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able over a wide temperature range, specifically between 60 and 90°F. To do this, hydrogenated or hard fats are added to formulations to extend the plastic range. Solid shortening actually is a suspension of high-melting-point fats in liquid fat or oil. Different fats will melt or solidify as is shown in the solid fats index (SFI) charted in Table 3. At room temperatures of 68°F (20°C), the typical shortening is only about 30% solid.

Two methods commonly are used to achieve desired crystal structure, size, and dispersion when producing plasticized shortening. One is for full-scale production (Fig. 22); the other, for limited "in house" applications.

For normal production, an A or scraped-surface heat exchanger is used to cool oil from 140°F (60°C) (no solids) to 60-70°F (16-21°C). This subcooling is done rapidly, and only about 5% of the fats are crystallized. The shortening then enters a B or pinworker unit (Fig. 23), which acts as an agitated holding tube of about 3-min duration. During this time, the remaining crystals form with the latent heat of crystallization causing the shortening to heat up. Typically, if the shortening enters at 65°F (18°C), it will exit at 80°F (27°C). The now plasticized shortening is ready for filling into 50-lb cubes. Most plasticizing systems usually include a shell-and-tube exchanger to precool product from 140°F (60°C) to 120°F (49°C) and reduce the heat load on the SSHE. Nitrogen also is usually added before the SSHE to give the final product a white color.

For bakeries that plasticize shortening for in-house use, a single unit combining the features of A and B machines commonly is used. As shown in Figure 24, this involves an open type dasher shaft. The dasher is hollow and has wide slots at regular intervals. A set of beater bars acts as the pinworker, holding and working the shortening to a high degree in order to produce a soft, readily flowable texture that can be handled in pump lines and tanks downstream.

Marshmallow

Marshmallow is an aerated food product typically formulated from 45% sucrose, 27% corn syrup, 23% water, gelatin, invert sugar, and flavoring. (See Fig. 25.)

As shown in the flow schematic, a production system generally employs multiple tanks, an aerating mixer, and a scraped-surface heat exchanger. A premix tank first is used to blend and heat sugars to 245°F before the solution is transferred to another blending tank for cooling to 155°F. Gelatin and 150°F tempered water, meanwhile, are thoroughly mixed by means of a high shear agitator and blended with the sugar solution. The resulting syrup is charged to a surge tank and pumped as required to a high shear aerating mixer where it is beaten to a weight of between 35 and 48 oz/gal (specific gravity of 0.26-0.36). The 155°F marshmallow mix then flows to an SSHE for chilled water cooling to 90°F.

Marshmallow is a difficult duty for a scraped-surface heat exchanger because of the extremely high viscosity and the low heat-transfer properties of the product. Operating pressures in the mixer and heat-exchanger range from 150 to 300 psi to handle product viscosity and to force the marshmallow through an extrusion head to a belt where it is shaped and dusted with starch. The low product density results in a low U value and because of viscosity, a high torque is required to turn the dasher. Note in the Figure 19 dasher horsepower—rpm chart how dramatically the dasher hp changes relative speed. Too high an rpm results in excessive motor load that makes cooling more difficult because the motor heat has to be removed by the heat exchanger. Since even the optimum dasher speed results in about one half of the cooling load coming from the motor input, selecting the proper dasher speed for this application is very important. Care also must be given to the selection of shaft seals. Since marshmallow is mostly sugar, seals with abrasive resistant, high hardness qualities should be used and the seals should be water flushed to prevent crystals from building up on the seal faces.

Cooking-Cooling Ground Meat

While ground beef generally is cooked in steam-jacketed kettles for moderate size production runs, the tendency is to install scraped-surface heat exchangers for both cooking and cooling duties when production requirements exceed 10,000 lb/day (Fig. 26). Not only does the SSHE provide faster cooking; it also improves product yield since moisture and fat loss is far less than the 15-20% decline in product volume that occurs with the open-kettle method (See Fig. 27.)

The typical meat cooking system consists of a single auger feed hopper with a leveling ribbon followed in line by a two-cylinder SSHE, one cylinder with 275°F steam and the other with 0°F ammonia. Both the auger feeder and SSHE are hydraulically driven from a central power source. After meat is ground, spiced, and blended, it is fed to the heat exchanger by means of a single-discharge auger designed specifically to move viscous, sticky products directly into a rotary pump. The auger maintains a constant stuffing pressure at the rotary pump inlet by varying the operating speed while a leveling ribbon maintains a uniform meat level in the feed hopper. The scraped surface blade and dasher design promote rapid removal of product from the cylinder walls and enhance agitation and mixing during both cooking and cooling phases. Assembly and disassembly for blade inspection and/or replacement is quick and easy. Meat remaining in the auger feed pump, pipelines, or the scraped surface heat exchanger is easily recovered and, once clear of product, the closed system and piping are easily cleaned with a pump circulation loop.

Products with Particulates

When processing sauces, gravies, or juices containing pieces of meat, vegetables, or fruit, one of the key elements is maintaining product identity by reducing or eliminating particulate breakage. This becomes more of a concern if the particulates exceed 1/4 in. in size and the carrier is a thin liquid such as broth or soup. Thick sauces, on the other hand, seem to have a cushioning effect with less product damage occurring during passage through the system.

While the dominant type of heat exchanger used for particulate-type products is the scraped-surface unit (Fig. 28), each case still must be considered individually. Many processes require multiple SSHE cylinders that should be piped in series rather than in parallel. This eliminates the need for individual product pumps while ensuring higher heat-transfer rates and an equal flow to each cylinder.

To reduce particle breakage, three design areas must be considered—the gap between the dasher and scraper blade, the sizing of SSHE inlets and outlets, and the dasher operating speed.

The distance between the underside of the blade and the dasher shaft (Fig. 29) should be equal or greater than the largest particle to be processed. This allows passage without particle damage as the blade sweeps by. The disadvantage of a wide annular space between the dasher and the cylinder wall is a longer product retention period and a reduction in exchanger efficiency. The faster the product travels through the cylinder, the higher the U value attained. This results in improved exchanger performance with lower residence time and less product damage.

The sizing of inlets and outlets is directly related to product quality. Using the largest possible ports reduces not only pressure drop through the system but also shearing effect. Furthermore, passage of product is eased and the possibility of contact with inlet walls is minimized.

Finally, there is the matter of dasher speed. As shown by the curve in Figure 30, while higher dasher speed results in a higher U value, it also can cause increased particulate damage. Therefore, higher dasher rpm and the corresponding high heat-transfer efficiency will apply only to products with small particulates. When products contain delicate particulates of 1/2-3/4 in., a variable-speed drive is used to reduce dasher revolutions to about 120 rpm. While this protects product quality, it also lowers exchanger efficiency. Consequently, striking a balance between performance and quality should be determined by lab runs so that optimum operation at full scale becomes a matter of a proper dasher speed setting.

Peanut Butter

For the production of creamy or chunky peanut butter, a process system will consist of surge tanks, a deaerator, scraped-surface heat exchanger, ingredient feeder with inline blender, and transfer pumps.

After mixing roasted nuts with a stabilizer, salt, and sugar to the desired formula, the product is ground and discharged at a temperature of about 150-200°F. Deaera-tion follows to eliminate air pockets, which initiate oil separation. During cooling from an SSHE inlet temperature of 140-190°F to a discharge temperature of 85-95° • F, the stabilizer is solidified in finely divided crystalline form and uniformly distributed throughout the mixture. At the proper temperature, the peanut butter becomes a viscous, extrudable mass. Crystal change continues and further solidification occurs after filling. For chunky style, a chunk feeder is located between the SSHE and filler or the transfer pump and deaerator.

Table 4 gives a listing of other products routinely processed using SSHEs.

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