DRYING OF MEATS Rate of Dehydration of Uncooked Cured Ground Meats E. C. RITCHELL', EDGAR L. PIRET2,AND H. 0. HALVORSON Division of Chemical Engineering and Department of Bacteriology, University of Minnesota, Minneapolis, Minn.
D
ESPITE the present importance of dehydrated meat products, very little published information regarding the rate of dehydration of such materials under various drying conditions is available. The following data have been selected from experiments carried out over the past few years in the chemical engineering laboratories of the University of Minnesota an the preparation of various types of meat products. These data are principally concerned with the rapid drying in air of uncooked ground pork and beef such aa is usually found in dry sausage meats. Some data are also presented on the drying of relatively lean, precooked ground beef.
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DEHYDRATION OF UNCOOKED CURED MEATS
The dehydration of uncooked meat containing a cure of salt, sugar, nitrates, etc., was investigated. Such meats (for example, , comminuted pork and beef mixture) have long been dehydrated in caamm to uroduce what is known& dry-or summer sausage, This product has been considered an excellent form of dehydrated meat and fhds a large market, However, &s produced by present methods, it is not a great factor in the current dehydration program, partly because production is limited by the length of time required for the drying p r a m s (30 to 90 days). Accordingly, it was believed that a thorough study of the mechanism and rate of drying of such mixtures might result in more rapid prooe&sing while retaining, or improving upon, certain qualities present in dry sausage-for example, established consumer acceptance, good keeping quality, and ability to be eaten directly without rehydration or cooking. Meat mixtures, when intended for use in dry sausage, are usually prepared by comminuting a blend of beef and pork meat, dry cure salts, and spices. After an aging or curing period of a few days at low temperature, the mixture is extruded through a nossle into casings. The m d t i n g sausages are then slowly dried. The initial composition of such mixturea oommonly includes 50 to 55 per cent water and as much as 20 to 30 per aent 1
a
Present addreea, Minnesota Valley Canning Company, Le Bueur, Minn. Preaent address, Minnenota Mining k Manufaoturing Compmv, Et.
Paul, Minn.
fats. The finalmoisture content varies with the type of sausage; an ordinary dry sausage may, for example: contain about 28 to 30 per cent water. In this experimental work no casings were used. Rather the ground mixtures of beef and pork containing cure were dried in various shapes without any covering. Under these conditions much more rapid and uniform drying takes place. Since the meat mixtures are quite plastic and adherent, the samples were forced into various shapes by extrusion. Cross sections of the shapes studied ranged from very small cylinders, which dried rapidly, to larger reotangular sections. The latter were investigated in an effort to make possible more efficient packaging than the large cylindrical casings used for present dry sausage and also to obtain a more coherent product. I n this investigation the presence of fat was found to influence greatly the method of preparation of the meat for drying, as well as the actual drying owration. Such factors as the conditions at which extrusion takes place-that is, meat temperature and amount of surface working-must be considered. The tendency for fat to cover meat particles and thus retard drying is considerable if these conditions are not proper. For the same reason drying operations must be carried out at relatively low temperatures, the range 50-75" F. being investigated in this work. These temperatures are also necessary to minimize microbiological and enzymatic action. The processes used in dehydrating precooked meat were disoussed by Kraybill (8). A common method for beef involves comae cutting, cooking in a steam-jacketed kettle, and fine grinding. At this stage the material differs considerably from the uncooked type. It is easily disintegrated into granular particles presenting a large specific surface. Drying air temperatures are usually in the 140-170" F. range with the meat temperature between 98' and 150' F. Preoooked beef may be dehydrated rapidly, especially when the meats are lean; however, a high fat content causes noticeable interference with the drying rate. In drying precooked pork, this factor becomes more troublesome despite the removal of fat in preliminary operations. The drying of precooked meat is carried out to a find moisture oontent of 10 per cent or less. For comparieon some data are
An investigation has been made of the air drying under various conditions of samples of uncooked cured meat to form products having properties similar to dry, or summer, sausage. The rate of drying is materially affected by the handling operations and temperature conditions, which may cause the fat to film over the material. As contrasted to the usual &y sausage manufacturing operations, shorter times of drying are necessary to obtain a product containing 25-30 per cent moisture. This is particularly true when the material is dried in the form of small cylinders. The products obtained are similar in color and palatability to the usual dry sausage sold on the market. The small particles, however, do not adhere to form a solid hard mass. If larger samples-for example, 4 X 11/8 inches in cross section-are dried, solid firm products are formed in about 15 days. Such observations as could be made over the period of a year indicate that the keeping qualities and bactericidal properties of the dried products are decidedly favorable.
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Some results are given on the keeping qualiticq of vacuum-tinnod uncooked 8e.mples 01 dried cured meats. These t a t s show B decrease in bacterial oount with time, the absence 01 rancidity, and the sbeence of mold growth in the product. The last is often troublesome with ordinary dry %a"S;tge.
An additional possibility, whiah was not fully investigated, was to reduoe the concentration 01 erne in the rapidly dried meat fiom the rather high m o u n t present in ordinary dry saamge. The latter p r d uct is often considered t o be highly seasoned by the consumer, and % product with B lower level of cure could be attractive. It is believed likely that such large amounts of cure are unnecemary t o preserve cured meat when it is in the final dehydrated st,qe. However, the regulnr o m is neeemaw for Dreservation durine B h a d N ing process, pnrticulsrly when the meat msy be rather wet for several weeks in the early stages of drying. It ahould be possible, &T B consequence of a great reduotion indrying time, t o have greater freedom in the choice of cure.
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Figure 1.
Experimretwl Dryer
presented in this paper on tha dryirig ,,ipreaookad hecf. In the e&se of uncooked cured meat. auit:hli. kcepinp qualities %re obtnined by drying L a a. f i n d moisture content of 25WO per cent. As a C O I ~ R ~ U ~ I I C Conly , the latter products R T ~r e d l y ediiile n-itliout being rrhydrnted. Tbe dried pr0duct.s d>tniord i n this investigstion were aimiiai in rolor rind tiutc t o t.hose obtained in the usual proceau lor dry s;tnsago mnnufaeturr. The ndhercnt nature of the individual pitrticles aiso eqLuair tillit of dry snusago. If, however. very small exfrwions having B l:rrge specific surfwe wc dried rapidly, it is not readily possible tc, rogrind m d tlm iecomhine them to form n In?gc!r ahajxt possessing adberenee. Such products caitnut be diced but e m he spread. Fiwtber work mily lead t o IL method 01 ubtnining tire former property. The principle 01 recombining melat aitm grinding i8 of portance, iiinee it offers B s:bi tion of small, rapidly dried sirrapes. Uii: ot the lirtter is advantageous since the drying time increases enoimousiy with an ineresse in cross seetiun. For examplo, '/s-inchdiameter cylinders may be dried t o 28 per eont moisture in nhout 10 hours, while I x 2 inch sI& requiio I t days. KEEPING QUALITY O F UNCOOKED CURED MEATS I.
llie keeping qualities 01 dry, uncooked, cured meat may be ostirnated on the basis of the many y e w s of experience tho indnstry has had in the msnufiicture and handling of ~ u m m o n dry sausage. Like the latter, it is not n sterile pmduct but shows similar bi~ctuiulo~icill jruyulntiur*. Storage temperatures should probddy equal those employed fur dry sausage with the same moisture content. I1 t,hc product is canned and pressure-cooked to obtain the oomplete steriiiiv required for universal military use, most of its attractive qualities me lost.
Various Methods of Plotting Data
Figure 2.
SampleNo.
% H.0
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% Fat
'OY fO6? 53.$ 55.1 19.0 24.0 Dipbulb tcmpraturr. 16' C.; reletirs humid.*)., 3Q%t roloclty. 5 fr./sae.
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November, 1943
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TEMPERATURES =-BULB 0-79QE O -6IOF:
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DRY-BULB TEMPERATURES 0-97O IF
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HUMIDITY-0.005 LB./LB. VELOCITY-7 FT./SEC. INITIAL MEAT TEMP.- 41° F:
HUMIDITY -0,005
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DRYING
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Effect of Air Temperature on Drying Rate
EXPERIMENTAL PROCEDURE
Experiments were conducted in a test dryer (Figure 1) ca able of producing a constant stream of air of variable and controiable temperature, humidity, and air velocity. This ap aratus was It condescribed in detail by Ritchell, Piret, and Mann sisted essentially of an insulated sheet-metal duct system fitted with an adjustable speed fan. A contracting section placed immediately ahead of the drying chamber provided a uniform air flow through the drying chamber in which the meat samples were placed on glass plates or screen trays. The air flow was usually parallel to the surface of the material, although throu h-circulation could be obtained. Temperature and humidity a4ustments were made by variation of dam ers'which allowed a portion of the air to by-pms a refrigerate3 coil. Manual and automatic oontrol of resistance heaters and steam 'ets then allowed reheating and rehumidifying of the air to desired dry- and wetbulb temperatures (accuracy of control waa ~ 0 . 1 "(3,). For higher temperature drying, the refrigeration system was not used. The cooling and dehumidifying load waa taken up by admission of outeide air. The meat samples used in the experiments'on uncooked cured meat were taken at various times over a eriod of two months from several large commercial production tatchea of meat mixture being prepared for the manufacture of salami sausa e. These batches had been prepared in a rotary cutter in which &e cure salts used for sausage manufacture had also been added. Following the cuatomary commercial procedure, all sam les were aged or cured by storing for two or three da s a t 4 0 4 5 " before use. Previous to drying, the meat was &ought to a definite temperature. This temperature was important in order to control during sample shapin operations, the spreading of a film of !at on the. meat particks; as pointed out reviously, this procedure decidedly affected the rattf of drying. $or preliminary experiments it was found convenient to prepare samples b repidly.httening a weighed quantity of meat (usually 75 gramsy on a bmch square glass plate. Other shapes on which data are
8).
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HOURS
DRYING
TIME
Figure 4. Effect of Extreme Air Temperatures on Drying Rate
given were extruded through nozzles under pressure to form shapes of desired cross section. A few typical curves for precooked meat are given. The meat in this cme was chopped into approximately 1/2-inch cubes and cooked with a small amount of added water in a steam-jacketed kettle (30 minutes at 212' F.). The meat was slowly stirred during the cooking operation. After regrinding through a inch plate, the matenal was dried in the apparatus mentioned above. The requisite amount of salt for keepin qualities (3.5 per cent in the final product) was added before cooeking.
METHOD OF CORRELATION. Correlation of the data derived from samples on glass plates was difficult because of the variable nature of the material dried. As a result of the long time required for the experiments and the undesirability of long storage periods, it was necessary to use samples from different large commercial batches of meat mixture. These samples varied sufficiently in moisture and fat content to render unsuitable the usual chemical engineering methods of plotting drying rate or drying time against residual moisture content. For example, when two samples having 53.4 and 55.7 per cent initial moisture were dried under identical conditions, the plots of drying data gave two rather widely separated curves (I and 11, Figure 2). However, in plotting cumulative moisture ioss against time, the points fell more nearly on a single line, showing that drying took place at nearly the same rate for equal times of drying (111, Figure 2). The data plotted aa drying rate (pounds per hour per square foot of surface) against drying time (hours), also fell on a common line, justifying the choice of these coordinates in correlating the data (IV, Figure 2). Figure 2 also includes a graph (V) of drying rate against total moisture on the fat-free basis
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(pounds water per pound dry fat-free meat). This plot was unsuccessful in correlating the data, but was attempted because analyses showed that the fat content increased roughly in inverse proportion to the water content. On the basis of these and similar results, in order to illustrate as clearly as possible the effectof the variables studied, the data are presented in this paper as plots of drying rate against time. In a few instances where samples having the same initial moisture content could be obtained, plots of total moisture against time were made since they can be used more directly,
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EFFECT OF TEMPERATURE AND SURFACE CONDITIONS ON DRYING RATE
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