Browning of citrus juices

This study was conducted in an attempt to understand better the effect of oxygen on non-enzymatic browning of citrus juices and the involvement of metals in this process. 

In order to better understand the effect of ascorbic acid on non-enzymatic browning of citrus juices, we oxidized endogenous ascorbic acid in grapefruit with ascorbic acid oxidase. The enzyme oxidized ascorbic acid to dehydroascorbic acid and H2O without forming H2O2 (19). 

The sugars, which contribute much to the acceptability of citrus juices, under adverse conditions can play a major role in the formation of off flavors that reduce the acceptability of the citrus juices and their products. The sugars, primarily the hexoses, can participate in "browning" reactions that cause darkening of the juice and these reactions give rise to components that are described generally as apricot-like or pineapple-like in flavor. In general,the more processed flavor that a citrus product exhibits, the less acceptable it becomes to the consumer. 

Another predominant furan, namely furfural, is described as sweet and bread-like caramellic. This furan can interact with hydrogen sulfide of juice to produce thiofurfural, a compound with a skunky odor (35). Furfural has an important role in the monitoring of citrus juice quality (36), and has an especially significant relationship to browning (34). The main source of furfural in aged citrus products is by oxidative degradation of ascorbic acid. Furans, such as deoxyfuroin, furoin and furil are probably formed by self-condensation of furfural (37). 

Citric juices contain iron and copper ions (30,31) at a concentration which could catalyze ascorbic acid oxidation. Contamination of citrus juices by transition metals will affect the rate of oxidative browning, especially in products exposed to air, such as citrus juices stored in plastic metallized laminate bags which are permeable to oxygen. 

When ascorbic acid is heated in the presence of acids, furfural is formed. The latter, either by itself or after reacting with amino compounds, polymerizes to brown products. Citrus juices, especially their concentrates, develop browning, which has been attributed to ascorbic acid degradation. 

In neither process is it commercially feasible to produce a dehydrated juice without the addition of a drying aid, although 100% orange juice has been produced with the continuous vacuum belt dehydrator (42,A3). This product is extremely hygroscopic and very temperature sensitive. As a result, the product "cakes" or hardens if exposed to moist air or to temperatures much above 24°C. The product is also subject to browning if not stored at refrigerated temperatures. Dehydrated citrus juices are produced on a vacuum belt dryer at Crystals International, Plant City, Florida, and are items of commerce. 

The piece of apple that is rubbed with lemon juice does not turn brown because molecules in the lemon juice change the enzymes in the apple. When the enzymes are changed, they cannot cause the oxygen molecules to add to molecules in the apple. Lemon or citrus juice may be added to fresh fruit in a fruit salad to keep the fruit from turning brown. 

Although the inhibition of ascorbic acid oxidation in citrus juices (22,23) and other foods by EDTA was reported by several researchers (22-24), the possibility that EDTA could inhibit non-enzymatic browning in grapefruit or other citrus juices has not been explored. 

Many furan derivatives have been identified as degradation products associated with browning in citrus products and model systems. Generally, furan derivatives are considered important aroma constituents from a sensory point of view. They are mainly associated with sweet, fhiity, nutty or caramel-like odor impressions. Furans apparently determine the overall browning of storage-abused citrus products 16, 17). Among them, hydroxymethylfurfural (HMF) and furfural are two major furan derivatives in citrus juices, and both HMF and furfural are considered as useful indicators of temperature abuse in citrus juices. 

Several types of Brown extractors are used in the citrus industry throughout the world. The Model 400 produces a juice that is low in peel oil content and high in juice quality. The fruit is halved and the juice removed by a rotating reamer that exerts pressure to effect extraction. 

The protein in citrus is generally associated with the solid portions of the fruit, i.e., the seeds, flavedo, albedo, chroma-tophores, and pulp. Some of these components find their way into the juice along with the available free amino acids during extraction and processing and storage. Studies conducted in our laboratories (42,43,44) and by others (45) have shown that reductions in the pulp content of juice slow the rate of browning. 

Thus, the potential role of DMHF as a flavor impact compound and as an intermediate to form additional flavor compounds in heat-abused citrus products should be carefully considered. 2,3-Dihydro-3,5-dihydroxy-6-methyl-4-H-pyran-4-one was also isolated from dehydrated orange juice (37) and is known as a novel nonenzymic browning reaction product. This pyranone has a high threshold value, over 200 ppm (38), and appears to possess negligible odor character. 

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