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Soft serve is a type of ice cream that is softer than regular ice cream, as a result of air being introduced during freezing. Soft serve ice cream has been sold commercially since the late 1930s.
Over Memorial Day weekend of 1934, Tom Carvel, the founder of the Carvel brand and franchise, suffered a flat tire in his ice cream truck in Hartsdale, New York. He pulled into a parking lot and began selling his melting ice cream to vacationers driving by. Within two days he had sold his entire supply of ice cream and concluded that both a fixed location and soft (as opposed to hard) frozen desserts were potentially good business ideas. In 1936, Carvel opened his first store on the original broken down truck site and developed a secret soft serve ice cream formula as well as patented super low temperature ice cream machines.
Dairy Queen also claims to have invented soft serve. In 1938, near Moline, Illinois, J.F. McCullough and his son, Alex, developed their soft serve formula. Their first sales experiment was August 4, 1938, in Kankakee, Illinois at the store of their friend, Sherb Noble. Within two hours of the "all you can eat" trial sale, they had dished out more than 1,600 servings (more than one every 4.5 seconds).
During the late 1940s, future UK Prime Minister Margaret Thatcher worked briefly as a chemist for food manufacturer J. Lyons and Company, at a time when the company had partnered with the US distributor Mr Softee and was developing a soft-serve recipe that was compatible with the American machines. Thatcher's precise role at Lyons is unclear, but she is reported to have worked on the company's ice cream products, as well as cakes and pies. A common anecdote in British left-wing circles is that by inventing soft serve ice cream, Thatcher "added air, lowered quality and raised profits".
In the 1960s, ice cream machine manufacturers introduced mechanized air pumps into vending machines, providing better aeration.
Soft serve is generally lower in milk-fat (3% to 6%) than ice cream (10% to 18%) and is produced at a temperature of about −4 °C compared to ice cream, which is stored at −15 °C. Soft serve contains air, introduced at the time of freezing. The air content, called overrun, can vary from 0% to 60% of the total volume of finished product. The amount of air alters the taste of the finished product. Product with low quantities of air has a heavy, icy taste and appears more yellow. Product with higher air content tastes creamier, smoother and lighter and appears whiter. The optimum quantity of air is determined by the other ingredients and individual taste. It is generally accepted that the ideal air content should be between 33% and 45% of volume. More than this and the product loses taste, tends to shrink as it loses air and melts more quickly than that with less air.
All ice cream must be frozen quickly to avoid crystallization. With soft serve, this is accomplished by a special machine at the point of sale. Pre-mixed product (see definitions below) is introduced to the storage chamber of the machine where it is kept at 3°C. When product is drawn from the draw valve, fresh mix combined with the targeted quantity of air is introduced to the freezing chamber either by gravity or pump. It is then churned and quick frozen and stored until required.