Nichols of Nichols & Green PLLC demonstrating this phenomenon with a slice of potato bread and a PBT.