The consistency of starch-milk puddings. I.—Preparation of puddings and measurement of consistency

THE CONSISTENCY OF STARCH-MILK PUDDINGS. I .-Preparation of Puddings and Measurement of Consistency

By R. RUTGERS

A standardized procedure for the preparation of starch-milk puddings is evolved and a study is made of various types of apparatus for the determination of the viscosity and consistency of the products. The limitations and applications of the instruments are discussed.

Introduction In the Netherlands, custard puddings containing 4-0-4.5% of starch in milk are widely

consumed, the majority of which are factory products. The puddings consist of maize starch with added colour and vanillin, cooked in milk, cooled to 60-85", and bottled. The bottles can then be pasteurized or sterilized, after which they are kept at 8-10' until they reach the consumer, usually within a few days. The pudding should not show wheying-off (syneresis) and should not be too thick, and after shaking, it should flow freely from the bottle. The consistency should be maintained independent of length of storage.

Much has been published about the gelatinization of starch and the consistency of starch pastes and gels, but there is little about thin starch-milk puddings and their consistency after storage at room temperature. An investigation was therefore made concerning the influence of the variable cooking factors and keeping conditions on the consistency of these puddings and of methods of determining their consistency.

I. Preparation of the puddings For comparative purposes a reproducible method for preparation of the puddings is essential.

Different methods of heating are used in various literature reports, e.g., over a flame, by heating on a water- or steam-bath or on an electric hot-plate at a temperature usually of 95". Starch concentration varies from 0.6 to 13%, although for the study of gelling properties, concentra- tiops above 4.0 to 4'5% are used. The cooking time varies from I to 15 min. an3 the mixtures are cooled in air or water, usually without stirring. Viscosities are often measured on the hot suspensions, and consistency by various methods after cooling for 4-24 h.

Preliminary experiments, in which products of variable consistency were obtained, indicated the necessity for standardization of the conditions of heating, stirring and cooling. In the standardized procedure finally evolved, regard was had to the method used in the commercial production of these puddings. Cooking vessels used are often of 1000 1. capacity, steam- jacketed and having a stirrer with one blade, placed about one-third of the way up the tank and having a speed of 50 r.p.m. Velocity at the circumference of the stirring-blade is about 150 m./min.

Standardized method of preparation of the puddings The pudding mixture is cooked in a 400-ml. aluminium beaker (99 mm. high x 72 mm.

diam.) placed in a bath of CaC1, solution. Over the beaker is a cover through which passes a filling funnel, the thermometer and the stirrer, The mechanical stirrer has two blades, each of three fins, the lower blade being just above the 'bottom, the higher somewhat below the surface of the liquid. Using 265 g. of milk and the stirrer set at 250 r.p.m., the stirrer draws a slight vortex in the gelatinized paste but the upper fins are still under the surface. The velocity a t the circumference of the fin is about 60 m./min. The milk is initially heated with stirring at 140 r.p.m.

A suspension of the starch and sugar in 20 g. of cold milk is added through the funnel. The beaker and, funnel are rinsed clean with 5 g. of cold milk and then with some water. The addition takes 1-14 rnin., the temperature falling to 89", gelatinization starting immediately. After the addition of the starch, the rate of stirring is increased to 250 r.p.m. After 1-1) min. the liquid has again reached 95" at which it is kept for 7) min. The heating bath is removed and the pudding cooled in the air

Procedure.-qo g. of milk are heated in 4 min. to 95'.

J. Sci. Food Agric., 9, February, 1958 E

62 RUTGERS-CONSISTENCY OF STARCH-MILK PUDDINGS. I

for 18 min. with stirring until its temperature reaches 70°, after which it is poured immediately into another vessel (400-ml. squat beaker with cover) for storage at the desired temperature. Without sterilization the puddings can be kept for 24 h. a t 25" without bacterial spoilage.

This loss depended on cooking time and temperature and on the method of cooling (see Table I), so that the amount of water added was varied according to the cooking in later experiments and the cooking loss was thus kept to within -+0.3-0.4~~.

The water added compensates for the low cooking loss.

Table I Cooking losses of starch-milk mixtwes under diffeevent conditions

Temperature, Time of heating, Cooking loss, " C min. % 95 74 86 74 77 71) 95 60 7'0 95 74 4.0;

* cooled in air to 70' (18 min.) t ,, ,, ,. ., 30'' ( 1 1 0 min.)

3.0'

0.6 1.2

11. Measurement of consistency Viscosity of pastes

As starch-pastes are non-Newtonian masses, more than one shear-rate should be measured to obtain a rheogram. For foodstuffs, sensory impressions must be the reference for the measurement with instruments' and this organoleptic judgment should determine which range of the rheogram is important.

These include the Amylograph, V.I. Viscograaf,z Corn Industries Viscometer,3 Epprecht R h e ~ m e t e r , ~ ~ Brookfield Meter,4 Stormer Meter,4 and the Haake Rotaviskometer.7 With several of these instruments it is possible to apply a number of shear-rates and thus obtain a rheogram. The rotating body is usually a cylinder, but spirals,s wire frames and a tuning-fork shapes have also been suggested.

A number of studies have been reported on the measurement of the consistency of starch suspensions during the heating process, and tests are usually made on hot pastes using only one shear-rate.

Another principle of measurement used is the rate of flow from a small orifice in a pipette or cup such as in the Stern meter,S Scott meter,4 Fordcup, Clinton funnel,1° or a Hagberg funnel." As the paste runs out under its own weight, the applied force varies continuously throughout the measurement, and a rheogram is not obtained.

Viscosity can also be measured using falling bodies, e.g., the Hoeppler viscometer with spheres falling in an inclined tube, and the Haake Visko-Waages in which a sphere is forced upwards through the paste. The Gardner Mobilometer makes use of a falling, perforated disc. Spreading methods are useful for thicker pastes such as porridges and thin jellies. The Bostwick meter12 was originally designed for tomato puree and used in the official U.S. method for determination of the consistency of this material. I t consists of a device whereby the paste is con- fined in a container closed by a lock gate which, when raised, allows the paste to flow d o m an inclined plane. The distance moved in 30 sec. is measured. With the sag method,I3 the flow is in four directions and the spread is measured. These methods do not differentiate between viscosity and structure, although the latter should be taken into account.

Firmness or rigidity of gels

the deformation before the gel breaks. i.e., resistance to reversible deformation.'$ resulting deformations gives a multipoint rheogram. recovers, and such an unbroken gel is elastic but not completely so.

For thinner pastes, rotation-viscometers are used.

Starch and pectin gels are tested using an apparatus which applies pressure and measures This is a measure of the strength or rigidity of the gel,

Use of different pressures and measurement of the When the pressure is released, the gel

The deformations are

J . Sci. Food Agric., 9, February, 1958

RUTGERS-CONSISTENCY OF STARCH-MILK PUDDINGS. I 63

measured on an instrument such as that of Brabender (see Chichester & Sterling15). A cylinder of about 20 mm. diameter is generally used and it should make good flat contact with the surface of the gel, which should have no skin or holes. The gel is usually measured free-standing, and so it must be fairly rigid. If, as has been suggested, gels are measured in a vessel containing a tube16 or a fin13 which is given a torsion, the resulting figures might be more difficult to interpret.

Breaking strength can be measured using the same instruments as for rigidity, by loading until breaking occurs. Some such instruments are the Gelograph (Brabender),"Y 1.9 Delaware Tester,19 and the Dorner Gelometer.20 Another method is the old Saare disc method2l with new variations such as that of BechteP2 and Hjermstad,2s in which a round disc enclosed in the gel contained in a beaker is displaced until breaking occurs. There seems to be no strict correlation between rigidity and breaking strength.'">

A body falls through the gel which is ruptured initially, and this measures firmness. The instruments used vary considerably, eg., some use a loaded cylinder or tube, and others, a thin bar or needle, a cone or a wire frame. Examples of these are the Suctilometer and the Bloom Gelometer'a which is also used to measure breaking strength. The time for a particular penetration with constant load is measured, or the depth of penetration with constant time. The penetration for different loads can also be measured and a form of rheogram obtained.

Using the penetration principle, higher concentrations can be measured and compared than with reversible deformation. The firmness is not always directly related to the rigidity and/or breaking strength, nor is there any definite correlation between hot-paste viscosity and gel strength or firmness, except in a broad sense.

Another measurement may be taken using the sag method, in which the deformation of a free-standing gel under its own weight is measured, but the consistency range to be measured is very small.

Whatever type of measurement is taken, it can seldom be repeate; on a particular sample of starch-paste, pudding or gel because it shows irreversible thixotropy, and changes by the application of a shear or pressure. At very low concentrations, or after a paste has been vigorously stirred, duplication is possible. Starch-pastes always show structural viscosity and have a yield value depending on the concentration. There is no mention in the literature of dilatancy or rheopexy.

Other rheological properties about which little has been written include shortness, which Kerr4 suggested measuring using a flow-out meter ; Lindemanr~ ,~~ using a rotation meter, measured the increase in viscosity of gelatinized starch-paste on cooling. Ropiness is very difficult to measure.25

It will be seen from the above that there is considerable variety in methods of measurement of viscosity and consistency applicable to starch-pastes and gels, and each measures a different rheological pattern. The correlation between the different measurements is not always good.

Experimental In the present work, hot-paste viscosities were not measured, since the important features

of the milk puddings were the consistency after storage a t room temperature or below and the effect of shaking and pouring out from the container. In this paper, the terms used are ' initial state ' to indicate the condition of the pudding after cooling and keeping, and ' end state ' the condition after a mechanical treatment. The intermediate stage is referred to as ' thinning out ' as the breakdown is irreversible.

As judged visually, on pouring or slow stirring, the initial state of a 4.2% potato-starch pudding was practically non-gelatinous but flowing, whereas that of a 4.2% maize-starch pudding showed gelatinousness with a very low yield value. I t was desired to measure this gelatinous initial condition.

I t was found that a rotation viscometer with a cylindrical body and rotation velocities above 20 r.p.m. was not suitable for this measurement. With a flowing-out meter (e.g. Ford cup) or a falling-body meter (eg. Gardner Mobilometer), potato-starch pudding did not give

J. Sci. Food Agric., 9 , February, 1958

Gels can also be tested using the penetration principle.


lower values, but could show even higher figures for the ' consistency ', than did maize starch. One cannot differentiate with one ' viscosity ' measurement between the resistance given by a thick liquid or by a feeble gel.

The Bostwick meterI2 The pudding is poured into the container of the meter immediately after cooling in the

cooking apparatus to 70" and is kept there for 24 h. at 25". The slide is then raised and the progression of the pudding-front is followed on the measuring scale at the bottom of the inclined plane.

Good flow a t 25" was observed for puddings made with non-gelatinous potato starch, tapioca and mechanically-treated maize starch, but puddings from untreated maize starch a t 25" and from potato starch at 3" showed

very little flow. These samples gave figures according fairly well with the subjective visual impressions of gelatinousness of the puddings.

For comparison of maize-starch puddings prepared with various modifications, the Bostwick meter did not prove suitable, as they may show tearing and falling down in stages or as a whole mass, instead of flowing. These difficulties can mostly be avoided by taking readings after 5 sec., but it is then evident that the meter is not sensitive enough for measuring small differences between thin puddings and gels in the initial state. For puddings differing considerably in gelatinous properties on visual inspection, 20 mm. at 25" or 16 mm. a t 3" were generally found.

The Bostwick meter seems suitable for thick porridges and purees with not too much thixotropy or structural viscosity, but not for milk puddings containing approx. 4.2% of starch.

Fig. I shows some curves obtained with this apparatus.

Timc,rcc.

FIG. ~ . - ~ o s i w i c k meter j a w c w v e s Starch concentration 4.2%

Sinker apparatus An attempt was made to measure the gelatinousness of the initial state of starch puddings

with a penetrating body of rather light weight and rather greater contact surface than a needle or conus and using only the start of the penetration.

An aluminium sinker with a flat circular base (20 mm. diam.), a small cylindrical lower part (5 mm. high) and a conical upper part (10 mm. high) is guided vertically. The cylinder has a rounded edge to prevent cutting the puddings. The sinker is loaded with an appropriate weight (in this case 6.4 g./cm.2) and placed on the surface of the pudding. The time for penetration by I cm. of the cone is measured. The cone does not come in direct contact with the pudding which is cut by the cylinder. With a cylindrical vessel of 65 mm. diam., there is room for the pudding to be pushed away by the sinker and the sinker then does not penetratethe gel. If the sinker stops, the measurement is discarded. Thicker maize puddings (above about 46% of starch) allowed no movement of the sinker at all.

Using a layer of petroleum minimizes the danger of skin formation but the sinker slips and mixes petroleum with the pudding so that the resulting times were much lower and more irregular.

With this sinker method, the results shown in Table I1 were obtained with puddings kept for 24 h. a t 25".

The difference between the two types of starch puddings is very obvious. Potato-starch puddings with 4'5% of starch are extremely slimy and have a very thick appearance with some gelatinousness. Higher figures of 3-10 sec. were found when some gelatinous part of the pudding slowed the sinker. The sinker was often slowed by the slimy threads of the pudding and so showed rapid decrease of the sinking velocity.

centrations and of different starch-types. and slow turning of a rod.

With a vessel of 35 mm., true penetration occurs, and this method was used.

A flat pudding surface without skin formation is necessary.

Measurements were taken using puddings made under different cooking conditions, con- - These puddings were inspected visually by tilting

This subjective

J. Sci. Food Agric., 9 , February, 1958

Points were given for gelatinousness of from o to 10.


Table I1

Results with the sinker apparatus on starch gels Starch type Starch concentration, Sinker time, sec.

O/ / O

Maize 4'2 4'0

Potato 4.2 0 2 4'5 11'0

4'5 1.5 and higher

judgment of the psycho-rheological impression should be the basis for instrumental measurements, although it is not accurate. Fig. 2 showed the relation between sinker time and subjective gel-marks. When the log of sinker time against gel-marks from the average curve of Fig. 2 is plotted, a straight line is obtained, as would be expected according to the law of Weber- Fechner. In comparison with the instrument, the eye is more sensitive to differences in very low gelatinousness or ' overrates ' them ; with stiffer gels the eye differentiates less. (It may be recalled that the puddings are all rather thin.) From a consumer's point of view, too much importance, however, should not be given to differences between higher figures.

I t must be admitted that the accuracy of the method is not very great and modification of the apparatus is possible.

The rheological basis of this type of measurement of the initial state is not too clear. With pastes showing little or no gelatinousness, but showing structural viscosity, e.g., 4.2% potato- starch puddings kept at 25', very low sinker times were observed, without intermediate stopping of the sinker. With gelatinous .$ 1

maize-starch puddings the sinker times are much higher, although the values found with a Ford cup or a Mobilometer are lower than those for potato-starch. As penetration occurs, no simple yield-value is measured. Gelatinousness, however, is indicated and measured. as used, figures above 0.5-1 sec. imply gelatinousness, and some connexion with Maxwell's relaxation time can be assumed.

The Ford cup A roo-ml. Ford cup with an opening of 6 mm. was used. This flow-out meter cannot

distinguish between thick liquids and weak gels, and couid not be used to measure the initial state. The apparatus may be of value for examination of gels prepared from one kind of starch under standardized conditions. Milk puddings containing 4.4-5% of starch gave only very slow extrusion, mostly in drops and clots, e.g. 30-300 sec. for 50 ml. Repeating the measurement on the same preparation gave much lower times, the pudding having been mechanically treated by the shear force at the first running out.

In Fig. 3 is shown log time plotted against extruded volume for some maize-starch experiments using a Ford cup, the time for the extrusion of 50 ml. being measured. For water, the time was 1.9 sec. at 25". The relation was more or less logarithmic, except a t the beginning. For very thin puddings, the Ford cup with a 6-mm. opening is not sensitive enough and a smaller opening must be used. Using a bigger cup, according to Posthuma,*o 150 mm. high and with an opening of 8 mm., fairly gelatinous puddings could be made to flow out in a stream a t the beginning of the run. The apparatus was then less useful, however, as smaller differences between different samples were found for the initial state as well as for end states.

Kerr suggested measuring paste shortness by observing, with constant flow-out time, a t what volume or time the stream changes into drops or clots, but the addition of a gelling sub- stance to maize starch could lower the transition point at 3' from over 50 ml. to nil (extrusion time of 21 sec. for 50 ml. of both puddings).

This apparatus appears to be useful for thin, mechanically-treated puddings, and all

J. Sci. Food Agric., 9, February, 1958

20

15 4

I t may be concluded that in the viscosity range Visual appearance .gel marks

9.--Sinker-limes and visual gel marks


puddings could be examined by its use after such treatment (e.g. by shaking or stirring), but the use of a Gardner Mobilometer (below), with which it is possible to treat and measure in one instrument, is preferred.

Gardner Mobilometer This apparatus, which is used in the paint and solvent industries, consists of a round plate

with four circular openings of 4 mm. fixed to the end of a thin rod, which is guided in a tube containing the pudding. The rod is loaded with weights, usually 130-180 g., and the whole kept in a water thermostat. The plate is placed about 20 mm. below the surface of the gel to be treated and the time is taken for the plate to fall 91 mm. with the applied weight. For water at 25" the time is I sec.

The pudding is kept at the desired temperature in a beaker and then transferred to the apparatus with the minimum amount of disturbance. An initial reading is taken and the pudding then mechanically treated by raising the rod and letting it fall under a load of I kg., the plate remaining below the gel surface. This pumping is repeated and measurements are taken after, e.g., 5, 10 and 25 strokes. For these measurements the rod is not loaded, as the consistency of the pudding has been considerably reduced. If the pudding is rather gelatinous or contains lumps, the plate stops at the first measurement. After 5-10 pumpings a smooth paste was always obtained. . Measurements were made on a series of puddings made from different starches, after 5 , 10 and 25 pumpings. The puddings were kept for 24 h. at 25" or 3 days at 3". The differences between the experiments will be discussed in the following paper.

Typical results are given in Fig. 4, with sinking times on a logarithmic scale.

F I G . 3.-Ford cup mzasurements on mcchani- cally treated maize-starch pzrdda~gs at 25'

FIG. 4.-Mobilomeler measurements OIZ 4.25b starch-milk puddings - after 24 h. at 25' ----- after 72 h. at 3'

Ten or 25 purnpings were generally used and the condition of the pudding then is termed

The reproducibility for four puddings from one boiling was reasonable, especially after The apparatus is fairly accurate and has a reasonable range, but it is not suitable

potato-starch


the ' end state ' even though after 25 pumpings no stable end state was reached.

pumping. for measuring gelatinousness and differentiating it from higher viscosities :


pudding gave higher values at 25" than did maize-starch puddings. As the following figures show, the Mobilometer appears to be more sensitive than the Ford cup (6 mm.) for treated puddings.

Affer 25 pumpings Mobilometer Ford cup

1 1 sec. 5 sec. 20 ,, 7 # I

200 ,, 30 .,

The first Mobilometer measurement is difficult to interpret. It cannot measure true initial state because the pudding must be disturbed when filling the meter.

firit measurement figure after 5 or 10 pumpings The relation - -- did not prove to be an indication of gelatinous-

ness, but it is suggested that the ratio of the value obtained initially to that after 10 pumpings and also that art& 5 pumpings to that after 25 pumpings can be-taken as a measure of the thinning of the puddings.

The sinking time after, e.g., 10 pumpings is taken as a measure of the consistency after mechanical treatment.

Comfiarison of sinker and Mobilometer In Fig. 5 are compared the sinker times, which measure gelatinousness, with the Mobilometer

figures after 10 pumpings, which indicate the viscosity after mechanical treatment. The measurements were made on puddings with maize starches of different origins, cooked at different concentrations and cooled under various conditions. The points lie fairly near to a straight line although there are some wide deviations. For any one type of starch, there can be found a correlation between initial state and end state conditions as shown by sinker and Mobilometer respectively, if the conditions of preparing the puddings do not differ much.

FIG. 5.-Sinker ccnd Mobilomelev measurements on maize-slavch

puddings kept at 2 9 , made under standard

kept at 3', made under standard

X

0 conditions

conditions different compositions or cmking

conditions

Mobibmtrr timc,scc.,aftcr 10 pumpings

With different types of starches or widely differing cooking conditions, the correlation breaks down, as is seen from some points indicated in the figure. The initial state or gelatinousness and end state may differ much more-in other words, the two measurements indicate different properties of the puddings.

Reproducibility of cooking and consistency measurements The reproducibility for different batches of the same composition cooked in the same way

proved to be considerably lower than that of the measurements of puddings from one boiling batch. These differences are compared in Table 111 for puddings with 4-2 and 4.5% of the same sample of maize starch.

The spread of the figures can be considerable, especially for the sinker and first Mobilometer measurement, in other words, for the initial state.



Table III Reproducibility of consistency measurements on different maize-starch puddings

kept for 24 h. at 25' Apparatus Range of values, Average value, Average value,

4.276 starch, sec. 4.27; starch, sec. 4.50/0 starch, sec. (4 s a m p l 4 ( 2 samples)

Sinker, initial state 1.5-4'0 2'5 10'0

initial reading 7.0-26.0 I 7.0 200

Ford cup, after 25 pumpings 5.0-7.5 6.5 26.0 Mobilometer,

after 10 pumpings 26.0-58 44'0 250

The accuracy of the average of duplicate estimations by the sinker method is 1-2 sec., and for the Mobilometer with 10 pumpings, 50 f 15 sec. or zoo & 50 sec. In terms of starch content, the accuracy for the Mobilometer with 10 pumpings is approx. f0.04% and for the sinker &o.I% starch. I t should be noted that for a concentration of 4.2% starch, f1.07~ moisture difference in the starch would mean a difference of fo.04y0 starch in the pudding.

Conclusions (I) The Bostwick meter is suitable for purees or porridges but is not suitable for comparison

of maize-starch puddings or for thin gels. (2) The sinker method is suitable for maize-starch gels, and there appeared to be a straight-

line relation between log of time for the sinker to depress the gel surface I cm. and the gelatinousness assessed visually on an arbitrary numerical scale. The method differentiates between the consistencies of maize- and potato-starch puddings.

(3) The Ford cup is useful for puddings of thin consistency or mechanically worked (shaken or stirred) puddings. For maize-starch gels there is a very approximate linear relation between log of time and volume extruded.

(4) The Gardner Mobilometer is suitable for examination of mechanically treated puddings with good and with thin consistency. Ry repeated movement of the plunger and measurements before and after this treatment, an estimate can be obtained of the extent of thinning out of the puddings

Acknowledgments Thanks are due to Mr. G. Blonk for his valuable help in this investigation.

Laboratory Royal Mills Laan Bros.

Wormerveer, Holland Received 31 Octobcr. 1956; amended inar~uscript 24 July, 1957

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The consistency of starch-milk puddings. I.—Preparation of puddings and measurement of consistency