《食品脱水技术》（英文版） 8 Spray Dried Products.pdf_大学文库

powder will not reconstitute so well, the solids tending to settle out rather than remain in homogeneous suspension. The cold break method has some merit in the manufacture of tomato juice, in which it often produces a better colour and flavour but it invariably has a tendency to cause separating out STRAINING From the pulper the material passes to a holding tank, thence to a series of strainers with perforated plates, reducing consecutively from 1mm holes to 0. 7mm and finally to 0. 4mm. This process removes the skin and seeds, amounting to 5 percent of the weight of the fruit. Strained pulp is then ransferred to storage tanks EVAPORATING The juice is concentrated under vacuum from 5 to 30 percent solids, as a preliminary stage to drying. A double-effect evaporator is normally used, h a finishing pan in the final stage. From here, the paste is transferred to a feed tank where it is constantly mechanically stirred SPRAY DRYING OF TOMATO POWDER The flow-sheet drawing Fig 8.1 shows an operation whereby the production of tomato powder is continuous and concurrent with the evaporation of the fresh pulp. In practice, however, some producers of tomato powder make their product out of season, and in the case of a plant in Portugal, which the author has visited, the company bought in most of their concentrate from a nearby tomato paste factory where there was a surplus, over and above what was contracted for export, as concentrate in 5kg cans The cans were delivered to thedrying plant in standard 5kg packaging, opened by a piston-type automatic can opener, which ejected the paste into the storage vat serving the dryer, at the same time crushing the empty ca and baling them for disposal. In this case the process commenced at Figure 19 on the Flow Sheet- Feed Tank for the Spray Dryer In this particular instance the concentrate was made by the hot break stem with concentration to 30-32 percent solids. Cold break pastes, concentrated to 36-38 percent concentration before drying, are sometimes

powder will not reconstitute so well, the solids tending to settle out rather than remain in homogeneous suspension. The cold break method has some merit in the manufacture of tomato juice, in which it often produces a better colour and flavour but it invariably has a tendency to cause separating out. STRAINING From the pulper the material passes to a holding tank, thence to a series of strainers with perforated plates, reducing consecutively from 1 mm holes to 0.7mm and finally to 0.4mm. This process removes the skin and seeds, amounting to 5 percent of the weight of the fruit. Strained pulp is then transferred to storage tanks. EVAPORATING The juice is concentrated under vacuum from 5 to 30 percent solids, as a preliminary stage to drying. A double-effect evaporator is normally used, with a finishing pan in the final stage. From here, the paste is transferred to a feed tank where it is constantly mechanically stirred. SPRAY DRYING OF TOMATO POWDER The flow-sheet drawing Fig. 8.1 showsanoperation whereby the production of tomato powder is continuous and concurrent with the evaporation of the fresh pulp. In practice, however, some producers of tomato powder make their product out of season, and in the case of a plant in Portugal, which the author has visited, the company bought in most of their concentrate from a nearby tomato paste factory where there was a surplus, over and above what was contracted for export, as concentrate in 5kg cans. The cans were delivered to the drying plant in standard 5kg packaging, opened by a piston-type automatic can opener, which ejected the paste into the storage vat serving the dryer, at the same time crushing the empty cans and baling them for disposal. In this case the process commenced at Figure 19 on the Flow Sheet - Feed Tank for the Spray Dryer. In this particular instance the concentrate was made by the hot break system with concentration to 30-32 percent solids. Cold break pastes, concentrated to 36-38 percent concentration before drying, are sometimes used. 193

heater(28)is supplied from the cooling air wall jacket supplemented by atmospheric air intake, The location of the atomiser within the roof air disperser creates optimum spray /air contact conditions, Moisture evaporation is rapid but controlled Product settles out of the air-flow on the chamber wall, building up to loose layers(15-25mm thickness) before breaking away and falling as nodules to the base ofthe chamber. The build-up is important forcompletion of evaporation. For the removal of the remaining moisture from a tomato particle, much resistance to mass transfer is apparent. The necessary long second period of drying is accomplished by the residence time on the cooled would result. 15-20 percent of the throughput does not settle on the walland passes out of the chamber with the exhausted drying air. The entrained product is recovered in a cyclone, and conveyed from the cyclone base in dehumidified air The bulk of the production falls from the chamber base into an enclosed band conveyor (36). Cool dehumidified air flows counter-currently slowly over the surface, and the product nodules are cooled on the conveyor Atatemperatureof24-30C, the nodules become brittle, and readily shatter into powder as they fall from the conveyor on to a sieve(39). This conveyor exit and sieve are installed within an air conditioned packing room kept at a low humidity. The plant under review dehumidified this area to 30 percent RH, with the temperature at 15C nsome plants this dehumidified area is treated with sodium fluorate, which is introduced into the air flow to provide a sterile atmosphere Final moisture of the powder is 3 to 3. 5 percent, and to maintain this low level in a hygroscopic material, it is preferably packed in nitrogen-flushed sealed polyethylene-lined drums or tins. Whatever container is used it must be air and moistureproof. Atmospheric packing in very dry air conditions was ised at the factory visited but this is recommended only fo storage periods. Anticaking dessicants can also be used, in the form of silica gel envelopes placed in each pack Overall Ratio: range from 20: 1 to 22: 1(Raw material to powder) Whilst the drying plant was equipped to process from 5kg cans of concentrate, a more economical method of packing of the concentrate by the asceptic 225 litre barrels. These are filled at the concentrate plant by flash sterilising the paste at 95C, cooling to 40C and filling into presterilised barrels underreduced pressure. A vacuum is drawn through a 20mm aperture in the top, and filling is through a second 50mm

heater (28) is supplied from the cooling air wall jacket supplemented by atmospheric air intake. The location of the atomiser within the roof air disperser creates optimum spray/air contact conditions. Moisture evaporation is rapid but controlled. Product settles out of the air-flow on the chamber wall, building up to loose layers (15 - 25mm thickness) before breaking away and falling as nodules to the base of the chamber. The build-up is important for completion of evaporation. For the removal of the remaining moisture from a tomato particle, much resistance to mass transfer is apparent. The necessary long second period of drying is accomplished by the residence timeon the cooled wall. Increased drying temperatures cannot be used as heat degradation wouldresult.15-20percentof the throughputdoesnot settleon thewalland passes out of the chamber with the exhausted drying air. The entrained product is recovered in a cyclone, and conveyed from the cyclone base in dehumidified air. The bulk of the production falls from the chamber base into an enclosed band conveyor (36). Cool dehumidified air flows counter-currently slowly over the surface, and the product nodules are cooled on the conveyor. Ata temperatureof24"-3OoC, the nodules become brittle,and readily shatter into powder as they fall from the conveyor on to a sieve (39). This conveyor exit and sieve are installed within an air conditioned packing room kept at a low humidity. The plant under review dehumidified this area to 30 percent RH, with the temperature at 15°C. Insome plants this dehumidified area is treated with sodium fluorate, which is introduced into the air flow to provide a sterile atmosphere. Finalmoistureof the powder is3 to3.5 percent, and to maintain this low level in a hygroscopic material, it is preferably packed in nitrogen-flushed sealed polyethylene-lined drums or tins. Whatever container is used it must be air and moistureproof. Atmospheric packing in very dry air conditions was practised at the factory visited but this is recommended only for limited storage periods. Anticaking dessicants can also be used, in the form of silica gel envelopes placed in each pack. Overall Ratio: range from 20:l to 22:l (Raw material to powder) Whilst the drying plant was equipped to process from 5kg cans of concentrate, a more economical method of packing of the concentrate by the supplier would be in asceptic 225 litre barrels. These are filled at the concentrate plant by flash sterilising the paste at 95"C, cooling to 40°C and filling intopresterilised barrels under reduced pressure. A vacuum is drawn through a 20mm aperture in the top, and filling is through a second 50mm 196

INSTANT COFFEE CLEANING AND BLENDING The first step in the processing line is a thorough cleaning of the green coffee beans, to remove defective beans and extraneous matter. Blending is usually carried out to achieve optimum flavour development in the roasting stage ROASTINGAND GRINDING Roasting enhances the flavour and aroma and may be carried out batch-wise orcontinuously. The lattercontinuous method is more cost effective. Grinding to a particle size most suitable for the extraction process follows EXTRACTION This is either continuous or batch. The continuous method employs a tiltable jacketed pressure vessel containing two helicoidal conveyors. Hot water enters the top end and the extract flows through the trough by gravity. The dwell time is from 30 to 40 minutes. In two stage extraction, atmospheric ressure is used in the first stage and pressure Batch extraction is carried out in a counter-current column battery unit featuring split extraction under closely controlled conditions, and yields of over 48 percent can be achieved. The first stage produces a prime quality extract, which is held separately pending final pretreatment and drying. The second stage produces a high overall extract yield but with low solids and this has to be subsequently concentrate CONCENTRATION recovery section, or arotary thins stage falling fiImevaporatorwitharoma evaporator designed uniquely by niro for heat sensitive products Both types of evaporator operate under vacuum, thereby maintaining low extract temperatures SPRAY DRYING The concentrated extract is pumped to the pressure nozzle in the dryin chamber where it is atomized and contacted with hot air Drying temperatures are low in order to preserve aroma and flavour in the dry product. An in-lir sparge system operating with inert gas in the feed system allows adjustment of powder bulk density and colour. The enlarged conical section of the drying chamber separates the dried coffee from the drying air so effectively that virtually all the powder leaves the base of the chamber, where it is cooled, screened and passed to storage, packing or agglomeration

INSTANT COFFEE CLEANING AND BLENDING The first step in the processing line is a thorough cleaning of the green coffee beans, to removedefectivebeans and extraneous matter. Blending is usually carried out to achieve optimum flavour development in the roasting stage. RO&TIJXGANDGRINDING Roasting enhances the flavour and aroma and may be carried out batch-wise or continuously. The latter continuous method is more cost effective. Grinding to a particle size most suitable for the extraction process follows. EXTRACTION This is eithercontinuousorbatch. Thecontinuous method employsa tiltable jacketed pressure vessel containing two helicoidal conveyors. Hot water enters the top end and the extract flows through the trough by gravity. The dwell time is from 30 to 40 minutes. In two stage extraction, atmospheric pressure is used in the first stage and pressure in the second. Batch extraction is carried out in a counter-current column battery unit featuring split extraction under closely controlled conditions, and yields of over 48 percent can be achieved. The first stage produces a prime quality extract, which is held separately pending final pretreatment and drying. The second stage produces a high overall extract yield but with low solids and this has to be subsequently concentrated. CONCENTRATION Two methods are used, eithera two stage falling film evaporator with aroma recovery section, or a rotary thin film evaporator designed uniquely by Niro for heat sensitive products. Both types of evaporator operate undcr vacuum, thereby maintaining low extract temperatures. SPRAY DRYING The concentrated extract is pumped to the pressure nozzle in the drying chamberwhereit isatomizedand contacted with hot air. Drying temperatures are low in order to preserve aroma and flavour in the dry product. An in-line spargesystem operating with inert gas in the feed system allows adjustment of powder bulk density and colour. The enlarged conical section of the drying chamber separates the dried coffee from the drying air so effectively that virtually all the powder leaves the base of the chamber, where it is cooled, screened and passed to storage, packing or agglomeration. 198

FREEZE-DRYING OF COFFEE Whilst it may be pertinent to describe the principles of freeze-drying coffee, it is a high capital investment project, spearheaded originally by General Foods Corp in the USA in 1964. Subsequently, production was extended in he States and Nestle entered the field and developed the process in Brazil, in the USA. whilst it i as a premium priced product, on account of the relatively high operating costs compared with spray dried coffee, freeze-dried coffee will undoubtedly hold its share of the market but it is not an area into which many newcomers may venture, as the market has been cultivated by the multi-nationals who were in from the beginning and have dominated the trade by expensive advertising and marketing devices which only massive organisations can sustain Technically, the first stages of the processes are similar to those applying to spray drying: blending, roasting, extracting and concentration. At this point the 30 percent solids extract is sometimes freeze-concentrated to 40 percent solids, and then frozen to about minus 50"C. The product is hen granulated, transferred into trays and dried in a vacuum chamber in an 10 hour cycle to about 2 percent moisture. End temperatures in the granules range from 37 to 40C A two stage compressor is needed to maintain a temperature of minus 50C in the condenser coils of the freeze dryer, and either ammonia or Freon is used as a ref A standard batch chamber dryer has a tray space of 93sq m and requires a 66ton two stage recirculating installation to operate it. Most commercial units operating on the batch system have at least 4 chambers of 93sq m in service, which today could involve a capital outlay of some $3m for equipment alone, not including installation. This is only a modest percentage of the total investment involved. Some plants are completely automatic, adding considerably to the investment cost but not always saving much on the operating costs per kg of coffee produced PACKAGING This is invariably in retail or catering glass packs, hermetically sealed and nitrogen-flushed, as the product is extremely hygroscopic