EVALUATION OF STARCH HYDROLYSING ENZYMES TO IMPROVE DRAINAGE OF RECYCLED PULP

Authors

Henny Jansen van Vuuren1 and Francois Wolfaardt1,2

Company/organisation and addresses

1Department of Microbiology and Biochemistry, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa.
2Sappi Forest Products, Technology Centre, P.O. Box 3252, Springs 1560, South Africa

emails

VuurenH@sci.uovs.ac.za and WolfaarF@sci.uovs.ac.za

Keywords

amylase, biotechnology, drainage, enzyme, recycled fibre, starch

ABSTRACT

Fibres from recycled boxboard, contains starch that contributes to lower drainage rates and reduced machine speed. The residual starch does not contribute to strength properties and can potentially be degraded by amylases to improve drainage. Lascaris et al. (1997) reported an increase in paper production of up to 6,8% after amylase treatment. The aim of this study was to evaluate different commercial amylases for the degradation of starch in fluting grade pulp in order to improve drainage. Different amylases from Novozymes were compared by treatment of pulp at high and low consistencies.  These enzymes were BAN, Fungamyl, Duramyl and AMG. Pulp was provided by the Sappi Cape Kraft mill.  Residual starch content of the pulp was determined before and after laboratory scale enzyme treatment, using the Tappi test method for starch in paper (T419).  Canadian Standard Freeness test, a Vacuum Drainage Evaluation test and a revised Schopper Reigler Drainage Evaluation test was used to quantify the changes in drainage. Further evaluation of drainage improvement was done with a pilot scale paper machine.  Enzyme treatments at high consistency (20 %) and low consistency (0,5 %) were done before papermaking (30 minutes at 40 C). Paper was made from pulp at a consistency of 0,5 % and sampled after pressing to determine the water and starch contents of paper.  Samples were also collected to determine the starch, maltose and glucose content of the backwater.  All the amylases degraded residual starch in pulp at high and low consistencies, but the low consistency pulp treatments were more effective than the high consistency treatments.  The different methods to quantify drainage of treated pulp did not yield repeatable results. The pilot trials showed drainage improvements of 1 to 2 % that could potentially result in 3 to 6 % increase in production.  Fungamyl was the most efficient enzyme to reduce the water content of paper when high consistency pulp was treated, and for low consistency treatment it was a combination between BAN and AMG.  Duramyl was the most efficient to degrade starch at low and high consistency. We concluded that further small-scale studies were required to optimise enzymatic treatments before mill trials can be considered. The relatively low cost and the small volumes of enzyme used to treat pulp would however make this process economically feasible, even without optimisation.

INTRODUCTION

Application of enzymes for the improvement of paper is a relatively new technology.  According to Kirk & Jeffries (1996) initial research was based on fibrillation using cellulase. The most extensively studied enzyme for application on pulp and paper is xylanase, which is also the enzyme with the most applications in the industry.  For drainage improvements, most researchers investigated the use of cellulase, where fines are degraded to decrease water retention and improve web formation. Amylases have also been used successfully in drainage improvement of recycled pulp (Lascaris et al., 1997).

The mechanism of drainage improvement with amylase is based on the removal of residual starch that restricts the free drainage of water on the wire.  Recycled fibres from boxboard contain starch that lower drainage rates and reduce machine speed.  The residual starch does not contribute to strength properties and can potentially be degraded by amylases to improve drainage. Lascaris et al. (1997) reported an increase in paper production of up to 6,8 % after amylase treatment in mill trials.

The aim of the present study was to evaluate different commercial amylases for the degradation of starch in fluting grade pulp in order to improve drainage. The improved drainage could result in an increase in machine speed or a decrease in energy use in the drying section of paper machine.

MATERIALS AND METHODS

Pulp

Recycled pulp for the production of fluting was provided by the Sappi Cape Kraft mill. The pulp was in noodle form at a consistency of 24 to 30 %.  Only fresh pulp was used in the different experiments.  The consistency of the pulp was adjusted to low consistency (2 to 5 %) or high consistency (20 %) for enzymatic treatment.

Enzymes

Different amylases (1,4-"-D-glucan-hydrolase (EC 3.2.1.1)) from Novozymes were compared by treatment of pulp at high and low consistencies.  The three products were: BAN 480 L, Fungamyl 800 L, and Duramyl 300 L (Table 1).  These enzymes were selected based on their specificity towards starch, low cost, commercial availability and wide ranging applications in other industries.

Table 1. Properties of "-amylases used in starch degradation trials.

 

Fungamyl

BAN

Duramyl

Origin

Fungal

Bacterial

Bacterial

Optimal temp.

37 °C

37 °C

70 °C

Cost (R/l)*

117,26

102,07

180,00

*June 2002

Enzyme activity

The activities of BAN, Duramyl and Fungamyl were determined using a standard Novo Nordisk analytical method (Anonymous, 1978). The method is based on the hydrolysis of potato starch by the enzyme.  Starch and iodine form a complex that has a dark blue to purple colour. As the enzyme degrades the starch, less starch is available for complex formation with iodine, which results in a decrease in colour intensity.  The change in colour is measured spectrophotometrically at 580 nm.  The time taken for the completion of the reaction is then used to calculate activity.

Enzymatic efficiency on pulp

The residual starch content of the pulp was determined before and after laboratory scale enzyme treatment, using the Tappi test method for starch in paper (T 419). The treatment of pulp was done at low (5 %) and high (20 %) consistency for 30 min at 40 C. 

Ranges of enzyme charges were evaluated to optimise enzyme dosages on pulp.  These charges were between 0 U/g (control) and 2000 U/g.  Pulp was treated with the enzyme for 30 min at 40C before determination of the starch content.

The activity of Duramyl on pulp was evaluated at different pH values (5 to 8) by suspending 30 g of pulp in a Britton -Robinson buffer at 1 % consistency before treatment at a charge of 70 U/g and incubated at 40 C. Triplicate samples (100ml) were removed after 1 min to determine the starch content of the pulp.

Pulp properties

Handsheets of treated pulp were made (Tappi. T 205) and the burst and tear strengths tested according to Tappi. T 220. The influence of enzyme treatment on drainage was initially evaluated by testing Canadian Standard Freeness (CSF) (Tappi. T 227).  A revised Shopper Riegler drainage evaluation was also used, where the time was recorded for all the water to drain from the forming sheet while the bottom orifice of the Shopper Riegler funnel was blocked.  In a further test, Vacuum Drainage evaluation was used, where the water was drained with the assistance of a vacuum and the time was recorded from application of the vacuum until the breaking of the vacuum through the web.

Pilot-scale trials

The influence of the three "-amylases on pulp was evaluated on a pilot-scale paper machine.  The experiment was replicated three times and consisted of untreated control runs that were compared to different runs where the pulp was treated with the three different enzymes.  The Fourdrinier machine (Brüder-Kämmerer, Osnabrück, Germany) has a plastic forming wire with an open area of 13 %, and runs at 1,42 m/min while making paper with a width of 210 mm. 

For low consistency pulp treatment, one kilogram of pulp was repulped and then made up to a consistency of 2 % in the machine chest. The pulp was heated to 40 C and enzymes added at the predetermined dosages.  After 30 min the pulp consistency was reduced to 0,5 %. With the addition of backwater the pulp consistency in the headbox was 0,3 % for production of paper with an average basis weight of 122 g/m2

High consistency enzymatic treatment was similar to that of the low consistency treatment but the pulp consistency was adjusted to 20 % by adding water and the predetermined enzyme charge.  The pulp and enzyme was then mixed with a mechanical mixer for 90 sec.  The pulp was placed in sealed plastic bags and incubated at 40 C for 30 min before repulping, dilution in the machine chest to 0,5 % and papermaking.

Paper samples were taken every two minutes after the pressing section to determine starch and moisture content (Tappi. T419 & T 550 ).  Backwater samples were also collected every two minutes to determine the starch content and chemical oxygen demand (COD) (High range: 0 to 1500 mg/l) (Hach, 1997).

Statistical analysis

Completely randomised designs were used for replicated trials and the data subjected to one-way analysis of variance.  Means of different treatments were tested for significant differences with Tukeys test (Winer, 1971) at 95 % confidence.

RESULTS

Enzyme activity

The activities of the three enzyme samples were found to be lower than those specified by the supplier (Table 2). These differences can possibly be attributed to the temperature of 40 C and a different type of starch used in the present study. The calculated activities were used for determination of the correct enzyme charges in the different experiments.

Table 2. Activities of enzymes as provided by Novozymes supplied and as determined in this present study.

 

Activity (KU/ml)

 

Novozyme

This study

BAN

480

303

Duramyl

300

239

Fungamyl

800

793

Enzymatic efficiency on pulp

All the enzymes successfully degraded the starch in recycled pulp using both low (5 %) and high (20 %) consistencies (Figures 1 & 2). Over the range of enzyme charges, Duramyl was the most effective at low consistency with more than 67 % removal of the residual starch, followed by Fungamyl with 39 % removal and BAN with 32 % removal (Figure 1).  The high consistency treatments showed that Duramyl removed up to 76 % of the residual starch, BAN 44 % of the residual starch and Fungamyl only 26 %. These results were used to determine the correct enzyme dosage to use during the various other tests. These charges for high consistency treatment were: Duramyl, 140 U, Fungamyl, 400 U and BAN, 400 U.  For low consistency treatments, the charges were: Duramyl, 140 U, Fungamyl, 175 U and BAN, 140 U.

Figure 1

Figure 1.  Starch removal by different amylases at low consistency (5 %) for 30 min at 40oC.

Figure 2

Figure 2.  Starch removal by different amylases at high consistency (20 %) for 30 min at 40oC.

Duramyl was active at pH levels as low as pH 5 and as high as pH 8 (Figure 3). The enzyme degraded the largest amount of starch at pH 6, which correlates to its optimal pH (Novozymes 2001), and degraded the least amount of starch at pH 8.

Figure 3

Figure 3.  Amount of starch degraded by treatment with Duramyl at different pH values.

Pulp properties

In most cases, the enzymes caused increased burst and tear indices (Figure 4) and enzyme treatment should, therefore, not have any adverse effect on the paper quality. Experiments to study the effect of enzyme treatment on drainage using CSF, the revised Shopper Riegler drainage evaluation and the Vacuum Drainage evaluation yielded no repeatable results.

Figure 4

Figure 4.  Effect of different low consistency enzyme treatment on Burst and Tear indices at standard moisture and temperature

Pilot-scale trials

The paper samples of high and low consistency enzyme treatments, contained up to 2 percentage points less water after pressing when compared to control samples (Figure 5).  Fungamyl with an improvement of 0,79 percentage points was the most effective at high consistency and Duramyl performed the best at low consistency treatment with an improvement of 2 percentage points decrease in water content. All the enzymes performed relatively better at low consistency, because of a higher rate of enzymatic diffusion between the fibres bringing it into contact with more starch, and less oxidation takes place because of lower oxygen concentration within the sample.  These results indicate that enzymatic treatment can be introduced in low-consistency and high-consistency unit operations.

Figure 5

Figure 5.  Reduction in water content after enzyme treatment at different consistencies

The starch content of the backwater samples from low consistency treatment were all reduced by the enzymes, with Duramyl performing the best under these conditions (Figure 6). The starch content of the water from the high consistency treatment showed that Duramyl caused a significant reduction in the starch content, but BAN and Fungamyl increased starch levels (Figure 6). The increase in starch content was probably due to BAN and Fungamyl partially degrading some of the starch, thereby releasing it from the web and increasing the concentration of the starch in the backwater.

Figure 6

Figure 6. Change in starch content of backwater after enzyme treatment at  different consistencies.

The enzymatic treatments did not cause any significant changes to the COD values of the drained water (Figure 7).  The extremely low volumes of enzyme used to degrade the starch had no direct effect on the COD, but when more fines are released from the web due to starch breakdown COD values could increase.

Figure 7

Figure 7.  Chemical oxygen demand of the backwater samples from enzyme treated pulp and untreated control.

CONCLUSIONS

With the removal of starch from recycled fibre, the effect on drainage was relatively small.   Slight improvements have however been found in the drainage on the pilot-scale paper machine and the removal of up to 2 % water could translate to an increase in production of between 7 and 14 %, or savings in energy consumption in the drying section of a paper machine. 

A mill trial is required to optimise enzymatic treatments and to evaluate the effect of the enzyme treatment on drainage for a specific mill. We recommend Duramyl for application at a mill trial, because of its high stability over different pH levels, its wide temperature tolerance, and its efficiency to degrade the residual starch in the pulp.  The present study indicates that the relatively low cost and the small volumes of enzyme used to treat pulp would make this process economically feasible, even without optimisation.

REFERENCES

1. Kirk K. & Jeffries T.W. Roles for microbial enzymes in pulp and paper processing. Enzymes for pulp and paper processing.  Chapter 1. pp3-14 L. Viikari & T.W. Jeffries 1996.

2. Lascaris, E., Mew, L., Forbes, L., Mainwaring, D., Lonergan, G. Drainage improvement of recycled fibre backwater following α-amylase bio-modification. Appita Journal 50(1) pp51-67 1997.

3. Hach. Oxygen Demand, Chemical Method 8000.  Hach Water analysis Handbook pp943-975 1997.

4. Winer B.J.  Statistical principals and experimental design, 2nd ed. 1971 Mc Graw-Hill Book Co., USA.

5. Anonymous. Novo Method for Determination of Fungal/Bacterial Alpha-Amylases. Analytical Method. Novo. 1978.

6. Anonymous. Duramyl. Product Sheet.  Novozymes 2001.

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