Involved in the catabolism of ascorbate to tartrate. The enzyme has no activity with NADP(+). L-idonate + NAD(+) = 5-dehydro-D-gluconate + H(+) + NADH Carbohydrate acid metabolism; L-idonate degradation. Peak of expression 4 weeks post flowering, at the time when tartaric acid biosynthesis occurs. Belongs to the zinc-containing alcohol dehydrogenase family.