Pulp production

Floating pulp, i.e. the larger solid particles in the juice, mainly consist of small pieces of rup tured cell sacs and segment walls. They are sep a rat ed from the juice in fi nishers. (The very small pulp particles fl ow with the juice stream from the fi n- ish er. These fi ne particles tend to sed i ment at the bottom of the juice and are re ferred to as sinking pulp.) The pulp stream from the fi nisher is han dled in different ways de pend ing on the end use of the pulp. The alternatives are:

• Recovery for production of commercial pulp. Pulp is used as add-back in juice and juice drinks.

• Production of pulp wash, the juice sugars ob tained by washing pulp with water. The re main ing material is sold as “washed pulp” or tak en to the feed mill.

• Routing to the feed mill for drying into pel- lets for animal feed.

In the past, most pulp went to pulp washing and the feed mill. However, now that the current mar- ket trend is to add more pulp cells to the fi nal juice, the proportion of pulp from the ex trac tors going to commercial pulp production is increasing. For most processors, however, more pulp is obtained from the fruit than is re quired by the juice industry for add-back to juice.

The extractor type and operation will in fl u - ence the quality of the pulp produced. In some plants, the extractors used for pulp production are adjusted to optimise pulp quality rather than to maximise juice yield. The visual difference be- tween pulp from reamer-type extractors (Brown) and stand ard squeezer-type extractors (FMC) is il lus trat ed in Figure 5.22.

Fig. 5.22 Illustration of relative pulp sizes after extraction.

Reamer-type extractor

Standard squeezer-type extractor

Reclaimed juice to evaporator

Defect removal Pasteuriser Drying finisher

Box/drum filler Primary finisher

Pulpy juice from extractors

Aseptic bag filler 5.10.1 PRODUCTION FACTORS WHICH

AFFECT COMMERCIAL PULP QUALITY Some of the process conditions which have a sig- nifi cant infl uence on pulp properties are giv en in Table 5.1.

5.10.2 PROCESS STEPS IN PULP PRO DUC TION

The exact confi guration of the pulp line will vary from plant to plant, and its design will de pend on the type of extraction system and processor preference. The basic pulp production steps are shown in Figure 5.23.

Instead of pulpy juice from the extractors, pulp from the primary fi nishers in juice clar i fi ca tion is sometimes taken as feed to the pulp pro duc tion lines. Dilution with juice prior to the defect re- moval step may then be needed.

Extraction

During the juice extraction process, segment walls and cell sac walls are torn into pieces. Both the reamer-type extractor and specially de signed squeezer-type extractor used for pre mi um pulp put less shear force on the pulp com pared with the standard squeezer-type ex trac tor. This results in larger and less frag ment ed pulp pieces. How- ever, defects such as core and seeds also end up in the pulpy juice from the extractors. This imposes greater de mands on the defect removal system. Some times, pulp from the primary fi nishers in juice clarifi cation is conveyed to feed the pulp pro duc tion line. Dilution with juice prior to defect removal may then be required.

TABLE 5.1 INFLUENCE OF PROCESS CONDITIONS ON PULP PROP ER TIES Pulp properties Process conditions

Cell length and - fruit variety and fruit maturity

fragmentation degree - size of the holes in the strainer tube (squeezer-type ex trac tors) - extraction pressure

- use of paddle or screw fi nisher

- back-pressure applied to the primary and fi nal fi nishers (screw type) - equipment and operating conditions for the pulp stabilisation step. Oil content - extraction pressure. High pressure gives higher juice yield but also higher oil content in the pulpy juice stream.

Defects in fi nal product - depends on what type of equipment is used to separate defects from the pulpy juice stream.

Pulp concentration - tightness applied in the fi nishers. (i.e the concentration

of pulp particles in pulp slurry)

Fig. 5.23 Pulp production steps.

Defect removal

Defects are normally described as small frag- ments of peel, membrane or seed. As the ab sence of defects in the fi nal product is an im por tant quality parameter, they have to be re moved from the pulp/juice slurry.

Defects are removed in a series of separa- tion steps. The fi rst step may be a classifying fi n ish er. This is a paddle-type fi nisher in cor - po rat ing screens with large perforations that will allow juice and cells to pass through but retain large seeds and pieces of membrane. The pulpy juice stream then goes to one or more hydrocyclones. If there are a lot of defects, two or more hydrocyclones are used in series. Hydrocyclones are based on gravity separation and remove defects which have a higher density than the pulp slurry.

Figure 5.24 shows the liquid and particle fl ow in a cyclone. The infeed, which is tan gen - tial ly introduced into the cone, starts moving in a down ward spiral along the cyclone wall. As it nears the cone outlet, some of the product leaves

through the underfl ow orifi ce while the majority changes its direction and fl ows upward to the cyclone overfl ow, taking an inner spiral path. If the density of the particles is higher than that of the liquid, the centrifugal force press es the particles against the cyclone wall from where they are pushed down and out through the bot- tom opening.

Separation in a cyclone is improved with low er solids concentration and lower liquid vis- cos i ty. As small immature seeds are lighter than pulp slurry they are diffi cult to remove. Thus the quality of fruit delivered to the processor is important for the results of defect removal. Concentration (primary fi nishers)

The “cleaned” stream from the defect removal system is normally concentrated prior to heat treat ment. The reasons for this are two-fold: • energy is saved by heating/chilling

less prod uct

• less juice is subjected to additional heat treat ment

Concentration is done in a screw-type or pad dle- type primary fi nisher. Paddle fi nishers treat pulp particles more gently. The operation of the fi n ish er can be adjusted so that the pulp con cen tra tion of the discharged pulp slurry is at the re quired strength for the downstream pas teur i sa tion step. In Florida, most processors operate so that the pulp slurry from the primary fi nisher has a typical pulp concentration of 400–500 g/l. In Brazil, there is a difference between plants – from 150-200 g/l up to 500 g/l pulp con cen tra tion. The lower range is due to using plate heat exchangers in the pasteuriser.

The pulp stream from the primary fi nisher to pasteurisation cannot be kept constant, but will vary in both fl ow rate and pulp concentra- tion (10–15 %) during a production shift. Over a sea son, different fruit varieties and extractor set tings will give wider variations.

Cleaned pulp slurry

Pulp slur ry

Defects stream

Fig. 5.24 A hydrocyclone used for removing defects.

Heat treatment

The two objectives of pulp slurry pasteurisation are:

• to inactivate enzymes

• to destroy relevant mi cro or gan isms

The necessary degrees of enzyme inactivation and microbial reduction depend on how the pulp will be further processed and stored. The required inactivation determines the pas teur i sa tion condi- tions (temperature and time).

As the enzymes in oranges are located in the fruit cell walls, the enzyme concentration is sig nifi cant ly higher in pulp slurry than in clari- fi ed juice. To achieve complete inactivation of en zymes, more intensive heat treatment is needed for pulp slurry than for juice. However, com plete enzyme inactivation is normally not re quired. En- zyme activity should be reduced to such an extent that the pulp:

• is stable during bulk storage

• will not lead to cloud separation in reconsti- tuted juice

If the downstream handling of heat-treated pulp is nonaseptic (e.g. the drying fi nisher), com plete killing of microorganisms is not required. This is the case for pulp stored frozen, the most common storage method. In this case the heat treat ment is referred to as “stabilisation”. Typical heating con di tions are 90–100 °C for 30 sec onds.

When pulp is to be stored chilled in aseptic bag- in-box containers, heat treatment may be re ferred to as “pulp stabilisation/sterilisation”. Tem per a tures in excess of 100 °C are normally used. A high er de- gree of enzyme in ac ti va tion is re quired for chilled storage compared with fro zen storage. Aseptic storage also requires that heat-treat ed pulp has no mi cro bi al activity. Fur ther more, downstream equipment must not recontaminate the product.

Which heat exchanger?

The heat exchangers used for pasteurisation of pulp slurry are typically of tubular type. Any ob struc tions on the product side, such as con- tact points in a plate heat exchanger, should be avoided. Often the heat exchangers incorporate a single product tube. With this type there is no risk of uneven product fl ow. However, through put is limited due to the pressure drop.

A multitube heat exchanger (see Figure 7.12) can process high pulp fl ow rates without the drawback of excessive pressure drops. The inlet to the parallel tubes requires careful design to ensure that pulp does not stick to tube en tranc es thereby causing blockage and uneven fl ow rates through the tubes.

Heat treating pulp at concentrations much above 500 g/l is not really feasible in tubular heat exchangers because heat transfer co ef fi cients rapidly decline above this concentration. Effi cient heat transfer is inhibited by the high cellulose content of the product. If tubular heat exchang- ers are used for higher pulp con cen tra tions, they become very large, which entails slow heat-up and cool-down times resulting in a loss of product quality. A pasteurisation system for pulp using multitube heat exchangers can also have the dual function of pasteurising NFC.

The nature of the pulp recovery process tends to entrain air into the product stream. This has to be considered in the design of the heat treat ment process.

5.10 Pulp production

“The enzyme concentration is

higher in pulp slurry than in

clarifi ed juice

Concentration (drying or fi nal fi nisher)

Traditionally, the heat-treated pulp is further con cen trat ed up to 950–1,000 g/l using a fi nal or drying fi nisher. Although still wet it is called “dry” pulp because it will not release any free liquid when pressure is applied to it. The re sid u al liquid is mainly adsorbed onto the cel lu lose membranes.

The concentration of dry pulp is measured for product specifi cation by a special method called Quick Fibre (described in Section 8). The liquid in the pulp, essentially NFC, typically cor re sponds to 5–8 % of pulp mass for standard pulp, and 9–13 % for premium pulp. Thus when pulp is added during reconstitution at the juice pack er, the juice still present in the “dry” pulp will provide additional NFC.

Packing in boxes/drums for frozen storage The concentrated pulp is normally packed in 20 kg corrugated cardboard boxes lined with a polyeth yl ene bag which are then frozen. Freez ing can take several days. Pulp may also be packed in drums (200 l/55 gal.) for frozen storage. How ev er, drums are not often supplied to juice pack ers as they are usually too large for the batch es of re- con sti tut ed juice.

Packing in aseptic bag-in-box containers for chilled storage

If the stabilisation process is modifi ed to be come a stabilisation/sterilisation process, it is possible to pack pulp aseptically and store it re frig er at ed. Packing is done directly after heat treat ment. Hence, the aseptic pulp will be bulk stored at a much low er concentration than fro zen pulp.

The disadvantage of packing pulp asep ti cal ly at 500 g/l concentration is that a larger stor age (and shipping) volume is needed for the same amount of dry pulp. The advantage is that the pulp is much easier to handle as it is pumpable and needs no thawing or crushing. It also gives the possibility of enhancing the fi nal product. When the aseptic pulp is added back to juice

reconstituted from concentrate, juice present in the aseptic pulp (effectively NFC) may provide some of the desired fl avour as so ci at ed with NFC products. Aseptic pulp is pro duced by several processors in Florida.

Fig. 5.25 Flow chart of pulp wash production. Pulp from juice

clarification

Water Washed pulp

Enzymes Multistage counter

current wash system Pulp wash heater Enzymatic reactor Pulp from juice

clarification

Box/drum filler

Evaporator feed tanks

Debittering Evaporator

Blending

Drum filler Clarifier

Pasteuriser

Orange juice concentrate