The rapid visco analyser (RVA) can be used to assess the quality of any product where the cooked viscousity is important. The precise linear ramped heating and cooling abilities of the RVA, along with steady state temperature control, allow careful control of the cooking environment, whilst changes in viscousity are continuously recorded.

The pasting properties of starch and starch-containing products are readily assessed in the RVA. During the test, the starch is gelatinized with consequent rise in viscousity, subject to high temperature and controlled shear during which it's stability is revealed, then cooled to provide an indication of set back during gelation . Samples can be assessed for pasting temperature, peak paste viscousity, time to peak, temperature at peak, hot and cold paste viscousity breakdown, set back,  and final viscousity

Apparatus and Reagents
·        Rapid visco analyser model Rva-3D.
·        Computer IBM compatible, capable of running RVA control software
·        RVA canister and stirrer .
·        Balance that can weigh to   0.01g.
·        Adjustable dispenser or pipette to deliver 25.0  ml of water or buffer.
·        Laboratory mill with screen where sample grinding is required.

Procedure for dried material

1.                  Milled sample to a fine powder
2.                  Determine  sample moisture content
3.                  Weigh 3g ( on 100% dry matter basis) flour into the cannister.
4.                  Place the paddle into the cannister and insert the cannister into the  instrument.
5.                  Initiate the measurement cycle by depressing the motor tower of the instrument when the computer says “press down the tower”.
6.                  Remove cannister on completion of test.

Calculation of dried sample weight
Since at 100% DM weight of flour required is 3g
  100 x 3g
                        RVA  weight   =          Sample D.M
                                      RVA wt- 3=Y
Volume of water =25.0ml –Y.



Procedure for fresh material

Determine moisture content of material
Calculate weight equivalent to 3g on dmb.
Blend sample with appropriate amount of water
Add 3 drops of antifoam (where the blended sample has foam) and to allow for disappearance of the foam.
Transfer 25ml of slurry into RVA cannister.
Initiate the measurement cycle by depressing the motor tower of the instrument when it says “press down the tower”.
Remove cannister on completion of test.

Calculation of fresh sample weight

Since at 100% DM weight of flour reqiured is 3g
  100 x 3g
RVA  weight ( fresh sample)     =      Sample D.M           =       Pg
                                      P - 3     =    Y (weight of water in Pg fresh sample)
     25.0ml –Y   =       Weight of water to add to Pg fresh material

If dry matter is 30%
                                    100 x 3
Pg material will be       30                 =         10g     
                                  P – 3       =      10g – 3g         =          7g

N.B 7g water is equivalent to 7ml water. 
25.0ml – 7ml       =   18ml

So weigh 10g fresh material, add 18ml of distilled and blend.
Pasting Characteristics
 When starch granules are heated in water beyond a critical temperature, the granules absorb water and swell to many times their original size. Around a critical temperature range the starches undergo a reversible process known as gelatinisation. Gelatinisation is characterised crystalline melting, loss of birefringence and starch solubilisation. As soon as the temperature rises above the gelatinisation temperature the starch granules begin to swell, when most of granules have become swollen, there is a rapid rise in viscosity if there is sufficient concentration of starch (about 10% w/v). As the temperature increases further, the granules rupture the more soluble amylose leaches out into solution followed in some cases by the amylopectin fraction The granule rupture and subsequent polymer alignment due to mechanical shear reduces the apparent viscosity of the paste. These combined processes that follows gelatinisation is called pasting. The similar characteristic viscosity curves produced during heating and cooling of starches is called pasting curves.
The pasting characteristics of starches has been correlated with cooking quality and texture of various food products (Moorthy, 1994), therefore it can be a good index of textural quality in most starchy food.

1.      Objective
The objective of this experiment was to determine the pasting characteristics of starch and flour in other to know if the pasting properties of the starch can be a determinant of textural quality in food product
The pasting profile was studied using a rapid visco Analyser (RVA) series 4 (Newport Scientific, NSW, Australia)
  3g was weighed and 25mls of (Dh2o) was dispensed into a cannister. Paddle was placed inside the canister this was placed centrally onto the paddle coupling and then inserted into the RVA machine. The measurement cycle was initiated by pressing the motor tower of the instrument. The profile can be seen as it is running on the monitor of a computer connected to the instrument. The 12 minute profile was used, the time-temperature regime used was: Idle temperature 500c for 1min, heated from 50octo 95oc in 3 min 45s, then held at 95oc for 2 min 30s the sample was subsequently cooled to 50oc over a 3min 45s period followed by a period of 2 minutes where the temperature was controlled at 50oc.

The pasting temperature gives an indication of the minimum temperature to cook a sample. Peak viscosity is a measure of the ability of starch to form a paste on cooking.
  Peak viscosity occurs at equilibrium between granule swelling which increases viscosity and granule rupture and alignment, which cause its decrease. viscosity rises only slowly and remained comparatively low. The holding strength is the ability of the granules to remain undisrupted when the Starch or the Flour paste was subjected to a hold period of constant high temperature (95oc for 2min30s) and mechanical shear stress (by rapid constant and continuous mixing).
The hold period is often accompanied by a breakdown in viscosity; it can also be called shear- thinning to a holding strength, hot paste viscosity, paste stability or trough. It measures the ability of the paste to breakdown during cooking. The ability of a starch to withstand this shear- thinning or breakdown in viscosity (that is high breakdown value) is of high industrial significance in starches.
On cooling of the starch mixture there is re-association between the starch molecules, this occur to a greater or lesser degree. Where there is sufficient concentration it causes the formation of a gel and this is indicated by increase in viscosity called final viscosity.  This increase in viscosity is not only caused by simple kinetic effect of cooling but also due to re-association of molecules (particularly amylose) (Anonymous, 1990). Final viscosity can therefore be an important parameter in predicting and defining the final textural quality of foods especially pounded yam in terms of its hardness and elasticity.
The phase of the pasting curve after cooling of the starches (cooling to 50oc) is called setback region. This stage involves re-association, retrogradation or re-ordering of starch molecules. It shows the tendency of the starch to associate and retrograde. A high setback value has been associated with a cohesive paste while a low value is an indication that the paste is not cohesive (Kim et al. 1991).
High setback value is useful for domestic products such as pounded yam, which requires high setback, viscosity and high paste stability at low temperatures. 

starch is responsible for the doughy, firm, elastic and cohesive nature of the pounded yam produced from them. Storage of the yam varieties also enhances the textural attribute of the pounded yam by making it to be moderately soft elastic, more deformable and more cohesive. Pasting characteristics of the starch of Flour can therefore be an important indicator of textural qualities in food.


Interpretation of results. In rapid visco analyser, manual. Section 5, pgs 25-28.( Newport Scientific Australia (1990)
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