Foam stability test

Figure 2.17 Illustration of a dynamic foam stability test apparatus in which foam is generated by flowing gas through a porous diffuser. Not drawn to scale.

The traditional kinds of physico-chemical characterization methods have been applied to beer foams, but potentially surface-active compounds in beer are so numerous, and their interactions so complex, that complete brewing and pouring/dis-pensing tests are still needed. There are also many foam stability tests available (see Section 2.6.2) but none has been universally accepted in this area [852], Tests that employ natural pouring tend to be inconsistent, while those employing porous... 

Similar three phase foam stability tests have been performed for presence of Salem (light oil) and... 

Table I. Antifoaming Efficiency of Different Types of Antifoaming Agents Measured by Foam Stability Tests...

One approach to determining the influence of an oil phase involves bulk foam stability tests conducted in the presence and absence of specified volume fractions of oil. An example of this kind of testing, in the... 

Foaminess. A measure of the persistence of a foam (the time an average bubble exists before bursting). Ideally independent of the apparatus and procedure used, and characteristic of the foaming solution being tested. In practice these ideals have not been achieved but some approaches to determining foaminess using dynamic foam stability tests have been reviewed by Bikerman [J3]. See also Dynamic Foam Test. 

The properties of the finished beer vary with the type of beer and place of origin. The figures in Table 1 do not, however, show much about the quality of the beer this can only pardy be expressed in figures based on objective measurements. The quality consists of aroma, taste, appearance, (color, clarity) formation, and stability of foam. Of these, the first two are still inaccessible to objective measurement. Although the aroma of a product is determined by the quantity of volatile alcohols, etc, the quality of the product cannot be expressed in those terms. Appearance, foam formation, and foam stability can be evaluated more easily. For judgment on taste and aroma, taste-testing panels are the only method. 

Foam stability is usually tested by one of three methods [4,6,13] ... 

The foaming capability and foam stability obtained from sparkling wines is usually tested by a dynamic foam stability method, as discussed in Section 2.6.2. Because these foams are evanescent and not really very stable, at least compared with the foams found in other industries, dynamic rather than static foam tests are the most suitable. In one version of the dynamic foam test, the Mosalux method, the foam heights are automatically measured using infrared beams and sensors [848],... 

A measure of the effectiveness of a defoamer. First, a column of foam is generated in a foam stability apparatus and the foam height is recorded. A measured amount of defoamer is added, and the reduction in foam height over a specified time period, for example, 2 s, is noted. The knockdown capability is the reduction in foam height. This test has many variations. Sometimes referred to as initial knockdown capability . 

A method for assessing foam stability in which one measures the rate of collapse of a (static) column of foam that has been generated by allowing a certain quantity of foaming solution to fall a specified distance into a separate volume of the same solution contained in a vessel. This technique is ASTM method D1173-53. See also Static Foam Test. 

Any of several methods for assessing foam stability in which one measures the rate of collapse of a (static) column of foam. See also Dynamic Foam Test, Foaminess. 

Cellular foam occurs at low vapor velocities in small columns, where the wall provides foam stabilization. It occurs with some systems or tray designs but not with others and is promoted by surface tension effects such as the Marangoni effect (99). Cellular foam is uncommon in industrial columns. The foam that causes problems in industrial installations is mobile foam, where the bubbles are in turbulent motion. Mobile foam is associated with the froth and emulsion regimes. Cellular foam is encountered in bench-scale and pilot-scale columns. If cellular foam occurs in the test unit, caution is required when scaling up the results. 

The quantity xp is a much better defined characteristic of foam stability (since the pressure in the borders along the height of the foam column remains constant during its destruction). This parameter is also much more sensitive to the kind of surfactant, electrolyte concentration and other additives, compared to the lifetime of the foam in gravitational field, with an averaged pressure value from 0 to pgH. Estimation of the stability of foams from different surfactants by xp and by the Ross-Miles test has been reported in [16]. The results are discusses in Section 7.6.1. The advantages of the Foam Pressure Drop Technique and, respectively, xp as a characteristic of the foam stability, are clearly shown. 

The increase in foam stability with the decrease in antifoam efficiency can be expressed as the difference between the relative foam stability at the end and at the beginning of the test... 

The antifoams were studied on a foam from 1% commercial sulphonol solution. The rate of foam breakdown, caused by the antifoam sprayed, is determined by the foam volume destroyed for 1 min and by the lifetime of one half of the foam column. As tests showed the change in the content of the foaming composition affects the activity of the antifoaming agent but the relative foam stability almost does not change. 

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