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Study of Mixing in Connection with the Rheological Properties of Biscuit Dough and Dimensional

Characteristics of Biscuits

Zoulikha Maache-Rezzoug,” Jean-Marie Bouvier,” Karim Allah’ & Christian Patras”

“UniversitC de La Rochelle, Laboratoire Maitrise des Technologies Agro-Industricllc:; (LMTAI), Avenue Marillac, 17042 La Rochelle, cedex 1, France

“CLEXTRAL I, Rue du Colonel Riez B.P. 1 k-42702 Firminy, France “Biscuiteric Nantaise (B.N.). 60200 Compikgne, France

(Reccivcd 20 July 1997; accepted 17 January 1998)

ABSTRACT

Impact of mixing time, and the role of premixing qf sugar and liquid ingredients were analyzed using deduced parameters of the mixograms such aJ instantaneous specific energy (ISE) and total specific energy (TSE). Rheological behavior of the dough was determined by uniaxial compression at constant crosshead speed of 0.7 mls; physical characteristics of the biscuits were also defined. Increasing mixing time resulted in the softening of dough, and reductions in both viscosity and relaxation time. It also caused an increase in the length and reduction in the weight of biscuits. The use of two protocols ,for mixing led to quite dtfferent doughs for the same formula. It was concluded thar a dough has good consistency when it is mixed with standard protocol und becomes vety softened when the liquid ingredients and sugar are pre-mixed. 0 1998 Elsevier Science Limited. All rights reserved

INTRODUCTION

Total quality control of the biscuit manufacturing process involves characterization of the product at all process steps. Analysis and quantification of this quality, for the purpose of controlling its parameters and acting on it in real time, remain the prime objectives of the biscuit industry. Accidental variation in ingredient quantities or poor quality of the flour may cause serious problems. Knowledge of the rheological behavior of the flour can be effective in controlling its technological aptitude.

44 Z. Maache-Rezzoug et al.

Mixing is the initial step in the biscuit-making process. This operation allows the mixing of flour, water and other ingredients so they form a coherent mass. In the mixing process, mechanical energy is applied to the dough. The energy input depends on the type of mixer and flour. The mechanical action providing the energy permits the establishment of numerous interactions between the constituents of the dough (Macritchie, 1985). During mixing, the mixture undergoes mechanical pro- cessing which determines the conformational arrangement of the biopolymers (especially gluten) and consequently, the rheological properties of the dough (Amend & Belitz, 1990).

Gluten is transformed into a network of small elastic fibrils, which gradually form a tissue in which the starch granules are embedded (Bernardin & Kasarda, 1973). In the presence of water, the protein macromolecules swell and become increasingly mobile above the glass transition temperature. The network is in a ‘rubbery’ state, and the mobility of macromolecular chains therefore allows numerous interactions with other constituents of the dough (Hoseney, 1986). Dough mixer recorders, such as farinographs and mixographs, can be used to follow rheological modifications during mixing (Finney & Shogren, 1972; Olewnik & Kulp, 1984; Mani ef al., 1992). Baig and Hoseney (1977) found that increased mixer speeds for the flour-water mix decreased mixing time. They showed that the effect of mixing time was less pro- nounced at higher than at low mixer speeds. According to Hoseney (1986), the mixing time of various wheat flours is genetically determined and is affected by protein content. Several studies have attempted to determine the mixing mechanism and correlate objective parameters with the quality of the finished product (Shuey, 1974). The quality of the final bread loaf is strongly dependent on the mixing (Eliasson & Larsson, 1993). However, in biscuit-making, the relationship between the mixing properties and the quality of the finished biscuits has not been elucidated yet.

In this paper, we investigated the effect of mixing time on the rheological proper- ties of the biscuit dough and dimensional characteristics of biscuits, and the effect of order of incorporation of the ingredients on dough quality.

MATERIALS AND METHODS

Mixing

A laboratory dough mixer (Fig. l), with similar operating principles to those of the farinograph (8 l), was equipped with a torquemeter and tachometer to follow the developed torque and mixer rotation speed. These measurements can be used to calculate the energy transmitted to the dough during mixing. Using a double jacket, the temperature of which was regulated to l/lO”C, dough temperatures at the end of mixing were controlled to the nearest 0S”C.

Determination of mixing characteristics

A data acquisition PC system was used to follow and record energy variations during mixing. Two main parameters were identified in order to characterize the mixing

Containkr 1 body

Transparent fide

Transmission shaft

/ Double jacket f

Thermostat water circulation

operation: insrantaneous specific energy (BE) and total specific energy (TSE), ISE, expressed in Jikg*s, is given by:

ISE= Torque (N-m) x r~~tatj~~n speed (radis)

Mass (kg)

TSE is the integral of the instantaneous specific energy during the entire mixing operation, calculated as follows:

‘l-SE (J/kg) = 1”’ ISE dr V 0

TSE= $ (ISE),At

where iz = ~,lA.f and Ar = 0.5 s. Ingredients are given in Table 1. Leavening agents (sodium bicarb~~n~te and

aml~onium bicarbonate) were dissolved in an aqueous solution of 0.23 g/l concentra- tion. Production af dough using the laboratory mixer followed a we~~-est~~~~l~shed protocol: the liquid pre-mix (water, brine, glucose, inverted sugar and solution of leavening agents) was placed in a heated container and stirred for approximately 2 min until complete homogenization. First, whey, flour and sugar are intr(~du~ed into the mixer, followed by the Iiquid fat at 50°C and the liquid pre-mix. ‘Tempera- ture of the ingredients, mrxing time (7 min) and mixer speed (I IS rpm) were kept constant.

46 Z. Maache-Rezzoug et al.

TABLE 1 Standard Formula

Ingredient

Flour Sugar Liquid fat Water Solution of leavening agents Glucose Inverted sugar Whey Brine

*Based on flour.

Part*

100 30.3 14 6.9 6 3.5 3 2.9 2.6

The curve recorded during mixing (Fig. 2) shows changes in dough characteristics (plasticity, elasticity and viscosity) induced by the mechanical energy transferred to the product. At the beginning of mixing, dispersed water is absorbed by flour constituents and solid ingredients. The transition from dispersed state to dough involves an increase in the mixing resistance reached by ISEmax. Thus, ISEmax corresponds to a complete hydration of mix constituents and a reduction in the quantity of free water in the system. In bread making, Faubion and Hoseney (1990) reported that this maximum corresponds to complete hydration of the flour particles and reduction in water mobility. Beyond this maximum, diminution of ISE is related

b 60 120 180 240 300 360 420

Mixing time (set) Fig. 2. Instantaneous specific energy versus mixing time.

l@ect of mixing on biscuit dough 47

to rheodestruction of the dough system; this phenomenon is manifested by a soft- ening of the dough.

Uniaxial compression measurement

Cylindrical samples of dough (25 mm diameter, 20 mm height) were compressed during a 30 min rest period. Rheological characteristics are obtained by uniaxial compression. The shaping and compression test takes place inside a temperature- regulated chamber (31°C) to maintain the sample at constant temperature during the test. The sample is compressed between parallel plates, lubricated with silicone oil to eliminate plate/sample friction (Bagley et al., 1985). Compression was carried out at constant crosshead speed of 0.7 mm/s. Samples were compressed to 50% of their initial height. The experimental device (Fig. 3) includes a differential cylinder jack controlled by a PC.

The rheograms obtained are curves plotting the changes in sample resistance versus time (Fig. 4). They show a compression phase when the plate deforms the cylindrical sample at constant crosshead speed, and a relaxation phase when dis- placement is zero (deformation maintained constant).

The convected Maxwell mode1 was used to determine the rheological charact.er- istics of biscuit dough. Maxwell behavior law uses measured stress, but in this case this quantity is not directly accessible. To transform force-time rheograms into stress-time, we considered the contact surface to be ‘ideal’. We assumed that the sample keeps its volume and a perfectly cylindrical shape during deformation. The surface area is therefore expressed at time t by the relation:

Constant temperature

chamber\

Movement sensor

Fig. 3. Experimental device.

48 Z. Maache-Reuoug et al.

0 10 20 30

Time (set)

Fig. 4. Typical rheogram for an apparent deformation of 50% and a crosshead speed of 0.7 mm/s.

ho S(t) = - so

h(t)

where S,, and S(t) are respectively the initial surface area and sample/compression plate contact surface area at time t, and h,, and h(t) are respectively the initial height of the sample and its height at time t. The stress-time curves were described using Maxwell’s convected model (Bagley et al., 1988):

where h= h(t)= ho-Vet. V,, is constant crosshead speed (mm/s), 3, and 11 are respec- tively relaxation time (s) and shearing viscosity (Pa/s). The numerical resolution of the differential equation has been realized using mathematical software in the Math/ PC Library from IMSL. The numerical calculation to obtain 1 and q is described in detail by Bagley et al. (1988).

Figure 5 shows that the model is adequate to describe the experimental data in the deformation range studied. However, Maxwell’s convected model diverges, par- ticularly for deformations larger than 50%. The problem is due to the non-linear response of the material, whereas the model describes linear behavior at small strains and strain rates (Petri, 1977).

G 6000

g

i 4000 3 v3

2000

0 5 10 15 20

Time (set)

Fig. 5. Experimental data for crosshead speed of 0.7 mm/s compared with results computed from Maxwell model.

Measurement of dimensional characteristics

The length and weight of the biscuits were measured during study of mixing time. Dough samples corresponding to various mixing times were rolled and baked in an industrial oven (after being left for 45 min). The pieces of dough were inserted into an industrial dough band, and worked successively by the rollers. The length and weight of 16 biscuits were then measured. Industrial rolling of dough is carried out lengthwise along the biscuit. Considering its variation due to shrinkage of dough after cutting, length is a very important parameter for quality control.

Experimental procedure

To study the influence of mixing time, doughs were mixed for 5, 7, 9, 11, IS and 25 min, corresponding to undermixing (5 min), optimal mixing (7 min) and over- mixing ( > 9 min) conditions. The formula (30.3% sugar, 6.9% water and 6% leavening agents solution) was kept constant for all experiments. Using this formula, two mixing methods were compared: (i) standard mixing according to the above- mentioned protocol; (ii) prior preparation of sugar syrup with the liquids (water, leavening agents solution, brine, glucose and inverted sugar), which were mixed for 190 s, followed by addition of flour and fat, and mixing for 230 s.

50 Z. Maache-Rezzoug et al.

RESULTS AND DISCUSSION

Effect of mixing time

In spite of identical proportions of ingredients, the characteristics of a dough mixed for 5 min differed from those of a dough mixed for 25 min (Fig. 6). Doughs obtained after 5-9 min of mixing took the form of scattered pieces and ISE values varied widely. For mixing times of 11-25 min the curves were smoother, especially for the dough mixed for 25 min, which was very soft. According to Hlynka (1962) during overmixing, some links between the water and the other constituents break, leading to an increase in free water. Webb et al. (1970) showed, using a Brabender farinograph, that the water in dough is held with various degrees of strength and that its distribution is dependent on the mechanical work input

Figure 7 shows a linear increase in TSE consumed by the dough when mixing time is increased. This increase is followed by softening of the dough and results in heating of the mix by viscous dissipation. The viscous dissipation is proportional to the shear rate, which is important for a soft dough, since the dough is in a continu- ous phase. A difference of 5.7”C was obtained between dough mixed for 5 min and one mixed for 25 min (Table 2).

Measurement of TSE alone is not sufficient to indicate the state of the dough. Knowledge of rheological properties, viscosity (ye) and relaxation time (3,) measured after mixing, completes the diagnosis of the texture of the flour-ingredient mix before baking. Increased mixing time leads to linear reduction in y (Fig. 8). i, is also

11 min It 15 min I ”

Fig. 6. Effect of mixing time.

Effkt of miving on biscuit dough

“b 60

20

0 5 10 15 20 25 30

Mixing time (min)

Fig. 7. Total specific energy changes against mixing time.

affected by mixing time; up to 1.5 min of mixing a quasi-linear reduction in 2 was observed. At mixing times of over 15 m, i. remained constant. Since 2 is a parameter intrinsically linked to dough elasticity, the greater the relaxation time, the more elastic the dough.

An interesting relation was established between the total specific energy (TSE) consumption and rl and jL (Fig. 9). Viscosity and the relaxation time decrease bhcn TSE increases.

When measured after baking (Fig. lo), biscuits showed a linear increase in length between 5 and 15 min of mixing; above this, the increase slows and tends towards a constant value. Increased biscuit length is a consequence of a reduction of the shrinkage phenomenon observed after cutting, which is closely linked to dough elasticity. The opposite effect was observed with biscuit weight, which decreased linearly up to 15 min of mixing and becames constant for longer times. The loss of

TABLE 2 Effects of Mixing Time on Temperature of the Mix and Dough Quality

Mixing time Temperuture (min) PC)

5 30.4 Very inconsistent 7 31.5 Inconsistent 9 31.5 Inconsistent

II 32.1 Firm and consistent 15 33.0 Slightly soft 2.5 35.7 Very soft

52 Z. Maache-Rezzoug et al.

12

10 -

8 -

6-L

4

2.5 ,

2.0 -

Gl.5 - X

l.O- L

0.5 ’ I I I I I

0 5 10 15 20 25 30 0 5 10 15 20 25 30

Mixing time (min) Mixing time (min)

Fig. 8. Evolution of viscosity and relaxation time with mixing time (rest time 30 min).

weight is certainly due to the fall in dough consistency. Soft dough results in thinner bands than firm dough.

A relationship was obtained between relaxation time and biscuit length (Fig. 11). A quasi-linear decrease in length was observed when 2 was increased. This relation clearly shows that shrinkage after cutting increases with dough elasticity.

Role of pre-mixing of certain ingredients

During mixing, the mechanical action of the kneaders and the energy input allow the establishment of numerous interactions between flour constituents and the other

10 G

2.0 -

s 1.5- x

1.0 -

0.51 ’ ’ ’ ’ ’ ’ ’ ’ ’

20 40 60 80 100 0 20 40 60 80

TSE (x 1O-2 J/kg) TSE (x 102 J/kg)

Fig. 9. Relation between q and I, and total specific energy.

100

64

63

62

61

60

59

Effect of mixing on biscuit dough

7.4

37.2 z

F7.0

IL

\ \

\ 6.8 1 I I I I I

0 5 10 15 20 25 30 0 5 10 15 20 25 30

Mixing time (min) Mixing time(min)

Fig. 10. Changes in biscuit weight and length with mixing time.

7.6 \

\

ingredients. These interactions also depend on the state of the ingredients when mixing starts, leading to major variations in dough consistency. On the mixing curves (Fig. 12), two separate developments of the dough can be identified. For standard mixing, the instantaneous specific energy profile fluctuates widely, and the mean value of ISE is approximately 30 J/kg-s. Conversely, with pre-mixing, ISE profiles are smoothed with much lower amplitudes (ISE ~35 J/kg*s). In this case, considerable heating of the dough through viscous dissipation was observed. The dough forms more rapidly when the sugar is dissolved in water than when the sugar is incor- porated as such at the beginning of mixing.

64

63

62

61

60

59 0.5 1 .O 1.5 2.0 2.5

h (9

Fig. 11. Effect of relaxation time on biscuit length.

54 Z. Maache-Rezzoug et al.

01 I I I

0 120 240 360

Mixing time (set)

0

Liquids i 1 premixing ; with sugar ; I

120 240 360

Mixing time (set)

480

Fig. 12. ISE behavior during standard mixing (left hand curve), and mixing with pre-mixing of ingredients (right hand curve).

For a given formula, the dough obtained with prior mixing of sugar and liquid is soft and displays good consistency, whereas standard mixing leads to inconsistent and crumbly doughs. Therefore, in spite of identical quantities of ingredients, the order of incorporation has a definite effect on flour hydration and consequently on the texture of the mix. To obtain identical consistencies, fewer binding agents should be used for mixing with prior mix of sugar and liquid ingredients.

Effect of pre-mixing of ingredients on rheological characteristics

Differences in rheological properties resulting from the two types of mixing were obtained by analyzing the compression rheograms at various rest times (Fig. 13).

12 2.0 I

0 10 20 30 40 0 10 20 30 40

Rest time (mm) Rest time (min.)

Fig. 13. Variation of viscosity and relaxation time for the two types of mixing: - o -, without pre-mixing; - x -, with pre-mixing.

&fect qf mixing OH biscuit dough 55

With pre-mixing, the dough had low viscosities, of around 6 x lo4 Pa.s, whereas those obtained by standard mixing had higher viscosities (about 10’ Pas). Concern- ing the relaxation time, there was a clear difference: dough produced with pre-mixing displayed lower elasticity than that obtained by standard mixing.

The availability of the water influences the nature of the interactions between the different constituents of the flour and the other ingredients. These interactions are directly involved in the formation of the dough structure. Spies and Hoseney (1082) showed that the role of sugar is linked to the water activity. Direct contact between the sugar and the water creates an interface between water and the flour constit- uents (proteins and starch in particular). Consequently, water mobility is reduced. Therefore, pre-mixing liquids with the sugar prevents the complete hydration of the flour constituents and consequently prevents the formation of the gluten network and its development. The dough produced is very soft and not very elastic.

CONCLUSION

Variation of the mixing time between 11 and 25 min led to widely differing shapes of the curves. Increased mixing time led to a reduction in dough consistency and significant heating. This implies lower viscosity and relaxation time, producing decreased shrinkage of the biscuits after cutting, and consequently longer biscuits after baking. Although the proportions of the ingredients were identical, dough obtained with prior mixing of sugar and certain liquid ingredients was soft with good consistency, with considerable heating of the dough occurring during mixing.

The mixograph provides important information on the reaction of the raw material with other ingredients in the biscuit formula, and on the state of the mix through the characteristics deduced from the mixograms. When associated with the rheological properties and dimensional measurements of the biscuits after baking, it represents an essential tool for predicting the technological behavior of the dough.

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