Limonin content

Limonin has been known to be an intensely bitter substance since it was first isolated in 1841 by Bernay (23). Beginning in 1966 the development of analytical methods for limonin allowed correlations of apparent bitterness with juice limonin content to be undertaken. The various reports of relative bitterness of suprathreshold limonin levels have been reviewed by Maier et al. 

The amount of limonin incorporated into the juice is influenced by a number of juice processing variables. Factors such as the maceration of albedo, central vascular bundle and carpellary membranes, the time of contact between the rag and the juice, and the amount of pulp incorporated into the final juice product are known to influence juice limonin content. The uneven distribution of XIV in the various tissues of the fruit (Table VII) is largely responsible for the effects of processing variables (33). 

Only about 6% of the total XIV content of the fruit is present in the juice vesicles. Any step in juice preparation that increases incorporation of the other tissues into the juice or that increases the extraction of XIV from those tissues will increase the limonin content of the juice. Consequently, techniques that increase juice yield, such as hard versus soft extraction, tend to increase limonin content (34). 

Table VIII. Limonin Content of Commercial Orange and Grapefruit Juices Throughout the Harvesting Season...

The inverse relationship between limonin content and taste preference was confirmed in another study (41) using a stepwise multiple regression analysis of data from 60 samples of commercial frozen-concentrated orange juice (FCOJ) packed during two seasons. This and a latter report (42) concluded that limonin content was highly correlated with the flavor quality of the juice. 

Determination of Limonin Content. A summary of the methods reported through 1976 for the quantitative determination of limonin in citrus juices was published previously (3 ) Since then improvements have been made in the high-pressure liquid chromatographic method (44, 45) and advances have been made in the development of enzymatic (46) and immunoassay (47, 48) (see Chapter 15) methods. The latter methods hold great promise for the future. However, in each case the required enzyme or antibody is not yet commercially available. Thus, while substantial progress is being made the need still exists for a simple, sensitive, and rapid commercially available method for routine industrial quality control purposes. 

Several postharvest treatments to citrus fruits have been tested in an effort to improve the quality of the extracted juice. Bruemmer and Roe subjected citrus fruits to anaerobic conditions for periods of 20 to 32 hours at 32.2 to 43°C (228, 229). This treatment reduced the titratable acidity and increased the Brix-acid ratio by about 10%. The decrease in acidity was accompanied, however, by a 20-fold increase in ethanol (229). Since the soluble solids-acid ratio is a major criterion of citrus juice quality, this procedure, if perfected, could allow earlier harvesting of fruit and a more consistent supply of fruit during the processing season. Bitterness of products from navel oranges, lemons, and grapefruit is related to limonin content. A 3-hour treatment of fruit with 20 ul ethylene/1 of air lowered the limonin content, reduced bitterness, and the juice was judged more palatable than juice from untreated fruit (230). 

HPLC/RIA Correlation. We compared the limonin content of 180 samples of canned, single-strength grapefruit juice which had been analyzed by HPLC at the Lake Alfred Experiment Station with our H RIA system (Fig. 5). A paired t test showed no difference between the two methods and a linear regression gave r = 0.794. 

Tatum JH, Berry RE (1973a) Method for estimating limonin content of citrus juices. J Food Sci 38 1244-1246... 

While our work on the RIA and EIA for limonin was in progress, a research group in Israel (35) reported on an immunoassay for estimating the orange juice content of commercial soft drinks and reconstituted juice. This is of great importance to the citrus industry since adulterated or improperly reconstituted products greatly affect the quality of the product and thus ultimately reflect upon citrus quality control credibility. 

Essential oils obtained by cold pressing or extraction contain, in addition to volatile substances, low volatile and non-volatile substances, such as oleoresins. For example, the dry matter content in citrus essential oils is about 4%. The main components are flavonoids, such as naringin, or metabolites of bitter triterpenes of limonin type. Coumarins such as scopolaron are present at a level of 1.5%, along with furanocoumarins (psoralens), such as bergaptene and bergamottin. 

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