Cottage cheese made with a food-grade acid must be labelled as "Direct Acid set". Usually, a small amount of low -producing citrate-fermenting lactococci or leuconostoc bacterial strains are added to the starter mix for the production of diacetyl for added buttery or creamy flavors. Producers must be careful that the final product contains approximately 2 ppm diacetyl, and that the ratio of diacetyl to acetaldehyde is 3–5 to 1, to achieve the typical cottage cheese flavor. Too small a ratio and the product tastes grassy, too much and the taste becomes harsh. Cottage cheese is naturally a yellow colour due to the cream dressing, but to increase consumer acceptance and appeal of the final product titanium dioxide is usually added to the dressing to make it a brilliant white colour and enhance marketability of the finished product. In the United States the FDA allows the additive in many dairy products (not whole milk) up to 1% of total volume by weight, but it must be labelled in the ingredient list. It may also be used in Canada and the European Union. Relatively recently producers have added this ingredient in nanoparticle form. In the USA the FDA does not restrict nanoparticle technology usage in food, but in Europe it must be first submitted for approval as a food ingredient. According to the Project on Emerging Nanotechnologies it is found in hundreds of products, not always labelled as such, including many organic products, however a number of large US producers have denied using it. Cottage cheese may be marketed as a small-curd (<4 mm diameter) or large-curd (>8 mm diameter). Cottage cheese is popular among dieters and some health food devotees. According to one person, it is popular among bodybuilders and athletes for its high content of casein protein while being relatively low in fat. Pregnant women are advised that cottage cheese is safe to eat, in contrast to some cheese products that are not recommended during pregnancy. The sour taste of the product is due to lactic acid, which is present at 124–452 mg/kg. Formic, acetic, propionic and butyric acid contribute to the aroma. In the United States, cottage cheese is popular in many culinary dishes. It can be combined with fruit and sugar, salt and pepper, fruit purée, tomatoes, or granola and cinnamon. It can be eaten on toast, in salads, as a chip dip, as a replacement for mayonnaise in tuna salad, and as an ingredient in recipes such as jello salad and various desserts.