Plate freezer

In the horizontal plate freezer (Figure 7.9a), the plates are arranged in a stack on slides, so that the intermediate spaces can be opened and closed. Trays, boxes or cartons of the product are loaded between the plates and the stack is closed to give good contact on both sides. When the necessary cooling is complete, the plates are opened and the product removed. 

The vertical plate freezer (Figure 1,9h) is used to form solid blocks of a wet product, typically fish. When frozen solid, the surfaces are thawed and the blocks pushed up and out of the bank. 

Figure 7.9 Plate freezers, (a) Horizontal, (b) Vertical (Courtesy of APV Parafreeze Ltd)...

Most vessels can now freeze their catch at sea, enabling them to stay offshore without the need to run back to a port within the limited life of the chilled product. If the fish is to be cleaned and processed later, it is frozen whole, either by air blast or, more usually, in vertical plate freezers (see Figure 1.9b), followed by frozen storage. Some fishing vessels and the fish factory vessels will carryout cleaning, filleting and other operations on board and then freeze and store the final product. 

Some freezing of fish fillets and other processed fish is carried out between or on freezer plates, in an evaporator assembly similar to that shown in Figure 7.9a. Flat cartons of fish and fish fillets are frozen in these horizontal plate freezers. 

In a plate freezer 2, on a belt in a flow of cold air 3, in trays in cold air 4, partially frozen in a soft-ice machine (continuously scraped cold surface) and finally frozen as in 2. 

It is important at this point to make the distinction between the terms freezing and frozen storage. The former may be regarded as part of the primary manufacturing process and uses, for example, fluidized bed freezers or plate freezers depending upon the nature of the food to be frozen. Frozen storage refers to the storage of food at... 

Plate freezers generally are limited in application to prepackaged products. In this system, the food substance is placed in direct contact with refrigerated metal plates (usually steel or aluminum). Cooling coils are located within the interior of the metal plaies. The required contact refrigeration time ranges from about 30 to 90 minutes, depending upon size and nature of food substance. 

Containerized ice cream is hardened on a stationary or continuous refrigerated plate-contact hardener or by convection air blast as the product is carried on a conveyor or through a tunnel. Air temperatures for hardening are —40 to —50° C. The temperature at the center of the container as well as the storage temperature should be <—26°C. Approximately one-half of the heat is removed at the freezer and the remainder in the hardening process. 

Continuous, plate and air blast freezers for ice-cream Low-temperature brine for lollipop freezing... 

Remove frozen assay plates from freezer and thaw at room temperature. Centrifuge plates at 700 for 1 min to collect siRNA at the bottom of the wells. 

Three to several colonies for each sample are inoculated into the same 600 pL of LB medium containing 50 pg/mL of kanamycin and cultured at 37°C overnight. For glycerol stock, 50 pL of the culture is added to 25 pL of Cell Stock Buffer in a 96-well plate. The remaining suspensions are applied to the plasmid preparation with the Wizard SV 96 Plasmid DNA Purification System see Note 6). One microliter of the resultant plasmid solution is directly applied to agarose gel electrophoresis for estimation of the size of plasmid in a form of covalently closed circular. Similarly, another 1 pL of the plasmid solution is treated with S fi/Pmel and analyzed by agarose gel electrophoresis for an insert size check. This plasmid solution set is the final product and is reserved in a freezer. 

The complete isolation, purification, and characterization of a-lactalbumin as described here require about 9 hours. The preparation of whey is completed in approximately 3 hours and the chromatographic step (Sephadex or affinity) requires another 3 hours. The analysis procedures require 3 hours. The whey fraction may be stored in a freezer for several weeks if desired. As an alternative to isolation, students may be provided commercial a-lactalbumin, which is then further purified by chromatography or analyzed directly by SDS-PAGE. The time for gel electrophoresis can be greatly reduced if commercially prepared gel plates are used. 

The ELISA procedure for the analysis of parathion as described above requires nearly eight hours, although many samples can be simultaneously assayed. However, incubation times can be shortened to one-half hour, in most cases, resulting in only a 10% reduction in sensitivity. Also the polystyrene microtiter plates containing bound RSA-AP can be mass produced and stored in a freezer. Since the enzyme-linked antibody can be purchased, the limiting factor of the applicability of the ELISA procedure, as well as the RIA procedures, for other pesticides is the development of the antiserum to the pesticide. 

Complete 96 well microtitre plates and their contents can also be frozen in a programmable freezer. The medium in the wells should be replaced with freezing medium ( 7.3.1) and the whole plate cooled on ice and taped shut and wrapped in plastic film prior to freezing. 

Equipment. Silver filters of 0.45-pm pore size and 47-mm and 142-mm diameter were obtained from Osmonics, Inc., Minnetonka, MN. Stainless-steel pressure filtration equipment (47-mm and 142-mm plate filtration units) was obtained from Millipore (Bedford, MA). XAD-8 resin was purchased from Supelco, Inc. (Bellefonte, PA) and cation-exchange resin was purchased from Fisher Scientific (Ag-MP-5, Springfield, NJ). A Millipore XX80 peristaltic pump was used in the isolation of humic substances (Bedford, MA), and the freeze drying unit was a FTS Systems Flex Dry freeze drier, an FTS Systems shell freezer, and an FTS model VP62A vacuum pump (Stone Ridge, NY). The specific conductance meter (model 604) was obtained from Amber Science, Inc., Eugene, OR. 

---