Parotid glands are the main saliva-producing organs. Located next to each ear, they are connected by a duct to the upper gingival area under the upper lip. Most people produce approximately 1.5 liters of saliva per day. Usually taken for granted until it's compromised, saliva is a clear, alkaline, semi-viscous liquid which helps both in the digestion of food as well as cleaning, helping to keep exfoliated epithelial cells, most bacteria, and food particles away from the teeth. Salaspuro's study proposes that the parotid glands are able to metabolize alcohol into acetaldehyde.
Oral microflora may also produce acetaldehyde. Every individual has about 300 hundred different bacterial species in their mouth. That number increases exponentially in saliva, even more on tooth surfaces, and even more on gingival scrapings. Everyone develops their oral microflora within a few weeks after birth; many live and grow in people's mouths on a platonic basis, but some are harmful, such as those producing tooth cavities or those producing acetaldehyde.
The study found that ALDH2-deficient Asians were exposed to two to three times higher salivary acetaldehyde levels than either Caucasians or Asians with normal ALDH every time they drank, and for as long as they had elevated blood alcohol levels. The ALDH2-deficiency seemed to prevent those subjects from eliminating salivary acetaldehyde. Those with the normal ALDH enzyme were able to remove the acetaldehyde, likely formed in the parotid gland, before it was secreted to their saliva. Which is not to say that normal ALDH levels completely protect heavy drinkers from salivary acetaldehyde; Salaspuro noted that Caucasians that drank heavily for a number of years had much higher rates (20 fold) of esophageal cancer.