Cloud, citrus juice

Similarly, there are present in citrus juices active enzyme systems that destroy the important pectic constituents. These enzymes have to be rapidly inactivated to protect the desirable cloud (turbidity) which is apparently stabilized by pectic substances. ... 

Juice Cloud. Mechanical extraction of citrus fruits yields a turbid suspension of wall fragments and cellular organelles in a serum composed primarily of cell vacuolar fluids. In most citrus juice products, such a suspension of fragments and organelles is a desirable component, since it provides most of the characteristic color and flavor (28). Essence and peel oils suspended in juice contribute desirable citrus notes to flavor, and these oils are rapidly adsorbed by juice particulate material shortly after extraction (29). 

If steps are not taken to stabilize cloud, most citrus juices will clarify when allowed to stand. Clarification occurs when native PE lowers the ester content of juice soluble pectin until it becomes susceptible to precipitation as insoluble pectates (23). If pectin levels are high enough, as in concentrates, these pectates may form a gel. 

Clarified Juice. Some citrus juices, notably lime and lemon, are in demand as clarified products. Natural clarification, in combination with filtration, is often used to achieve a sparkling clear serum. However, native PE action is slowed by the high acidity of these juices, and may not give satisfactory cloud removal. In this instance, modified pectins can serve to enhance juice quality by removing cloud. 

Also he speculated that citrus juices contain two cloud-coagulating enzymes of different thermostabilities. One enzyme, most active at low pH and temperature, appeared to be destroyed by heating the juice at 65 to 70°C (149 to 158°F). The second enzyme, most active at pH 3.0 to 3.3 and about 35°C (95°F), appeared to require heating to 888C (191°F) for inactivation (12). Stevens (13) described a rapid test for pectic enzymes in citrus juice. It involved adding pectin under controlled conditions of temperature and sample preparation, and measuring the time required for flocculation. Stevens, and coworkers (14) further elaborated on the patent work (12, 13). They produced a trend curve of the... 

A quantitative objective measurement of citrus juice turbidity was used by Loeffler (15, 16) to show that pectic enzyme changes occurred so rapidly after the juice was reamed from the fruit that at least a partial coagulation of the cloud occurred before the juice could be screened, deaerated and heated to a pasteurization temperature. He showed that juice turbidity was increased by flash-pasteurization and also by homogenization of the juice before pasteurization. Loeffler (15, 16) presented data on turbidity of flash-pasteurized citrus juices (heat exposure for 16 to 18 sec) after storage at several temperatures. He found that "samples pasteurized at 918C (196°F) lost their cloud when stored at 35°F (95°F) but others pasteurized at 93-95°C (199-2038F) retained their cloud almost indefinitely". 

Loeffler, H. J. Maintenance of cloud in citrus juice. Inst. Food Tech. Proc., 1941, 29-36. 

Endogenous pectin esterases play an important role in citrus processing they have therefore been studied intensively. They cause cloud loss in citrus juices, which is due to the calcium precipitation of enzymatically de-esterified pectin. This is desirable in the production of lemon and lime juices but undesirable in the production of orange juice. In orange juice concentrates strong calcium pectate gels may form which cannot be reconstituted... 

Cloud stability in citrus juices Pectin manufacture from citrus pomace Distillates from fermented fruit pulps Enzymatic maceration of fruits and vegetables Self-clarification of lemon/lime and apple juices Pressing characteristics of citrus pomace Ca2 -firming of fruits and vegetables... 

Figure 1 depicts the SEM of cloud substance from guava juice under various processing conditions. The cloud surface of the fresh sample (unheated and not pressurized) was irregular and with unique particle size (Fig. 1, A B). The particle distribution and appearance of pressurized juice (6,000 atm, 25 °C, 15 min) were similar to the fresh sample (Fig. 1, C D). Takahashi et al (1993) have reported that the soluble solid particle distribution in citrus juice does not change after high pressure treatment. However, the cloud surface of pasteurized juice was observed to be greatly different from the pressurized juice (Fig. 1, E F) due to coagulation of the small particles.

Pectolytic enzymes are described in section 4.4.5.2. Pectic acid which is liberated by pectin methylesterases flocculates in the presence of Ca + ions. This reaction is responsible for the undesired cloud flocculation in citrus juices. After thermal inactivation of the enzyme at about 90 °C, this reaction is not observable. However, such treatment brings about deterioration of the aroma of the juice. Investigations of the pectin esterase of orange peel have shown that the enzyme activity is affected by competitive inhibitors oligogalacturonic acid and pectic acid (cf. Fig. 2.51). Thus, the increase in turbidity of citrus juice can be prevented by the addition of such cort5)ounds. 

Because measurement of suspended pulp as volume/volume (v/v) in citrus juice is not precise, a mass relation was established by ultracentrifugation (250,000 X g in 2-mL minitubes) of juice to recover pulp (90% moisture) and remove all of the cloud. This pulp was added to pulp-free juice as percentage by weight of juice and the saturation point of t(-limonene in 11.8 °Brix juice at different pulp levels determined. Using a standard... 

Another very important development was the production of citrus comminutes. These were produced by mixing together, in appropriate proportions, the juice, peel components and essential oils of citrus fruits and comminuting the mixture in a stone mill. The resulting product delivered a more intense flavour and cloud than could be obtained from juice alone and allowed the creation of whole fruit drinks , which have dominated the concentrates market in the United Kingdom over the past 40-50 years. 

Citrus fruits, especially certain of their component parts, constitute one of the richest sources of pectin. On a dry weight basis, as much as 30% of orange fruit albedo may be pectin (8). The rag, comprising the fruit core and segment membranes after juice extraction, is also a rich source. Since pectin is a cell wall component, it follows that comparatively little would be present in juice expressed from fruit. For example, concentrations ranging from 0.01 to 0.13% in orange juice have been reported (15). Much of this would be present as cell wall fragments and particulate material in juice pulp and cloud. 

Versteeg et al. (50) have isolated various multiple forms of pectin esterases in oranges and shown that they differ in affinity to pectins and pectates and in heat stability and therefore may play different roles in cloud loss phenomena. One form (about 5% of the total PE activity) was found to be much more heat stable than the other forms it was also active at low pH (2.5) and at low temperatures. Another form did not cause self-clarification for this enzyme a similar mode of attack was proposed as for fungal pectin esterases which produce low-ester pectins less sensitive to calcium. Multiple forms of pectin esterases are present in the fruits of all of the orange varieties and citrus species tested. The two isoenzymes known to be responsible for cloud loss and gelation in citrus products were found to occur in all of the component parts of the orange fruit (51). In the French cider industry the endogenous PE of apple is used for the self-clarification of apple juice (52). 

Figure 7. a, Cloud loss in enzyme-inactivated orange juice by addition of citrus PE. Stabilization of cloud by adding PG together with PE. b, The activity of PE in both cases is shown by methanol release. (Reproduced with permission from ref. 52. Copyright 1974 Forster.)... 

For example, holding raw juice for a short time will allow destabilization of the cloud by citrus pectinesterase. This process is not reversible by mechanical homogenization or other treatment. 

Collet et al. (2005) stated that the study of pectinesterase inactivation behavior is important because pectinesterase is responsible for juice cloud stability loss, is composed of several isoenzymes, and occurs naturally in orange. Freshly squeezed juice of Pera orange (Citrus sinensis) was pasteurized at temperatures of 82.5, 85.0, and 87.5°C. At least five runs with different holding times were performed for each temperature. As the isothermal curves obtained showed deviations from the expected first-order kinetics, the data was statistically treated by applying a nonlinear regression, and the estimated best fit was a three-parameter-multicomponent-flrst-order model. At 82.5°C, the isothermal curves showed a nonzero asymptote of inactivation, indicating that at this temperature the most heat-resistant... 

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