Browning (food process)
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Browning is the process of becoming brown, especially referring to food. Browning foods may be desirable, as in caramelization, or undesirable, as in an apple turning brown after being cut. Foods, including beverages, can turn brown through either enzymatic or non-enzymatic processes.
Browning has an important economic cost, causing deterioration of the value of products in the market of food.
Enzymatic browning
Enzymatic browning is a chemical process, involving polyphenol oxidase, catechol oxidase, and other enzymes that create melanins and benzoquinone from natural phenols, resulting in a brown color. In general, enzymatic browning requires exposure to oxygen, for example the browning that occurs when an apple is cut.
Enzymatic browning can be beneficial for:
- Developing flavour in tea
- Developing color and flavour in dried fruit such as figs and raisins.
Enzymatic browning is often detrimental to:
A variety of techniques for preventing enzymatic browning exist, each exploiting a different aspect of the biochemical process.
- Lemon juice and other acids lower the pH and remove the copper cofactor necessary for the responsible enzymes to function
- Blanching or roasting, to denature enzymes and destroy responsible reactants, as used in the "kill green" phase of tea processing
- Low temperatures can also prevent enzymatic browning by reducing rate of reaction.
- Inert gas, like nitrogen, prevent necessary oxygen from reacting
- Chemicals such as sodium bisulfite and citrates
Arctic Apples are a suite of trademarked apples that contain a nonbrowning trait. Specifically, gene silencing is used to turn down the expression of polyphenol oxidase (PPO), thus preventing the fruit from browning.
Non-enzymatic browning
Contrary to enzymatic or oxidative browning, non-enzymatic browning is a chemical process that produces a brown color in foods without the activity of enzymes. The two main forms of non-enzymatic browning are caramelization and the Maillard reaction. Both vary in reaction rate as a function of water activity.
Caramelization is the pyrolysis of sugar. It is used extensively in cooking for the resulting nutty flavor and brown color. As the process occurs, volatile chemicals are released, producing the characteristic caramel flavor.
The Maillard reaction is a chemical reaction between an amino acid and a reducing sugar, usually requiring the addition of heat. The sugar interacts with the amino acid, producing a variety of odors and flavors. The Maillard reaction is the basis of the flavoring industry, since the type of amino acid involved determines the resulting flavor; it also produces toast.
The wine color is altered with wine aging by reactions between different active molecules present in the wine, these reactions, in general, giving rise to a browning of the wine, leading from red to a more tawny color.
Melanoidins are brown, high molecular weight heterogeneous polymers that are formed when sugars and amino acids combine (through the Maillard reaction) at high temperatures and low water activity. Melanoidins are commonly present in foods that have undergone some form of non-enzymatic browning, such as barley malts (Vienna and Munich), bread crust, bakery products and coffee. They are also present in the wastewater of sugar refineries, necessitating treatment in order to avoid contamination around the outflow of these refineries.
