Culled from How Stuff Works.
Mayonnaise is a white, creamy sauce that is made by combining egg yolk, oil, lemon juice or vinegar, and seasonings. Technically speaking, mayonnaise is an emulsion – a mixture of liquids that normally do not combine (like water and oil). Lecithin in the egg yolk is the emulsifier that binds the ingredients together so the mayonnaise remains as a thick sauce.
Although there is some uncertainty about the exact origins of mayonnaise, it is generally believed to have originated in France in the mid 1700’s. Today, it is one of the most popular condiments and sauces throughout the world. Note that there are regional differences in how mayonnaise is made. For example, in Russia, mayonnaise is typically made using sunflower oil which imparts a very distinctive taste.
In addition to being a sandwich condiment, mayonnaise is also used as a base sauce for other sauces such as hollandaise sauce, tartar sauce, ranch dressing, and thousand-island salad dressing. It is also used to create vegetable dips, potato salad, and a wide range of other foods. Mayonnaise is “crucial” for certain kinds of foods such as tuna sandwiches and BLT’s. They just wouldn’t be the same using mustard.
Mustard, sometimes called “prepared mustard”, is a condiment made from crushed mustard seeds combined with vinegar, wine, water, or other liquids plus seasonings such as salt, turmeric, and horseradish. It has a paste-like consistency and ranges in color from bright yellow to yellow-brown.
Mustard has been around for thousands of years and has been used as a medicine as well as a condiment. The ancient Greeks used mustard as a remedy for scorpion stings and as a treatment for tooth aches. The Romans were the first to develop prepared mustard by combining pulverized mustard seeds with wine.
Yellow mustard is the most commonly used mustard in the U.S. and Canada. This relatively mild mustard, which was introduced in 1904, is a staple condiment for hot dogs, hamburgers, pretzels, and sandwiches. It is also used in potato salads, salad dressings, barbecue sauces, and various other sauces.
Calories and Fat
Mayonnaise has a much higher fat and calorie content than mustard. Commercial mayonnaise is typically 70-80% fat, including lots of artery-clogging saturated fat. One tablespoon of mayo packs about 90 calories and 10 grams of fat versus 5 to 10 calories and less than one gram of fat for a tablespoon of yellow mustard.
So-called “low fat” mayonnaise products contain cellulose gel, cornstarch, potassium sorbate, or other ingredients to simulate the texture of real mayonnaise. Many of these products don’t taste as good as real mayo but they are healthier for you. As an example, Hellman’s Light “only” has 50 calories and 5 grams of fat per tablespoon.
For those with heart problems or those who are simply concerned about maintaining a healthy diet, mustard is a much better choice than mayonnaise. It is low in sugar, has almost no fat, and only 5 to 10 calories per tablespoon depending on the type of mustard.
Mustard reportedly has a number of other health benefits. It stimulates the appetite by increasing salivation. It also has anti inflammatory, antiseptic, digestive, laxative, and circulative stimulant properties. Mustard poultices applied to the chest aid in clearing sinuses and clearing the lungs. Some studies even indicate that the turmeric in mustard may have anti-cancer effects.
Mayonnaise has a relatively short shelf life. According to the USDA, commercially-produced mayonnaise can be kept in the refrigerator for two months. However, many people keep mayonnaise around for much longer than two months without any ill effects. Home-made mayo is a different story. It’s shelf life is no more than one to two weeks.
Mustard will last indefinitely without spoiling or growing harmful bacteria, although it will eventually dry out, lose flavor, and turn brown. Mustard does not require refrigeration but it will retain its distinctive pungency longer if kept refrigerated in a tightly sealed container.