• COLOURED JUICES • BUCHER • MEMBRANE FILTRATION • PRE-TREATMENT •

FILTRATION

MEMBRANE FILTRATION: PRACTICAL EXPERIENCE WITH JUICE PRE-TREATMENT AND FILTRATION OF COLOURED JUICES * Dr Edgar Zimmer

1. INTRODUCTION Crossflow membrane filtration (CMF) in juice production started about 20 years ago. It replaced in succession, traditional rotary vacuum-drum filters (RVDF), diatomaceous earth (DE) and sheet filters. The reasons for this success are the perfect and reliable clarity of the filtrate, the highly automated and easy-tohandle process and the cost savings. For apple and pear juice, CMF is today by far the most widely used filtration technology. It may safely be considered industrial standard. The benefits of the CMF technology contrast the weaknesses of the traditional filtration process:

TABLE 1: WEIGHT OF FIBRE, PECTIN AND ANTHOCYANINS IN VARIOUS FRUIT TYPES Fruit Black Red Apple Cherry Strawberry Component Currant Grape Fibres 18 – 25 15 15 – 20 40 – 70 8 – 14 g/kg Pectin 5–9 2–3 5 17 3–4 g/kg Anthocyanins traces up to 4.5 5‘000 to 10‘000 Dalton. Depending on type and maturity of the fruit and the processing conditions, the content of soluble colloids in the juice after pressing varies significantly. Conventionally pressed apple juice has low colloid levels of 0.2 – 1.6 g/L, typically. The values at the higher end of this range result from either overripe fruit or extravagant mash enzymation. After juice depectinisation, the levels drop to 0.1 – 0.7 g/L due to partial enzymatic degradation of the colloids. The colloid levels in cherry juice and most grape juice types also seem to be low. This may be explained by the low pectin concentration of these fruit types. On the other hand, the colloid levels in strawberry and black currant juice are extremely high, even after enzyme treatment. Another difference between the juice types can typically be found in sediment concentration. While apple juice extracted with a Bucher press shows low sediment levels, typically in the range of 1 %, cherry and especially strawberry juice may feature enormous sediments loads, up to 10 % or more. Black currants and grapes often show higher sediment ratios than apples but not by far as high as strawberries. What explains these high colloid and/or sediment concentrations? Three reasons stand out: TABLE 2: JUICE COLLOIDS (MW > 5‘000 – 10‘000) AND SEDIMENTS Fruit Black Apple Cherry Strawberry Component Currant

Mash heating to 50 °C is required for extracting the valuable colour (plasmolysis). Also mash enzyme treatment of most fruit types requires this temperature to provide acceptable yield and throughput levels during juice extraction as well as to improve colour extraction. Mechanical and thermal stress during crushing, pumping and agitating as well as the partial degradation of pectin and cell-wall material through mash enzyme treatment sometimes cause elevated sediment loads and colloid levels. Only a part of the colloids in the extracted juice is degraded by juice enzyme treatment because the enzyme products on sale do not feature the required activity. As a result, the colloid levels in some types of berry juice can be > 10x higher and the sediments levels in cherry and strawberry juice > 5x higher than in apple juice. Besides colloid and sediment levels, the colour (mostly anthocyanins) obviously differs from apple juice. The anthocyanins from coloured fruit are relatively small watersoluble molecules, which easily pass the pores of membranes used in juice filtration. However, especially at the beginning of the filtration of a new batch, some colour is retained by the membranes. How can this paradox be explained? The most likely explanation for this phenomenon is that the juice colour is only partially created by monomeric anthocyanins. Anthocyanins are reactive molecules. They go through numerous reactions during juice processing and storage. Many of these reactions are still not or not fully understood. They encompass hydrolysis, oxidation, condensation, polymerisation, etc. – sometimes termed “anthocyanin aging”. It is known that more or less polymerised or condensed reaction products are formed that still are Red coloured. Some of these secondary colour Grape bodies even seem to stabilise juice colour.

Soluble Colloids g/L 0.2 – 1.6 before enzyme treatment

?

?

5 – 15

?

Soluble Colloids g/L 0.1 – 0.7 after enzyme treatment

0.34

>6

2.7 – 10

50 % compared to CMF only, mainly due to the higher flux.

Often juice processors buy CMF systems primarily for apple juice filtration. If the CMF throughput is not sufficient for filtering the coloured juice (fluxes lower than with apple), implementation of existing traditional filtration equipment can help to overcome the filtration bottleneck.

7. SUMMARY AND CONCLUSIONS

One option is to filter the fined juice with the CMF as described above, but to not diafilter at all or only slightly, e.g. down to 10 Bx. The sediments concentrate (= residual retentate) is further filtered through a RVDF. The RVDF filtrate can be returned to the next CMF batch or filtered with a DE filter for polishing. Throughput increase compared to CMF only is approx. 20 % (Picture 5 option 1). A second option for including traditional filtration equipment is to wait for the sedimentation of the fining sediments (Picture 5 option 2). The partially clarified supernatant (typically 80 – 90 % of total juice) is fed to the CMF system. Due to the low sediments the flux typically is higher and higher CR can be achieved, minimising the water and the time required for diafiltration. The diafiltered sediments concentrate with approx. 2 Bx is discarded. Running two to three batches a day helps to protect the colour. The fining sediments (10 – 20 %) are filtered with a RVDF. Again, the RVDF filtrate can be send to the CMF or to a DE filter for polishing.

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To summarise: • The filtration and clarification of coloured juice can, in most cases, be significantly improved by reducing the colloid and sediment extraction during mash and juice preparation. • For Crossflow Membrane Filtration, wide bore tubular polymeric membranes are recommended because they feature greater tolerance for high sediment loads and abrasive bentonite. • The membrane cut-off is not decisive: Micro Filtration is preferable but Ultra Filtration is also feasible. • CMF juice is generally brilliantly clear. • With diafiltration and/or additional sediment filtration, maximum colour yield is achieved. The clever integration of crossflow membrane filtration into juice processing will allow the profitable manufacture of coloured juice and concentrate, featuring brilliant clarity and excellent colour. The question is not so much “if” but rather “how” a CMF system should be implemented in the processing set-up for coloured juice. AUTHOR Dr Edgar Zimmer Bucher Processtech AG CH-8166 Niederweningen – Switzerland www.bucherfoodtech.com

May/June 2007