Here's a quote from an outake from the blogger's accompanying WSJ Asia article: "Sour soup is found in many Southeast Asian countries (sinigang in the Philippines, samlass m'chou moan in Cambodia, tomyam in Thailand, sayur asam in Indonesia), suggesting that it may have developed separately in each locale, probably in response to climate. Souring agents such as tamarind, lime juice, and vinegar are believed to possess antibacterial (and other healthful) properties, which would have come in hand in the days before refrigeration, and sour flavors are thought to have a cooling effect on the body. 'It's got zip to it,' says Ms. Nguyen of canh chua ga, and indeed the light, clean, sharp edge that sourness lends dishes is refreshing and energizing in tropically torpid weather. Perhaps most importantly, 'sourness rouses the appetite,' she adds."