PT,ASTI('ITT O F STARC'H PASTE* BY CARL BERGQUIST
For many years. a chemical analysis of starch has been used as a criterion of quality. Acidity or pH value, protein, ash, moisture, fat and fibre have
been determined. These figures are a check on certain processes in the manufacture of starch. What the customer is interested in, however, is: How does the starch cook up when it is going t o be used? Simple jelly tests have been made, but they leave too much for individual judgment, The Mchlichael Viscometer has been recommended. This instrument is not practical, as starch paste actually thins down during the test. The Scott Viscometer is employed. The objection to this instrument is that it only gives one point on the flow curve. and that pastes of low concentration at fairly high temperatures must be employed. This is a general objection t o most investigations on the viscosity of starch. Very dilute pastes at high temperature have been used as a rule. I n practice, generally a concentration of one pound of starch t o one gallon of water is the rule. Everybody knows that a paste of thip kind does not flow a t ordinary temperature. It is not viscous, but plastic: it has both a setting value and a flow value, or, in other words, yield and mobility. These two values can readily be deterniined by means of the Bingham Plastometer. A paste containing 7c; dry starch IS prepared by mixing the starch with cold water, and then heating it t o 95" c'. for 4-i minutes, ujing a screw agitator. The paste is then poured into the containers for the plastometer, left t o set, over night at zj" C., and tested in the usual way the next day in the plastometer. Vsing a capillary 0.13 cin. in diameter and 2.6 c.m, long, and a pressure up t o 850 gnis. per sq. cm., t h e yield value for thick-boiling corn starch will vary between 2 5 0 t o 400, and the mobility between 0.030 t o 0.100. I t can definitely be stated that a starch having a mobility higher than 0.05 j is not n first grade starch. The yield value does not seem to he so important. This comparatively large capillary must be used on account of the nature of starch paste, A smaller capillary is more liable t o he clogged by impurities or small lumps. X higher pressure or a lower concentration would he required for a smaller capillary, and neither is desirable. Thus, for the first time. we have been able t o deteriiiine and express in niiinerical values two distinct qualities of starch paste. -1 complaint on starch being thin cpn he verified without calling on a man's personal judgment. .As mentioned I rcviously, the usual concentration of starch paste in practice is one pound t o a gallon. This paste will set to a jelly, but through con* Paper presented at the Plabticity Symposium, Lafnjettc Collegc~,0c.t 1 7 (1924).
PL.k>TICITT O F ST.kRCH PAsTC
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tinuecl stirriiig tlic paste will become semi-fluid. \\lien a starch pa$te is hot,, that i?, o i w 70' (*., stirring does not, thin it down. Below this temperature, stirring makes it thinner. If stirring is continued vigorously at, room teniperature, the paste will finally become as thin as water. In our opinion. this s h o w t'hat stmarchpaste is viscous at a temperature over i o o C'.> while helow that teiiiperature it is plastic. In practice, low temperatures are used. In several pla#ccs,st'arch paste is pumped around cold in pipe lines marly liundred feet long, and gradually thins down, causinp complaints. Rut this is the very nat,ure of starch paste. =1 fatigue value is thus introduced. This is also illustrat,ed in the use of dextrine. Aisis well known, heavily sized bond papers are hard t o stick together. Thin dextrines are used, but many times do not give satisfactory results. A stripping gum, that is, a fairly-thick-hoiling dextrine mixed with borax, readily binds this paper. On trying this 011 the envelope machine, the &ripping gum a t first gave good results. hiit in a short time it' becanie too thin. The dextrine had practically no yield value, but the st'ripping gum had a comparatively high yield value. The very rapidly revolving glue roll in t,he envelope machine destroyed t,he yield. and thus thinned down the paste. 11-e h a w tried the Murray Tube instead of the original plastonieter. This instrument is comparatively simpler to run, and less time is consuined in calculating results. The AIurray Tube, however, gives another set of values for yield antl mobility. We believe there is adhesion between the starch pastr and the glass, and this shows up in the long tube. The curves, where flow is plotted against shearing stress, all have a marked bend towards the origin. I t should be mentioned that, values for yield anti mobility do not indicate the adhesiveness of a paste. Surface tension has been mentioned as a possible factor. -1thick starch pa,ste will give high readings on du Nouy's instrument,,but as this paste does not form drops, it must be the tensile st,rength of the film antl not surface tension t,hat is measured. How this adhesiveness or internal cohesion can be measured is a probleiii for the fut,ure. The plastometer has given us the only values n-e have for the flow of a paste. Research Laborntory, Corn Products Refinina C o m m n i i . Edgeicater, X. J . S e p t . 30. 1.924 1
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