Nitrite either alone or in combination with nitrate is added to food to preserve the color and taste and to prevent foods from becoming rancid. They are also used in food for their anti-microbial properties. Higher levels in vegetables and leafy greens are possible from the use of nitrate fertilizers and/or livestock manure.
Nitrate can be reduced to nitrite at certain physiological conditions in the human body. Nitrite however can oxidize Fe (II) in hemoglobin to methemoglobin, an Fe (III) product. The oxidized product is incapable of binding molecular oxygen and high concentrations of methemoglobin can result in methemoglobinemia especially in infants. Nitrite can also react with secondary amines present in food products or in the digestive system to form nitrosamines, a class of carcinogenic compounds. Nitrite levels in food could also be produced by reduction of nitrate to nitrite during processing.
The AOAC Official Method 993.031 for the analysis of nitrate involves reduction using spongy Cadmium which is toxic and carcinogenic. The USFDA improved on this method by using Vanadium (III) chloride and heat2 for the post-column reduction of nitrate to nitrite. Nitrite reacts with this modified Griess reagent to produce a red chromophore with maximal absorbance at 535 nm. Pickering Laboratories Inc. has further improved this method by substituting the corrosive and volatile hydrochloric acid with methane sulfonic acid.