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Food Biotechnology S. Bielecki, J. Tramper and J. Polak (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
Application of bacterial cellulose for clarification of fruit juices
A. Krystynowicz, S. Bielecki, W. Czaja, M. Rzyska,
Institute of Technical Biochemistry, Technical University of Lodz, ul. Stefanowskiego 4/10, L6d2 90-924, Poland
The aim of our studies was to estimate the usefulness of bacterial cellulose synthesised by
Acetobacter xylinum E25 for the clarification of apple juice. The cellulose was applied either
as a membrane formed under stationary culture conditions or as a suspension obtained by disintegration of such a membrane. The membrane was used as a filter bed. The results of our studies confirm that cellulose membranes may be used as filtration ones. After filtration through cellulose membranes, apple juice displayed very high clarity, higher than its sample clarified under standard conditions, even after a stability test. Juice clarification only by filtration through cellulose membrane does not assure its stable colour, which darkens under the conditions of a stability test performance. This phenomenon is also observed for other products clarified by filtration only. Bacterial cellulose application as an adsorbent for fruit juice clarification yielded the product with demanded stability.
1. INTRODUCTION
Bacterial cellulose synthesised by acetic acid bacteria, especially strains of Acetobacter
xylinum, has attracted attention because of its unique properties, distinguishing this kind of
cellulose from the plant one, and determining its practical application. The differences lie first of all in thickness of elementary microfibrils comparable to thickness of fibrils present in primary walls of higher plants and equal to 3,0 nm [1,2]. The fibrils originating from 50- adjacent pores localised in bacterial cell membrane combine into bundles which form ribbons with the width of 40-60 nm, thickness of 10 nm and length of 10 ~tm [3]. These ribbons synthesised by many bacterial cells form a pellicle on the surface of a liquid but not agitated growth medium. Both shape and area of this pellicle depend on the size of the horizontal cross section of a culture vessel. The pellicles contain almost pure (about 97%) and highly crystalline (about 65%) a-cellulose [4]. The cellulose synthesis yield depends on the activity of a strain-producer, composition of growth medium and culture conditions, and may amount up to about 28g per 11 under stationary culture conditions or up to about 9g per 11 under submerged culture conditions [5]. The mechanism of cellulose fibrils formation results in a high dynamic durability of bacterial cellulose, equal to 54000 kg/mm 2. This factor is 3-4 times higher in comparison to Kebler fibrils and comparable to glass fibrils [6]. Pressing, squeezing or rolling in a definite direction may increase mechanical durability of membranes. After drying, cellulose membranes resemble a parchment paper with a thickness of 0,01-0,5 nm, and display porous
structure. Pores with diameters of about 3~tm may constitute up to 50-93% of the membrane surface [7,8]. Because of such structure, the cellulose membrane may be applied as a filter cloth [6,9]. Moreover, bacterial cellulose may be used in the form of a defibered suspension for the production of filter media, which due to the size of pores meet the requirements for dialysis, micro- or ultrafiltration membranes [10,11]. Bacterial cellulose structure resulting from the mechanism of its biosynthesis is characterised by a very well developed internal surface, markedly larger in comparison to the plant pulp surface, which gives it high absorption capacity. Since bacterial cellulose has been recognised to be safe for humans, it seems reasonable to use it in food industry [ 12]. The above mentioned advantages of bacterial cellulose prompted our research into its application for clarification of fruit juices.
2. M A T E R I A L S A N D M E T H O D S
2.1. Materials
Apple (Malus domes#ca, Cortland variety) juice after extrusion was depectinized with
Pektopol PT (lml/l) for 4 hours at 45~ and pasteurised for 1 hour. Clarifying agents: gelatine of food quality for clarification SIHA and silica gel for beverage clarification SHIASOL- were purchased from Begerow. Bacterial cellulose in the form of membranes produced under
stationary culture conditions by Acetobacter xylinum E25 from the collection at the Institute of
Technical Biochemistry, Technical University of Lodz. The following growth media were used: (A) The modified Schramm medium [4] containing glucose (2%), yeast extract (0.5%), Bactopeptone (0.5%), NaHPOa (0.27%), MgSOa'7H20 (0.05%), ethanol (1.0%), pH=5.5. (B) The modified Schramm medium in which glucose was replaced with 2% of fructose.
Acetobacter xylinum E25 was cultured under stationary conditions at 30~ for 6 days. The
synthesised cellulose pellicles were washed with tap water, boiled in 1% NaOH solution and again washed with water till NaOH removal.
2.2. Analytical methods Determination of water permeability was based on the measurements of time required for the filtration membrane under constant pressure. Determination of thickness and porosity was performed by the computer image analysis method, using the Carl Zeiss Jena microscope with a program IPS-512 (Imal system). Mechanical testing was performed with cellulose membranes in the form of 20 mm x 160 mm strips partly dehydrated by squeezing. The degree of squeezing off was determined by the per cent content of cellulose. The sample tensile strength was measured with an INSTRON TMM-1111 machine under the following conditions for all membrane variants under investigation:
- distance between jaws 10 cm, - tension rate 10 cm/min.,
- force measurement range 50 N,
- recording paper rate 50 cm/min.,
- temperature 20~
permeability and the smallest pores were observed in the membranes synthesised in the medium containing fructose. This fact probably confirms the conclusion that in this pellicle the cellulose microfibrils are packed more tightly. So one can modify properties of cellulose membranes by means of selection of the growth media composition.
Table 2 Determination of the porosity of cellulose membranes produced in glucose containing medium
Water permeability Pore diameter [lam] Sample coefficient [1/m 2 min] Wet membrane 6.4 1. Air-dried membrane 2.1 1. Membrane dried at 80~ 0.8 1. Freeze-dried membrane 2.2 1. Membrane dried at 80~ * 0.2 0. *from the fructose containing medium
The cellulose membranes produced in the glucose-containing medium were applied in the studies on apple juice filtration. The properties of the juice obtained in this way were compared to the properties of the juice clarified under standard conditions, by using gelatin (0,5 ml of 1% gelatin solution per 100 ml of the juice) or sol of silicic acid (0,5 ml of 3% sol per 100 ml of the juice), followed by filtration on the diatomaceous earth Hyflo Super Cel. Bacterial cellulose in the form of suspension was applied for juice clarification by adsorption using 3 g of the cellulose dry mass per 100 ml of the juice.
Table 3 C.haracteristics of the apple juice a~er filtration Before Filtration through membrane Assay clarification - wet Dried
Clarified by Standard adso .rption clarification Extract [%] 1 3 11.5_ 11.6 11.5 11. pH 3.8 3.9 3.9 3.9 3. Total acidity (g of malic acid per 1 1)
Total polyphenols (g/l)
Clarity (%T62o~m)
Colour
( A ~ )
Colour CIA (Z,- the lenght of maximal absorbance)
Y - palenes s 0.88 0.88 0.88 0.91 0. P e - purity of stimulation
The results of the studies are presented in Table 3 and 4. They prove that the application of this form of bacterial cellulose for filtration or adsorption in the process of juice filtration enables yielding of a product, which displays higher clarity than a juice sample clarified in a standard manner. The parameters characterising the colour saturation and paleness point to the fact that the juice clarified using bacterial cellulose was decolourised to a higher extent. It was particularly distinct in the ease of the sample clarified by means of adsorption, in which a decrease of polyphenol concentration (about 45%) was the highest. The loss of polyphenol stability confirmed the stabilisation of clarity of the examined juice samples. Darkening of juice samples clarified by means of filtration through cellulose membranes only was observed. This phenomenon points to the incomplete stability of polyphenols. When bacterial cellulose was used as an adsorbent for filtration stable juice was obtained. Since the application of adsorption resins for food production is still questionable [14], bacterial cellulose because of its purity and physicochemical properties seems to be a suitable material for fruit juice clarification. Table 4 Characteristics of the apple juice after stability test. Standard Filtration through membrane Assay clarification wet dried
Clarified by adsorption Clarity 92 95 94
Colour (A42o~) 0.24 0.28 0.28 0. Colour CIA (~- the lenght of maximal absorbance)
Y - paleness 0.9! ' 0.89 0.88 0. Pe - purity o f stimulation 0.14^ 0.19^ 0.18^ 0.
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