You've dined at Vietnamese restaurants many times and you've always noticed the small bowl of sauce that accompanied your fried spring rolls and noodle salad known as bún bò xào. You've been mystified by its complex flavors and you've wondered what's in it that makes it so good. Wonder no more.
To really understand the sauce, it helps to learn a few terms. Nước mắm is a fermented fish sauce. The ubiquitous fish sauce is known by different names throughout Southeast Asia. It is called nam pla in Thailand, nam pa in Laos, ngan bya yay in Myanmar and patis in the Philippines. It is used to season food during cooking and it is also a condiment served as a dipping sauce to accompany cooked dishes.
Just as the Italians grade olive oil in accordance with purity, so do the Vietnamese with their nước mắm. An article in a Vietnamese website describes the fermentation and grading processes in detail.
"As soon as fishing boats return with their catch, the fish are rinsed and drained, then mixed with sea salt -- two to three parts fish to one part salt by weight. They are then pressed into large earthenware jars, lined on the bottom with a layer of salt, and topped with a layer of salt. A woven bamboo mat is placed over the fish and weighed down with heavy rocks to prevent the fish floating when the water inside them is extracted by the salt and fermentation process. The jars are covered and left in the sun for nine months to a year. From time to time, they are uncovered to expose the mixture to direct, hot sunshine, which helps to ‘digest’ the fish and turn them into fluid. Periodic ‘sunning’ produces a superior fragrant fish sauce with a clear, reddish brown colour. Eventually, the liquid is removed from the jars, preferably through a spigot on the bottom so that it passes through the layers of fish remains. Any sediment is removed and the filtered fish sauce is transferred to clean jars and allowed to air in the sun for a couple of weeks to dissipate the strong fishy odour. It is then ready for bottling. The finished product is 100-percent, top-grade, genuine fish sauce.
"Second and third grade fish sauces are made by adding salt water to cover the fish remains, leaving them for 2-3 months each time, then filtering before bottling. Finally, the fish remains are boiled with salt water, then strained out and discarded, to produce the lowest grade fish sauce; or they may be added to other fish remains from the first fermentation in the process of making second-grade sauce. Because the flavour is substantially reduced with each fermentation, top-grade fish sauce is frequently added to the lower grades to improve their flavour. In practice, few manufacturers market top-grade fish sauce, mixing it with second and third grade sauces instead in order to produce larger quantities that can still qualify as genuine fish sauce."
It is interesting to note that it seems impossible to get access to premium grade nước mắm.
If nước mắm is the bottled fish sauce, what is dipping sauce that goes with fried spring rolls?
Nước chấm is a dipping sauce, in general. Nước mắm pha is a mixed fish sauce. At its most basic, nước mắm pha contains lime juice and/or vinegar, fish sauce, sugar, and water. Optional ingredients include bird's eye chilies and garlic.
Nước mắm pha is prepared differently throughout Vietnam. In the north, the basic mixture is diluted with broth. In the central region of the country, the sauce uses less water and is, therefore, bolder. In the south, coconut water is added to nước mắm pha. Some recipes recommend boiling the sugar in water to completely dissolve it; others instruct that all the ingredients be simply shaken in a jar.
The color and flavor of nước mắm pha are affected by the color and the grade of the nước mắm. Nước mắm pha in southern Vietnam also tends to be darker because palm sugar is used.