Protein in citrus

The amount of protein in citrus fruit is relatively low (Table II), and the juice and peel have about the same amount (29). Much of the value that is considered as protein is either free amino acids or non-protein constituents which contain nitrogen. 

The total nitrogen of orange juices was found to increase with the maturity of the fruit and ranged between. 068 to. 120 g per 100 ml (30). The actual protein values obtained by Clements (31) were about 20 percent of the acetone powder. Nearly 30 percent of the alcohol-insoluble solids of juice and about 20 percent of that of vesicular pulp were found to be protein as determined by the Kjeldahl procedure (32). These values are the actual protein that was precipitated by alcohol and are only a fraction of the total protein values usually reported for orange juice (8). The main source of proteins in citrus juice is probably in the form of enzymes and the plastids. At least 47 different enzymes have been reported to occur in citrus fruits (33). Citrus fruits also contain several phenolic amines (34), some of which such as syn-epherine, may have physiological importance (35). 

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. 

Citrus species are well-known for their accumulation of flavone- and flavanone-glycosides, and thus should contain all of the enzyme activities necessary for the synthesis of these compounds. Two tentative consensus sequences for FNS-II have been identified by in silico analysis of the CitEST database, apparently representing the first identification of putative FNS-II genes in this genus [29]. Biochemical determination of function and analysis of the proteins encoded by these genes will be an important step toward elucidating flavone synthesis in Citrus. 

Since the acetal exists in equilibrium with the aldehyde, it is possible for the aldehyde to be released when water is added in a mixed drink, changing the balance and giving a burst of freshness to a mixed drink. Ethyl esters of terpene alcohols in citrus oils and other botanicals, plus the ethyl esters of fatty and volatile acids, are formed during prolonged exposure to ethyl alcohol. Certain beverage alcohol products that need to contain milk, eggs, or other protein containing materials must be developed carefully and the added flavors must be considered to prevent the precipitation of the protein and separation of the product. 

Some authors have indicated that the sugar-amino acid reactions of the Maillard type are of minor importance in citrus juices because of the high acidities involved. Studies in our laboratories (42-44) would tend to indicate that, to the contrary, the amino acids and sugars are of more than just minor importance in the darkening of citrus juices. Huffman (42) treated citrus juices with cationic ion-exchange resins to remove amino acids, proteins, and the mineral cations, then restored the cations. 

Other investigators have used maximum and minimum concentration values of specific amino acids as a criteria of authenticity. Brenoe (55) reported means and confidence intervals for all of the amino acids in Greek orange juice. Special emphasis was placed on glycine since it is present in very low levels in citrus juices but is relatively abundant in protein hydrolysates. 

Vitamin C is used to metabolize carbohydrates, for tissue repair and capillary endothelium, and for synthesis of protein, lipids, and collagen. Vitamin C is also needed for absorption of iron and folic acid metabolism. Vitamin C is found in citrus fruits, tomatoes, leafy green vegetables, and potatoes. Excess serum levels of vitamin C are excreted without any negative effects. Vitamin C is commercially available as Ascorbicap, Cecon, Cevalin, and SoluCap C. 

Vitamin C traps radicals formed in aqueous environments (Section 9.8). It is an antioxidant because it prevents oxidation reactions by radicals. Not all the physiological functions of vitamin C are known. What is known, though, is that vitamin C is required for the synthesis of collagen, which is the structural protein of skin, tendons, connective tissue, and bone. If vitamin C is not present in the diet (it is abundant in citrus fruits and tomatoes), lesions appear on... 

But what are the elusive ingredients in citrus fruits and rice hulls that prevent these deficiency diseases The late 19th century wimessed trendy attempts to produce synthetic animal feed using pure proteins, carbohydrates, fats, and minerals. The results were vastly inferior compared to animals raised on namral food. In 1906, British biochemist Frederick Gowland Hopkins (1861-1947) (1929 Nobel Prize in physiology or medicine) recognized the existence in natural foods of accessory factors, beyond the major components, that are required for a healthy diet. 

Flavones consist chiefly of glycosides of luteolin, chrysin, and apigenin. They are less common in fruits [8]. Polymethoxylated flavones, the most hydrophobic flavonoids, present in citrus fruits (mainly in the peel, the nonedible part of the fruit) are tangeretin and nobiletin. Apigenin and chrysin possess anti-inflammatory and free-radical scavenging properties in several cancer cell lines and inhibit tumor cell invasion, metastasis, and mitogen-activated protein kinases (MAPK) and their downstream oncogenes [26]. 

NIR spectrometers based on InGaAs diode array have been realized and successfully used for the determination of nitrogen (protein origin) and water content in citrus leaves. Measurement in the 900-1800 nm spectral interval revealed a broad spectral band, with a maximum situated at 1425 nm. This band for fresh whole leaf differs significantly from one for dried powdered leaf due to the abimdant water masking the nitrogen features in the NIR spectrum. 

In the course of the investigation of the antioxidative activity of EOs obtained from three different citrus species (Rutaceae), p-bisabolene was also determined for its antioxidative property. The LDL oxidation was measured spectrophotometrically at 234 nm by the formation of TEARS. The resnlts were expressed as nmol malonic dialdehyde/mg of protein. In the test system, p-bisabolene inhibited only TEARS formation from the AAPH-induced oxidation of LDL (Takahashi et al., 2003). 

The accumulation of lycopene, at the expense of jS-carotene formation, on treatment of plants with the substituted triethylamine CPTA (Figure 4.9) has been demonstrated in several cases, including citrus fruits and Aphanocapsa cells and cell extracts. In Aphanocapsa, in vivo and in vitro I50 values for lycopene cyclase were 45 and 30 pM, respectively, and a noncompetitive effect on the enzyme was shown. However, in citrus fruits a range of onium compounds related to CPTA regulate the formation of all- carotenoids, in that not only is cyclization blocked, but also the formation of lycopene is stimulated. The latter effect requires protein synthesis. Indeed, the tertiary amine MPTA [2-(4-methylphenoxy)triethyl-amine] causes gene expression and translation of [poly(A) ]RNA on SOS ribosomes. Therefore, CPTA and related compounds may have multiple effects in plants. 

When cell-wall fragments are incubated in molar NaCl, ionically bound proteins are released into the incubation medium. All investigated crude cell extracts deesterified Citrus pectin (Table 2) but the deesterification rates were clearly higher when the enzymes were still bound to the cell walls, indicating a major loss of activity during the solubilization process. 

The basal medium of Mandels (Mandels et al., 1976) was used with the following modifications it was buffered with 3 g/1 of sodium nitrate to pH 5.5 and supplemented with 1% w/v citrus pectin " Sigma" or other carbon sources. For enzyme production, 50 ml medium in 250 ml erlemneyer flasks were inoculatedwith spores (10 spores /ml ) exept for the non sporulating Pol 6 strain, where mycelium was used. The culture were incubated at 30° C on a rotary shaker (150 rev mn -1) for 5 days. The culture broth was filtered (Millipore 0.45 pm ) and the supernatant was analysed for pectinolytic activities, reducing sugars and proteins. 

The strains were cultured on Mandels medium + 1% citrus pectin for 5 days and the enzymatic activities of culture filtrates were determined on three substrates citrus pectin, polygalacturonic acid and filter paper, (a) extracellular proteins are in p.g/ml. (b) p>ectinolytic activities on pectin (PC) and on polygalacturonic acid (TO) and Pectin esterase (PE) are in units/ml. (c) total cellulolytic activity (filter paper, fp) are in mg of liberated reducing sugars/ml. 

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