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Science Park Aarhus, Denmark
ISI 19-6e Determination of Viscosity of Starch by Brabender - Comments.

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6. Notes The Brabender Viscograph has for half a century been a de facto standard in the starch industry.
In a starch lab. it is most convenient to run the same profile on all samples and only adapt slurry concentration to suit the properties of the sample. The following notes state the reason for the profile used in ISI 19-6e.

 

Slurry Quantity

International Association for Cereal Science and Technology (ICC) Draft ICC-Standard no. 169 recommends a total weight of 450  g water + dry matter of sample. In case of high-fluidity starch the slurry may make up a total of 600 g. Corn Refiners Association (CRA) recommends 460 g including sample - but the recommendation is limited to native starch. Most European manufacturers use 500 g including sample. The differences are less important as the rotating spindle is fully covered with slurry anyway.

 

Slurry Concentration

Quantity of starch is ideal adapted to suit the range of the viscograph - in particular the original mechanical versions in order for the recorder to "catch" the peak viscosity on the paper. Extra weight (zero suppression devise) can extend the range during the critical pass of the peak, but it requires more attention. This means that the ideal concentration is limited to give a peak viscosity of less than 1000 BU - allowing for an occasional overshot now and then.

Potato starch

Potato starch in particular has a high peak viscosity. Even 15 g DM per 500 g (3%) often hit and overshoot the 1000 BU barrier at the peak with a hot paste viscosity just within range. 3% potato starch therefore is a well chosen level and so it was for nearly half a century. Sales people, however boast of the peak viscosity of their starch and even some raised the slurry concentration without pointing it out to the effect that inflation came to Brabender viscosity. In order to cope the more respectable manufacturers reluctantly increased concentration to 4% to produce competitive results. In fact a range of 3% to 5% is applied to day to suit different customers. It is obvious that a viscosity without stating the concentration is worthless, but nevertheless the concentration is omitted far too often.

New electronic versions of the viscograph allow for higher viscosity's with no extra attention and thereby facilitate the inflation tendency.

In staid of resist tooth and nail we have - from ISI 19 ver. 6 - adopted the present Danish Industry standard of 4% native potato starch.

Corn starch

Cereal starch do not have a peak viscosity nothing like as high as potato starch. Consequently higher concentrations are in use. In the trade concentration of dent corn starch slurry may vary from 5% to 8% in Brabender viscosity measurements.

The draft ICC-standard no. 169 recommend 35 g corn starch dry matter + 450 g water (equivalent to 7.2%), which does not give the percentages as a whole number and thereby reduces its chances of acceptance for this standard.

We have adopted 8% for native corn starch in compliance with CRA.

Waxy maize starch

We have adopted 5% for native waxy maize starch in compliance with CRA.

Wheat starch

The draft ICC-standard no. 169 recommend 40 g wheat starch dry matter + 450 g water (equivalent to 8,2%). We find it awkward with the skew percentage and a higher concentration than for cornstarch seems not justified.

We have therefore adopted 8% for native wheat starch by way of comparison to dent corn starch.

Tapioca starch

The draft ICC-standard no. 169 recommend 30 g tapioca starch dry matter + 450 g water (equivalent to 6.3 %). We find it akward with the skew percentage and so do most suppliers of tapioca starch.

We have therefore adopted 6% for native tapioca starch in compliance with what seems to be an Industry Standard in dominant producing countries.

Milo starch

We have adopted 8% for native milo starch by way of comparison to dent corn starch.

Rice starch

We have adopted 8% for native rice starch by way of comparison to dent corn starch.

Sago starch

Sago starch has greater similarity to root starches than to cereal starches.

We have therefore adopted 6% for native sago starch by way of comparison to tapioca starch.

Sweet potato starch

As a tuber starch we have adopted 6% for native sweet potato starch by way of comparison to tapioca starch.

Waxy sorghum starch

As a waxy cereal starch, we have adopted 5% for native waxy sorghum starch by way of comparison to waxy maize starch.

Waxy rice starch

As a waxy cereal starch, we have adopted 5% for native waxy rice starch by way of comparison to waxy maize starch.

 

Modified starches

A thin-boiling starch may require as much as 150 g starch dry matter per 500 g slurry to give an informative viscograph. As a rule of thumb the quantity of starch is chosen to suit the range of the viscograph.

 

Beginning of Gelatinization The temperature at which the viscosity has increased by 20 BU ("Gel Temp") is generally accepted as the temperature of beginning gelatinisation. Fortunately this occurs at a steep rise. The temperature of beginning of gelatinization is important to certain food products.

Starting Temperature

To increase the througput of samples the initial heating may be faster than 1 oC/min. When a temperature below the beginning of gelatinisation is reached the heating rate is reduced to 1 oC/min. This temperature is the starting temperature of the test. We have adopted 45 oC as the standard starting temperature and only make an exception as required by  special properties of the sample.

 

Gelatinization Temperature and Peak Viscosity Individual starch granules gelatinise over a temperature interval. As temperature increases more starch granules gelatinise. With increasing temperature the starch granules begin to break down and at peak viscosity this break down get the better and resulting viscosity starts to drop off. The temperature at peak viscosity ("Peak Temp") is defined as the gelatinisation temperature and it usually occurs before maximum temperature is reached (Elevated "gel temp" may appear after inhibition of starch by cross-linking)

 

Peak viscosity and the temperature at peak viscosity is the points attracting most interest in the trade of all.
Maximum Temperature Heating  from 25 oC to 92 oC in 45 min. is a classic procedure. 92 oC is high enough to paste the starch and has a good margin to the boiling temperature - at least in lowlands. The new electronic version of the Brabender viscograph cannot handle half degrees, hence the ICC recommendation of 93 oC. In lowlands like Denmark and The Netherlands some researchers have found the 92 oC to skew and prefer the more "round" 95 oC. A leading potato starch manufacturer only heat potato starch to 90 oC, for the simple reason that potato starch gelatinise so easily.

As no temperature prevails we have adopted 93 oC in compliance with the ICC and CRA methods.

Upper Holding Period ICC recommend a holding period of 15 min. or slightly more before cooling. CRA B-9 consider the completion of a 20 minute hold period as the end of test. A leading potato starch manufacturer hold (for potato starch) the temperature of 90 oC for 20 min. before cooling to 50 oC and for cereal starches the slurry is heated to 95 oC, holding for 10 min. before cooling to 50 oC. Whenever the nature of the starch can justify it temperature and holding time is cut to increase throughput of samples (A viscograph costs a fortune).

We have adopted 20 min. as a general holding time for reason of simplicity and in compliance with the ICC and CRA.

 

Cooling ICC stands quite alone with a recommendation of cooling to 30 oC and no subsequent holding period. Cooling to 50 oC with a subsequent holding period seems to be a de facto Industry Standard in Europe.

We have adopted cooling to 50 oC in compliance with what seems to be an Industry Standard.

Lower Holding Period

Maximum viscosity and more seldom the viscosity at the end of upper holding period are the points of commercial interest and for routine tests the subsequent profile is often omitted. Cooling is often also the end of test. Researchers may have interest in a holding period after cooling.

We have adopted a 20 min. holding period after cooling to 50 oC and the end of the that holding period is the end of test.

 

Transformation of Units Torque in Brabender Units (BU) may be transformed to Pascalsec or Poise by calibrating the viscograph with viscosity standard fluids.

700 cmg (standard) means that 0-1000 BU equals 0-700 cmg and 350 cmg means that 0-1000 BU equals 0-350 cmg.

For a Newtonian liquid and a range of 0-700 cmg centipoise (cP) = BU*3*cmg/1000. Example: 600 BU = 420 cmg or 600 BU = 420*3 = 1260 cP

Convert BU to cP. 

For non-Newtonian liquids the conversion may be slightly in error.

 

 


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