Soft serve, also known as soft ice, is a frozen dessert, similar to ice cream but softer and less dense as a result of air being introduced during freezing. Soft serve has been sold commercially since the late 1930s in the US.

In the US, soft serve is not sold prepackaged in supermarkets, but is common at fairs, carnivals, amusement parks, restaurants (especially fast food and buffet), and specialty shops. All ice cream must be frozen quickly to avoid crystal growth. With soft serve, this is accomplished by a special machine that holds pre-mixed product at a very low, but not frozen, temperature at the point of sale.

Charles Taylor of Buffalo, New York patented an automatic ice cream maker in 1926 that is reported to be the first soft serve ice cream machine. His Taylor Company continues to manufacture the McDonald's ice cream machine. 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.^[2] 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 once every 4.5 seconds.

It is a common myth that during the late 1940s, future UK prime minister Margaret Thatcher worked briefly as a chemist for food manufacturer J. Lyons and Co., at a time when the company had partnered with the United States distributor Mister Softee and was developing a soft-serve recipe that was compatible with the American machines.^[6] Thatcher's precise role at Lyons is unclear, but she is reported to have worked on the quality of cake and pie fillings as well as ice-cream and researched saponification.

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 percent) than ice cream (10 to 18 percent) and is produced at a temperature of about −4 °C (25 °F) compared to ice cream, which is stored at −15 °C (5 °F). Soft serve contains air, introduced at the time of freezing. The air content, called overrun, can vary from 0 to 60 percent of the total volume of the 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.^[7] Ice cream with higher air content tastes creamier, smoother, and lighter and appears whiter. The optimum quantity of air is determined by other ingredients, if any, and individual taste. It is generally accepted that the ideal air content should be between 33 and 45 percent of volume. If more than this, the product loses taste, tends to shrink as it loses air, and melts more quickly than that with less air. Less than 33 to 45 percent, the product will not melt as quickly but will not taste good.

Some forms of soft serve, like many other processed foods, have palm oil.

All ice cream and similar products must be frozen quickly to avoid crystal growth, moreover, when soft serve is stored below freezing temperature after dispensation for any substantial length of time it will quickly freeze solid. Thus, in order to sell and consume soft serve in its most palatable state, it must prepared 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 (37 °F). When the 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, quickly frozen and stored until required.

While the most basic machines only dispense one flavor of the mix at a time, certain models of soft-serve machines have an additional nozzle that dispenses a mixture of two separate flavors simultaneously. This mixture emerges in a distinct swirl pattern. It is classified as its own separate flavor on menus as swirl or twist.

Pre-mix can be obtained in several forms:

• Fresh liquid that requires constant refrigeration until needed. It can be stored for 5 to 7 days before bacteria spoil it. Quality can be severely compromised by bacterial contamination and handlers must exercise caution to maintain quality.
• A powdered mix. This is a dried version of the liquid mix. It has the advantage of easy distribution and can be stored for long periods of time without spoiling. Water must be added prior to being churned and frozen. The disadvantage is that water quality cannot be guaranteed and some operators can put too much water in to make it go further. It also should be refrigerated to 3 °C (37 °F) prior to use, as airborne and waterborne bacteria can infect it immediately and can grow quickly if the product is warm. Residual bacteria in the refrigerated storage compartment can also be activated by warm products being introduced.
• Ultra heat treated mix, a liquid that has been sterilized and packed in sealed, sterile bags. It can last a very long time without refrigeration and can be poured into the soft serve freezer immediately upon opening. However, it should be refrigerated to 3 °C (37 °F) prior to use for the same reasons mentioned above. At the time of opening, quality can be guaranteed and bacterial counts are zero. Where it is available, health authorities consider it the safest form of soft serve mix on the market. It was first developed for commercial use in New Zealand in 1988 in a joint venture between Tatua Foods, a dairy company, and Bernie Cook, owner of Blue Boy, a mobile franchise network.