5 Dehydration of Potato Products Potato Granules Potato granules are more usually commercially designated as Instant Potato or Potato Mash Powder. This product was first manufactured in the UK, on a commercial scale, some 55 years ago. As the name implies, precooked potatoes are dehydrated into granular form and, when hot water is added, they revert almost instantly to cooked mashed potato. This product has been widely developed as a convenience food in America and Europe, and is almost certainly the most popular forin of dehydrated potato on the market today. It features extensively in snack foods. There are several methods of producing granules but the generally accepted commercial method is the 'add-back' process. Several improvements have been made in the manufacturing processes since World War 11, and some of these are referred to in British Patents No.683,604 (1952) and N0.740~711 (1955). The basic process goes back to, and is described in, earlier Patents - No.496,423, No.525,043 and N0.601~152. The following description of the process explains the basic commercial add-back method but makes no reference in detail to the refinements and improvements, introduced in recent times, by individual pmcessors, many of which were originally patented. I05 Flow Sheet Feed to Line I Dry Cleaning I Destoner Washer I Peeling I Skin Elimination I Washing I Inspection - Trimming I Slabbing (Slicing) I I Mashing - Mixing I Add - Back Mix I Sulphiting + Additives I Granulating - Conditioning I + 1.5mm rejected I to mixer by bulk Secondary Drying (35%) by bulk I Final Sieving through cooking kreeningd 65% add-back granules 250 micron sieve I 7 Overtails return Primary Packing to mixer Nitrogen Flushing I Secondary Packing in poly-lined sacks I06 A Lockwood self feed hopper in to which potatoes are being tipped from a swivel head fork lift truck Varieties of Potato Suitable for Granules The main requirements are (1) high solids content, (2) low reducing sugars, (3) freedom from aftercooking darkening, (4) immunity to wart diseases, and (5) shallow eyes. In America and the UK, the preference is for white fleshed varieties, whereas in Western Europe, cream and yellow fleshed potatoes are more popular. The following varieties are, therefore, in general commercial use for this process. USA: Russet Burbank, Katahdin, Kennebec, Cobbler, Chippewa I07 UK: Aran Comet, Aran Pilot, Home Guard, Estima, Pentland Javelin, Pentland Crown, Maris Peer, Cara, Maris Piper, Pentland Dell, (West and East) Record, Uren, Wilja, Desiree. Europe: Bintje, Iris, Lenino PROCESS The important factor to be observed at all points of processing is the prevention of cell rupture in the raw material. Every individual cell that is damaged in peeling, cooking, granulating or drying will release free starch, which will mitigate against satisfactory reconstitution, and will produce a sticky gelatinous mass instead of a light fluffy mashed potato. Peeling should be by lye or the steam method, and the potatoes should be well sprayed with cold water afterwards. This can be done in a rod washer, or preferably in a brush washer as described in the chapter on preparation plant. Batch abrasive peelers are wasteful, and tend to create the very conditions of cell damage which should be avoided. More recently developed continuous abrasive peelers, with sophisticated control of the depth of peeling, may well meet the requirements of minimal cell damage, therefore the processor has a reasonably wide choice of equipment for this part of the process, but Flash Steam Peeling is preferred. From the peeler and washing plant, the potatoes pass over an inspection belt where they are trimmed. With average quality raw material, ten women at the trimming belt should handle 16 tons in eight hours. At the end of the trimming and inspection table, the small to medium size potatoes go straight to the cooker, whilst tubers of 44mm upwards are diverted to a slicing machine for slabbing into 20mm slices before returning to the main flow into the cooker. If required, the trimmed and sliced potatoes can be delivered into a surge tank prior to cooking; this has the effect of removing surface starch, and also provides buffer storage to keep the line running where there are breaks in the preceding process for some reason or another. Cooking, in continuous cookers, may be in atmospheric steam for 45-60 min, or alternatively the potatoes may be partly cooked in hot water, cooled and then transferred to the steam cooker (Cording and Willard method: 1955). From the cooker, which is usually a rectangular steam cabinet with a stainless steel mesh conveyor belt, the potatoes now emerge, thoroughly but not overcooked, and fall into a paddle mixer. This is normally of the U trough-type, with a rotating shaft fitted with T-shaped paddles at intervals I08 along the length. The angle of the paddles to the line of the shaft is variable, so that the flow of material can be accelerated or decelerated, as may be required, to thoroughly mix the product. Mashing, and mixing in the add-back granules or ‘seed’ powder at this point must be gentle to avoid cell rupture, and the speed of the mixer is critical. The seed powder is continuously fed into the mixer at the point where the cooked potatoes enter, at a rate in excess, by weight, of the potatoes. The average proportions might be 35 percent potato to 65 percent seed powder but this can only be established by practice, and will vary according to the solid matter in the raw potato, and, conversely the moisture content of the seed. The ultimate objective is to obtain a blend with 35-40 percent moisture; the arrangement whereby the seed powder is available continuously at the mixer is described later. The blend of cooked potato and add-back seed fills up the trough mixer, and is then allowed to fall over a weir, through an aperture in the end- plate, into a second mixer of longer but shallower proportions. The purpose of this second mixer is to extend the mixing period, at the same time allowing the blend to cool as it travels along the trough. As stated before, the rate of travel is controlled by the angle of the paddles, and thorough granulation will take 25-35 min. Sulphite and other additives, which may be required, are added to the blend by a suitable metering device, during the mixing process. No general rule can be applied as to the additives, as Food Laws differ from country to country, and the processor must familiarise himself with what is permitted, and what is not, in his own particular market. Additives may include sodium metabisulphite, acid sodium pyrophosphate, monostearates, anti-oxidants, flavourings and milk powder. Sulphur Pyrophosphate, anti-oxidants, and sometimes milk powder are mixed with potato granules to make a ’master mix‘ whereby the metering into the mash can be more accurately gauged and, by dilution, can be assimulated more evenly than small quantities of separate ingredients. Glycerol monostearate in solution is metered in. At the end of the second mixer, the blend again falls over a weir and passes to a conditioning bin, where the material cools to about 24°C in an hour. Two bins are normally used, one being filled whilst the other is ‘conditioning’. This conditioning is a most important stage in the process, as it assists granulation and retrogradation of the starch. From the bottom of the conditioning bin, the blend feeds into the first stage of drying. The primary dryer can be either (a) a pneumatic ring dryer, or (b) a thermal venturi dryer, both of which were described in Chapter IV. A suitable feeding arrangement is required for the particular type of dryer I09 used but it is important that this should provide a consistent, regular rate of feed, compatible with the rate of throughput from the mixing plant, and a proper balance must be achieved at this point. There must always be an adequate reserve of blend in the conditioning bins because, if the feed to the dryer is too fast or too slow, the whole system will break down. DRYING The air velocity of the dryer must be carefully controlled from the outset, otherwise cell damage can occur here. The function of the primary dryer is to reduce, rapidly the 35-40 percent moisture in the blend to 12-15 percent as the powder, which it now more correctly resembles, leaves the collecting cyclone. At this point, the powder is screened and coarse material Left: A Finex 22 sieving machine which is ideal for potato granules or vegetable powders l lO - plus 1.5mm mesh - is removed. Of the finer material which passes through this mesh, approximately two thirds is conveyed back by auger or belt as seed powder to meet up with the cooked potatoes in the first mixing process. The proportion fed back must be carefully metered to provide exactly the comt proportion in relation to the quantity of potatoes passing through the cooking stage. As explained before, this proportion will be of the order of 65 percent. The remainder of the powder, having passed through the screen, now passes to a secondary dryer, which ideally is (a) a fluidised bed dryer, or (b) a rotary louvre dryer. This secondary drying reduces the powder to 6-7 percent moisture content. The secondary dryer discharges into a collecting cyclone and it is customary to locate the latter in a cooling system. An air ring, similar in design to the ring dryer, but utilising cold air, is ideal for this purpose, and this cools the product down to about 16°C. From the cold air cyclone, the powder discharges through a rotary valve on to a final screen fitted with a 250 micron stainless steel sieve, and the 'through' material is the final product which then goes to pack-off. Any overtails from the final screening are returned back as seed. It is essential that a permanent cascade magnet be fitted on the outlet of the last screen, to remove any ferrous metal contamination. A Finex 22 sieve with 2 decks as illustrated is ideal for this purpose, and a Finex 48 sieve for the initial screening. From the foregoing description, it is seen that the process involves the continuous feeding back of part of the dried material, which is used to absorb in excess of 50 percent of the initial water content of the cooked potato. The processor must therefore, always retain a stock of seed granules to start up the system at the beginning of a new season. It must also be remembered that the rate of pack-off of the final powder can only equate with the input of raw potatoes, and if this is wrongly estimated, the seed will gradually bleed out of the system, and the process will eventually come to a halt. This balance can only be achieved with experience, and the constant attention of the dryer operative. Product should be packed in nitrogen-flushed drums for prolonged storage, or 25kg polyethylene-lined sacks for 6-8 weeks storage in temperate conditions. RAT1 0 Expected ratio of final product at 6 percent moisture content, using potatoes with 20 percent solids, would be expressed as about 6:l. 111 Single drum dyer for potatofldkes (courtesy ofMitchell Dryers Ltd) POTATO FLAKES Flow Sheet Feed to Line I Dry Cleaning I Destoner-Washer I Peeling I Skin Elimination I Washing I Inspection-Trimming I Slabbing (Slicing) I Hot Water Cooking I I Steam Cooking Cooling I Ricing I Additive Addition I I Flake Breaking I Inspection I Antioxidant addition or I I Nitrogen Flushing I Secondary Packing in poly-lined sacks Drying Antioxidant addition or Primary Packkg (Nitrogen Flushing The production of Potato Flakes was developed in 1954 by Cording and Willard at the Eastern Utilisation Research and Development Division of the Agricultural Research Service in Philadelphia, USA. Potato flour had been produced on single drum dryers for at least 70 years but it was not until 1954 that the technique was perfected, whereby the drum dryer could be used to produce a product, which, on reconstitution with hot water, gave a mash equal in texture and appearance to freshly mashed potato. The success of the process lay in minimising the rupturing of the starch cells, and the special steps, taken in precooking and cooling, to retmgrade or reduce the solubility of the amylose fraction of the potato starch. (See Potato Granules cooking method.) Varieties of Potato Suitable for Potato Flakes The required characteristics are similar to those for granules, therefore all the varieties listed for the latter product are suitable for flaking. Process Peeling is by lye or steam methods as a general rule, followed by a thorough brush washing and cold water spraying to remove surface starch. After inspection and trimming, which must be very thorough, so as to remove all eyes and blemishes, sizing and slabbing follows, as for granule I13 production. The cooking procedure is most important, and this is the main part of the process as set out by Cording and Willard. The first stage is precooking in hot water, and for this purpose, a continuous auger-type cooker is often employed. The dwell time is about 30 min at 71 °C. Cooling takes place in a similar vessel, circulating cold water to sustain a temperature of 10°C for 20-30 min. Section of the Erin Foods plant at Mallow, Eire - potato flake process showing Gouda roller dryer The potatoes then pass to a continuous atmospheric steam cooker with a cycle of 35-50 min. Overcooking destroys texture, and this must be carefully controlled. High solids potatoes require less cooking than low solids varieties. Ricing, or mashing, follows the cooking process. This is achieved by feeding the cooked potatoes through rolls into a rotatbg cylinder, perforated with 6mm holes. The cooked slices are gently forced through the perforations, and a ribbon screw on the inside of the cylinder discharges the product at one end. This equipment was designed in the US but there are equally acceptable alternative methods in Europe. Additives are introduced after ricing to improve stability, texture and colour. As in the case of granules, these may comprise sulphite, glycerol monostearate, sodium pyrosphosphate, citric acid, etc. Sometimes anti- oxidants are also used but legal restrictions on their use apply in some countries. Skimmed milk powder is used in small quantities by some manufacturers. l l4 Sulphur dioxide content in the final product, where permitted, is usually controlled at 250 ppm with a legal limit of 500 ppm in the UK. DRYING The riced potato is fed on to a single drum dryer, as described in Chapter 4. Single drum dryers are used mow frequently than double drums, as it is believed that a better bulk density is achieved by this method. Potato Flakes have a bulk density of about half that of potato granules, and this may be considered somewhat of a disadvantage in the context of packaging costs. The delivery from the dryer is usually by auger, which breaks down the curtain of dried potato to flake form, and the flakes then pass over an inspection belt for removal of any residual blemish. It is important, as in the case of all dehydrated products, that the flakes flow under and over permanent magnets, or other metal detection devices, before packing. Some drum dryer installations provide for a flake breaker, or Floconeuse, which is reputed to produce more uniformity in flake size. Packing This is usually in 15kg polyethylene-lined multi-ply paper sacks, or, for prolonged storage in nitrogen-flushed air-tight drums, with anti-oxidant added. Ratio Expected ratio is 6.5:l. POTATO DICE Feed to Line I Dry Cleaning I Destoner- Was her I Peeling I Skin Elimination I Washing I Inspec tion-Trimming I15 I Dicing I Blanching I Sulphiting + Additives I Dewatering I Drying I Conditioning I Screening I Inspection I Packing There is a growing market for Potato Dice (9.5mm cubes) half dice (9.5 by 9.5 by 4.8mm) and ‘Thins’ (9.5 by 9.5 by 2mm), and this product features prominently in most dehydrators’ production programmes. Apart from catering outlets, potato pieces figure prominently in soups and vegetable mixtures, and there is an off-take by the bakery trade for meat and vegetable pie manufacturing. The dehydrated product is very uniform in quality, and offers the manufacturer of composite food products some advantages over fresh potatoes, especially in the preparation stages. Varieties of Potato Suitable for Dicing There is a fairly wide choice open to the processor and, indeed, the latter may have to spread his production programme to take in the second early varieties, as well as the main crop varieties. The important requirements in potatoes for dicing are: (1) Good solids content (2) White fleshed, although the preference on the Continent of Europe is for cream or yellow flesh (3) Free from after-cooking darkening (4) Low reducing sugars, (under 2 percent) (5) Firm texture after reconstitution, with the dice retaining their shape after cooking. I16 To meet the latter requirement, some of the varieties with a high starch content, which are ideally suited for granules and flake production, are not so suitable for dicing because of their tendency to 'fall' or mush in cooking. Whilst high solids are economically desirable, a compromise may have to be made by selecting a variety which will not disintegrate in cooking but which will have a solids content of at least 20 percent in a normal growing season. Matching this potato against a variety with, say, 22 percent solids, the processor will lose about lkg of dehydrated product for each 5Okg of peeled potatoes through the production line but, if the texture stability is right, this compromise is worthwhile in the interests of improved product quality. Varieties, which are borderline in this respect, can be firmed up in processing by the addition of calcium chloride in the blanching process, and it is good practice to use this technique for all potatoes used for dicing, in varying degree, according to their natural texture and tendency, to ensure they keep their shape during cooking. The exact level of calcium in the blancher will be determined, therefore, in the light of experience of the stability of the finished product. Potatoes for dicing should be large and uniform. Small tubers are wasteful, in that they do not cut economically into dice, and they give a lot of bits and off-cuts, which, when dried, have a low sales value. The following varieties are used commercially for this product: USA Early varieties: Irish Cobbler, Chippewa. Main crop: Kennebec, Katahdin Russet Burbank. UK: Early varieties: Arran Comet, Arran Pilot, Home Guard. Maris Peer Maris Piper Pentland Squire Cara Pentland Dell Pentland Crown. Second Early: Estima, Pentland Javelin Main crop: Wilja I17 Process After washing and destoning, potatoes should be well peeled by lye or steam. If lye is used, an immersion time of 3 min at 92"C, and a sodium hydroxide concentration of 15-18 percent should give adequate msults but, again, times and concentrations will have to be varied according to the time of the season and the varieties. Stored potatoes are usually more difficult to peel than those freshly lifted. Peeling losses will range from 20 to 30 percent. Peeling is followed by washing under a powerful water spray in a brush washer or reel washer. The former method is preferred by some processors, as the brush action is particularly effective in scouring and cleaning growth cracks and other excrescences softened by the lye h-eatment. Alternatively flash steam peeling may be used, and this is now usually preferred. The peeled potatoes pass from the washer on to trimming and inspection belts. Inspection must be very thorough here, and particular watch must be kept for grit, or foreign matter, which may have escaped the precleaning operation, as, if these enter the cutters, considerable damage will be done to the dicing knives, which are quite expensive to replace. With good quality raw material, well peeled, each person on the trimming belt should be able to handle 170-180kg per hour. The trimmed potatoes are conveyed to the cutters. A standard size of dice for dehydration is 9.5mm cube but this can be varied by changing the first, or slicing, knife position to give a thinner initial cut (down to 2mm) or the spacing of the stripping and the dicing knives can be altered to change the other two dimensions. It must be remembered, however, that reducing the thickness of the cut may create a dense 'bed' condition in the dryer, necessitating a lighter loading per square foot of drying area. A 9.5mm cube, therefore, is the ideal size for optimum throughput, as this particle size forms a satisfactory permeable bed in the dryer with the minimum restriction to the air flow through the product. The dicer knives must be kept very sharp at all times and it is good practice to change these once per eight hour shift. They should then be honed and reassembled on the carrier block Eady for the next change. The dicers should be lubricated by a flow of water, through the feed hopper, to assist with the removal of starch from the cut surfaces of each potato. Blanching is carried out either in a steam or a hot water blancher. As sulphite is invariably added to the product, steam blanching will need to be followed by a sulphite spray. 3-6 min in flowing steam at 99°C should be an adequate steam blanch for potato dice, and this is followed by sulphiting to a level of 250 ppm in the final product. As, however, most dehydrators use additional additives, such as I18 calcium chloride, citric acid or sodium pyrophosphate, it is perhaps simpler to use a hot water blanch, whereby the additives can be drip-fed from an auxiliary tank, at whatever rate is required. Hot water blanchers should always be heated by a closed steam coil system, with live steam injection used only for boosting the blanching water to operating temperature. The liquor should not be recirculated through the additive make-up pans but allowed to over-flow to waste to avoid excessive foaming created by released starch. The blancher water should be changed at least once a day - depending on build up of excess starch. The adequacy of the blanch is constantly checked by the peroxidase test. One of the purposes of blanching is to inactivate the surface enzymes of the cut vegetable and, by using an indicator solution of Guaiacol and hydrogen peroxide on a sample of blanched vegetable, this can be gauged and, as long as negative peroxidase conditions are shown, blancher dwell times and temperatures are adequate. If the blanched dice discolour, turning brown when immersed in the Guaiacol solution for less than one minute, it is an indication of positive peroxidase, arising from underblanching. If no colour contrast develops within the minute, then peroxidase is negative and blanching conditions are adequate. Underblanched potato shows up immediately the product dries, and this is manifested by the white, chalky appearance of the dice half-way through the drying cycle. On leaving the blancher, the dice should be sprayed with fine jets of cold water, again for the purpose of removing starch, then dewatered. DRYING Suitable dryers for potato dice are (a) the tunnel dryer @) the through-conveyor band (c) stove or through-flow tray dryer. With all types of dryer, finishing is usually carried out in bin dryers. Drying temperatures and times will vary according to the type of dryer used but typical 'wet' inlet temperatures for an efficient 3-heat zone conveyor band dryer would be: Zone1 107°C Zone2 99°C Zone3 85°C Outlet temperature would be 65°C - 71°C with efficient evaporative conditions. Finishing temperatures in bins range from 57'C to 60°C. Sizing and Selection After drying, the dice are conveyed to a vibratory screen with an adequate sieving area and the 9.5mm dice or half dice are usually screened I19 out, standing on a 4.5mm or 6mm hole perforated sieve. The ‘throughs’ are the smaller cut-off pieces, which are diverted for use in soups or vegetable mixes where a very quickly reconstituted product is required. With careful attention to the dicers, however, this fraction only represents about 5 - 10 percent of the pack-off material. The main product, after screening, passes to inspection belts for manual selection and removal of blemished pieces, which may have escaped the earlier trimming operation. Or, as is more common practice today, the dice are electronically sorted by colour sorters. These machines have reached a very high standard of performance in recent years, and one machine has the capability of sorting as much weight of product as that sorted by eight to ten women in a given time. The product should pass under and over magnets before packing. Ratio The expected ratio from raw material with 20 percent solids content would be 7.5:l to 8:l. POTATO STARCH Starch is not a primary product of the dehydrator, as its production from ware grade potatoes is not by any means economically viable. Some simple means may have to be resorted to, however, to recover and process starch in a plant handling large quantities of potatoes for dehydration, solely for the purpose of relieving a critical effluent problem. The release of large quantities of starch into factory effluent systems can give rise to expensive treatment bills, and one method of dealing with this is to separate out the free starch in settling tanks, and to contrive some inexpensive means of drying it. The slurry can be centrifuged to dewater it, and from that point drying can be effected by any one of several types of dryer, which may be available, but the economics will preclude this product taking any priority in the overall dehydration programme. It is convenient if any waste heat in the factory can be ducted into a simple home-engineered dryer to cope with this problem. Potato starch production is, in itself, a highly specialised industry, and is only possible in areas where large tonnages of potatoes can be diverted, by giving subsidies to growers, for this purpose. Several starch plants operate in America, Germany and Holland, producing a wide range of high quality specialised grades for the paper, textile, adhesive and food industries. It is not an exercise, however, to be recommended to the dehydrator, other than as a means to an end, and that is invariably for the purpose of reducing effluent disposal problems. I 20