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