Cốt Nhĩ Hạ Thổ Thanh Quốc 45N is a traditional fish sauce produced by the Vietnamese company Thanh Quốc on Phú Quốc Island. It is made from fresh anchovies caught in the surrounding waters and sea salt sourced from Bà Rịa–Vũng Tàu. The fish and salt are combined in a ratio of three to one. The ingredients ferment for 12 to 15 months according to the traditional fish sauce production method of Phú Quốc. The first layer of pure concentrated sauce that naturally drains from the fermented fish mixture is collected for this product. The term cốt nhĩ refers specifically to this prized first extraction. Following the initial maturation period the sauce is stored underground for an additional two years. This process known as hạ thổ allows the sauce to continue ageing under more stable temperature conditions. The product has a natural nitrogen concentration of 45 grams per litre making it one of the most concentrated fish sauces in the company’s range. It is distinguished by its naturally dark amber to reddish-brown colour, full flavour and pronounced umami character.