There are five basic tastes that can be perceived by the human tongue. These are bitterness, sourness, saltiness, sweetness and savoriness. Bitter is the most sensitive among these tastes, perhaps an evolutionary response to the fact that a large number of toxic substances found in nature are bitter. Sourness, on the other hand, is indicative of acidity.

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A popular myth that has now been debunked is that different areas of the tongue are involved in perceiving different tastes. In reality, all taste buds can sense all tastes, and taste buds are found all over the surface of the tongue, as well as in the cheek and upper esophagus.

Comparison chart

Bitter versus Sour comparison chart
BitterSourExamples Taste threshold Taste
Coffee, bitter melon, beer, arugula, swiss chard, citrus peels Lemon, orange, grape, melon, wine and sour milk.
Bitterness of substances is compared with bitter taste threshold of quinine which is 1. Sourness taste threshold is rated with respect to dilute hydrochloric acid which has a value of 1.
Unpleasant and disagreeable taste. Sharp taste that indicates acidity of substance.

Examples of bitter and sour food

Examples of bitter food include unsweetened cocoa, coffee, marmalade, beer, olives, citrus peel etc. Lemon, spoilt milk, oranges, grape etc are examples of sour food.

Threshold Substances for Bitter and Sour Taste Comparisons

Taste is measured and defined using threshold substances. For example, the threshold for stimulation of bitter taste by quinine averages 0.000008 M (where M is molarity, indicative of how concentrated the solution is). Quinine is given and index of 1 and the taste thresholds of other bitter substances are rated relative to quinine. Brucine has a bitterness index of 11 implying that it is bitterer than quinine. Hence the bitterness of brucine can be detected at lower concentrations in solution. Denatonium is the bitterest substance known at a bitterness index of 1000.

Sour taste is measured against the sourness bitterness threshold index of dilute hydrochloric acid which is 1. Hence Thus tartaric acid has a sourness index of 0.7, citric acid – 0.46 and carbonic acid – 0.06 when compared to threshold index of hydrochloric acid..

How Bitter and Sour tastes are perceived

Bitterness is perceived to be unpleasant, sharp, or disagreeable. The perception of bitter taste is facilitated by the combination of a taste receptor (Type 2) and a G protein (gustducin). The type of taste receptor, monomeric or surface bound, determines the various bitter ‘ligands’. In genetic studies for bitter taste perception, synthetic substances like phenylthiocarbamide (PTC) and PROP or 6-n-propylthiouracil are used. To ‘supertasters’ both these substances are extremely bitter.

Sourness is detected by the concentration of hydronium ions in the hydrogen ion channels. Hydronium ions are formed from water and acid. Hydrogen ions formed permeate the amiloride channels allowing for detection of sourness. Apart from these mechanisms of sour taste detection, there are other mechanisms also suggested. Among these prohibition of potassium channels by hydrogen ions leading to depolarization of cell, conversion of CO2 to bicarbonate ions facilitating weak acid transport are notable.


Being able to detect bitter substances at low concentrations is considered to be a protective function and hence is used by health researchers. Bitter tasting food is generally considered to be toxic and hence food processing techniques are used to detoxify these foods and make them more palatable. For example, Denatonium (a bitter, aversive agent) is added to toxic substances so that accidental ingestion can be avoided.

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Sour taste is usually found in fruits like orange, grapes, lemon etc. Spoilt milk is also sour. People who like sour tastes can try the very popular Cry Babies, lemon drops, Shock tarts and Warheads.