Acid coagulation

Despite the anhydride stmcture, it is remarkably stable, surviving addition to the highly alkaline viscose, the acidic coagulating bath, and also resisting multiple laundering of the rayon fabric. The unusual stability may be attributed to the sulfur atoms, which enhance hydrophobicity, and to the stericaHy hindering neopentyl groups that retard hydrolysis. 

Dissolution of the cellulose in cuprammonium solution followed by acid coagulation of extruded fibre ( cuprammonium rayon —no longer of commercial importance). In this case the acid converts the cuprammonium complex back into cellulose. 

At the pH = Jt there is a balance of charge and there is no migration in an electric field. This is referred to as the isoelectric point and is determined by the relative dissociation constants of the acidic and basic side groups and does not necessarily correspond to neutrality on the pH scale. The isoelectric point for casein is about pH = 4.6 and at this point colloidal stability is at a minimum. This fact is utilised in the acid coagulation techniques for separating casein from skimmed milk. 

Destruction of the casein micelles in the milk with subsequent precipitation of the casein can be accomplished in a number of ways. The action of heat or the action of alcohols, acids, salts and the enzyme rennet all bring about precipitation. In commercial practise the two techniques used employ either acid coagulation or rennet coagulation mechanisms. 

There are at least 1000 named cheese varieties, most of which have very limited production. The principal families are Cheddar, Dutch, Swiss and Pasta filata (e.g. Mozzarella), which together account for about 80% of total cheese production. All varieties can be classified into three superfamilies based on the method used to coagulate the milk, i.e. rennet coagulation (representing about 75% of total production), isoelectric (acid) coagulation and a combination of heat and acid (which represents a very minor group). 

Figure 10.26 Examples of acid-coagulated or heat-acid coagulated or whey-based cheese varieties (from Fox et al, 1996a).

Figure 10.27 Protocol for the manufacture of fresh acid-coagulated cheese (from Fox et al ...

Acid-coagulated cheeses are usually produced from skim milk and are consumed fresh. Major varieties include quarg, (American) cottage cheese, cream cheese and petit suisse. These cheeses may be consumed in salads, as... 

Minerals found in milk which are insoluble remain in water in the curd and are more concentrated in the cheese than in milk. About two-thirds of the calcium and one-half of the phosphorus of milk remains in cheese. A major portion of the milk calcium is retained in the curd of cheese made with coagulating enzymes. Acid coagulation alone results in the loss of portions of both calcium and phosphorus salts in the acid whey, since these minerals are more soluble in the acidic medium. Most milk fat and fat-soluble vitamins are retained in the curd, but a considerable amount of water-soluble vitamins is lost during cheese manufacture. Retention of part of some B-complex vitamins in curd is due to their extended association with casein in the original milk. 

Bovine casein is listed by the International Union of Immunological Societies as a single allergen, Bos d 8. However, it contains four main protein components, a-sl-, a-s2-, 3-, and K-Casein, in approximate proportions of 40% 10% 40% 10%, respectively (Bernard et al. 1998). The fractions obtained after acid coagulation of individual caseins are cross-linked to aggregates called nanoclusters, which combine into micelles. Their central part is hydrophobic and peripheral hydrophilic parts contain sites of phosphorylation. 

Rayon. Viscose rayon, like cotton, is comprised of cellulose. In the manufacturing process, wood pulp is treated with alkali and carbon disulfide to form cellulose xanthate. Subsequently, the reaction mass is forced through a spinneret and precipitated in an acid coagulation bath as it is formed into a continuous filament. The fiber has a round striated cross-section. Rayon staple is made by breaking the continuous strands into staple-length fibers. Viscose rayon is conventionally produced in diameters varying from 9 to 43 microns. 

Hinrichs, R., Bulca, S., and Kulozik, U. (2007). Water-mobility during lenneting and acid coagulation of casein solutions a differentiated low-resolution nuclear magnetic resonance (NMR) analysis. Int J. Dairy Technol 60, 37-43. 

The expression transition refers to a change in physical state and, in a food, the transition of concern is often either from liquid to solid, solid to liquid, or solid to solid. It is caused primarily by a change in temperature (heating and/or cooling) or pressure (Roos, 1998). However, auxiliary conditions, such as pH and presence of divalent ions, as well as enzymatic action aid liquid to solid transitions. For example, gels can be created from Casein either by enzymatic action followed by precipitation with Ca + or by acid coagulation. 

Acid coagulation Solubilization of the colloidal calcium phosphate and aggregation when the i.p. is approached (charge neutralization)... 

Bringe NA and Kinsella JE. Eorces involved in the enzymatic and acidic coagulation of casein micelles. In Hudson BJF, Ed. Developments in Food Proteins, Vol. 5, Elsevier Applied Science Pubhshers, London, 1987, pp. 159-194. 

The separation of the rising suspension can be achieved by several methods. Diamadopoulos and Woods [28 ] recently evaluated an application of membrane filters for separating fulvic acids coagulated with aluminium chloride. They... 

Analogous to this processing, Korolczuk and Mahaut [80] report the necessity to use ceramic membranes for the filtration of acid-coagulated milk in order to produce UF-fresh cheeses with good taste. Typical fluxes, using Carbosep Ml (cut-off 50,000 D) increase from 10 to 20 1/m h at 40°C, with decreasing concentration factor. 

Fine wrinkling of the skin when pinched the acid coagulates proteins that ensure cohesion between the dermis and epidermis, and the epidermis slides on the dermis to which it is no longer firmly attached. 

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