Fruit juice, viscosity

An)nvay, the principal use of artificial tongues is within the food sciences. The applications concern almost exclusively liquid food mainly wine (about 18% of the studies examined), fruit juices (almost 15%), mineral water (about 13%), followed by infusions like tea and coffee, soft drinks, milk, beer, and other alcoholic beverages. All these liquid foods are characterized by both low-viscosity and high-polarity values. 

This unit describes a method for measuring the viscosity (r ) of Newtonian fluids. For a Newtonian fluid, viscosity is a constant at a given temperature and pressure, as defined in unit hi. i common liquids under ordinary circumstances behave in this way. Examples include pure fluids and solutions. Liquids which have suspended matter of sufficient size and concentration may deviate from Newtonian behavior. Examples of liquids exhibiting non-Newtonian behavior (unit hi. i) include polymer suspensions, emulsions, and fruit juices. Glass capillary viscometers are useful for the measurement of fluids, with the appropriate choice of capillary dimensions, for Newtonian fluids of viscosity up to 10 Pascals (Newtons m/sec 2) or 100 Poise (dynes cm/sec 2). Traditionally, these viscometers have been used in the oil industry. However, they have been adapted for use in the food industry and are commonly used for molecular weight prediction of food polymers in very dilute solutions (Daubert and Foegeding, 1998). There are three common types of capillary viscometers including Ubelohde, Ostwald, and Cannon-Fenske. These viscometers are often referred to as U-tube viscometers because they resemble the letter U (see Fig. HI.3.1). 

Capillary viscometers are ideal for measuring the viscosity of Newtonian fluids. However, they are unsuitable for non-Newtonian fluids since variations in hydrostatic pressure during sample efflux results in variations in shear rate and thus viscosity. This unit contains protocols for measuring the viscosity of pure liquids and solutions (see Basic Protocol) and serums from fruit juices and pastes (see Alternate Protocol). 

USING CAPILLARY VISCOMETRY TO DETERMINE THE VISCOSITY OF SERUMS, FRUIT JUICES, OR PASTES... 

Because of the high content of suspended solids and pectins in fruit juices, the use of a MF or UF pretreatment before the RO unit is also able to reduce the viscosity of the feed stream, increasing the transmembrane flux. 

The interactions between pectins and sugars (rhamnose, arabinose, and galatose) are principally responsible for the high turbidity and viscosity of fruit juice. Pectinases immobilized in membranes are used to reduce the viscosity of fruit juice [12, 13]. 

Pectic enzymes are used commercially in the clarification of fruit juices and wines and for aiding the disintegration of fruit pulps. By reducing the large pectin molecules into smaller units and eventually into galactur-onic acid, the compounds become water soluble and lose their suspending power also, their viscosity is reduced and the insoluble pulp particles rapidly settle out. 

Pectinex Ultra SP-L is a commercial enzyme preparation from Aspergillus acu-leatus that is used in the food industry for reducing viscosity in fruit juice processing. It contains different pectinolytic and cellulolytic enzymes [29]. In addition, the existence of fructosyltransferase activity in Pectinex Ultra SP-L has been reported by several authors [30-32]. In recent years, we have investigated the purification, characterization, and application of the fructosyltransferase from A. aadeatus contained in this commercial preparation [33]. 

Davidson, M.H. et al., A low-viscosity soluble-fiber fruit juice supplement fails to lower cholesterol in hypercholesterolemic men and women, J. Nutr., 128,1927, 1998. 

Unlike biopolymer dispersions where the intrinsic viscosity is known and the polymer concentration can be chosen a priori, often for fluid foods the concentration of soluble (e.g., pectins in fruit juices) and insoluble solids can be determined only posteriori, and the determination of their zero-shear viscosities is also difficult due to instrument limitation and due to the existence of yield stress. However, in many foods, it may be possible to identify the components, called key components, that play an important role in the rheological properties. 

Saravacos, G. D. 1970. Effect of temperature on viscosity of fruit juices and purees. J. Food Sci. 35 122-125. 

A new process uses ultrafiltration to clarify fruit juices (see below). The advantage of this method is that flocculation and fining aids are no longer necessary which results in cost savings. To optimise the throughput it is however recommended to treat the juice enzymatically in order to reduce its viscosity. Properly applied, this process can work very economically. 

The capacity can be increased further by reducing the viscosity of the liquid to be filtered. This can be achieved by treating fruit juices enzymatically and also by increasing the filtration temperature. The ideal temperature is determined by the temperature resistance of the membrane material and on the other hand by the fact that it must not impair the quality of the product. 

This process is used industrially to desalinate sea water for the production of potable water. However, it can also be used for fruit juice concentration. The achievable concentration depends very much on the viscosity of the concentrate. If the viscosity is too high the process is not economical. Since this process is rather expensive due to the high cost equipment it can only be employed for speciality products. In the future new membrane types with larger flow capacities could improve the economy of the RO process. 

In beverages, carrot pomace, or citrus waste will stabilize the natural color, improve the vitamin and fiber content, enhance the viscosity (mouthfeel) (Laufenberg et al., 1996), and enrich or adjust the cloudy appearance (Sreenath et al., 1995). The organoleptic and chemical properties offer a widespread use in healthy and functional drinks and selected fruit juices. 

It is natural that pectin, a normal constituent of fruits and berries, should be used with these foods when thickening and gelation are required. Although pectin does give nearly the same caloric effect upon digestion as does sucrose, it imparts a desirable body and viscosity to fruit juices at a much lower concentration than does sucrose. Knowledge of this fact is used in many canned fruits and berries prepared especially for dietetic purposes. 

Fruit juices Pectinases To improve fruit-juice extraction and reduce juice viscosity... 

Tara gum is useful in thickening various fruit drinks and diet drinks without sugar. Tara gum and carrageenan are used to stabilize chocolate syrups and chocolate powder mixtures. Nectars consisting of fruit puree, fruit juice, sugar, citric acid, and ascorbic acid get a good texture and a stable viscosity [85, 86] by the addition of 0.2% to 0.8% of Tara gum. 

For pure fluids (e.g. water, glycerine, ethyl alcohol, acetic acid) Bloembergen et al. [1] found theoretically and experimentally a correlation between the NMR relaxation times T and 72 > respectively, and the dynamic viscosity ii of Newtonian fluids, which is valid independently of temperature and pressure. Harz [3] could show that this correlation also holds for aqueous solutions like treacles, fruit juices, beer and wine. Further studies on silicone oil/glass sphere suspensions and beer mashes demonstrated that the 72-11 correlation, which originally was exclusively derived for Newtonian fluids, can also be applied to suspensions [4]. In contrast to solutions, the dependence is nonpotential. 

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