Lambic is a difficult beer to produce. Due to its spontaneously fermented nature it’s usually brewed between the months of autumn and spring. This is because strains of bacteria that are not desirable in fermentation are much more active during the hot summer months. The grist is generally comprised of two-thirds pale two-row barley and one-third unmalted wheat. The wort is boiled with aged hops that are at least a year old but, as is the case with Drie Fonteinen, can be much, much older. Once brewed, the finished wort is pumped into a large, flat tank known as a koelschip or coolship. Here it is left to cool overnight by the breeze that filters in through open slats or windows in the brewery wall.

With it, this breeze carries a unique culture of bacteria, such as Lactobacillus and wild yeasts such as Brettanomyces, that inoculate the freshly brewed wort. The exact microbes that ‘infect’ the beer will vary from one producer of lambic to another. This is how each producer can create a beer that has a distinctive signature. In a way, each lambic brewer has its very own terroir.

After a twenty-four hour cooling period, the inoculated wort is then moved into oak barrels or foudres where it will age for months, even years. Each of Belgium’s lambic producers will mark its barrels with the symbol of its brewery. For example, Drie Fonteinen simply marks its aging beer with a chalked "3."