M ICROWAVE P ROCESSING

The application of microwave energy to heat foods was patented in 1945, while the first commercial ovens were introduced in 1955.⁴ Microwave processing is based on the use of a portion of the electromagneticspectrum. The frequencies commonly used for microwave heating are 915 and 2450 MHz, with wavelengths of 32.8 and 12.25 cm, respectively.³ The final product temperatures attained depend on the energy input and usually are not higher than 100°C.² Most of the presently available appliances operate at 2450 MHz, which produces a single surface peak. Use of 915 MHz provides a more consistent product heating, as this frequency produces two peaks, one on the surface and one at the center. Use of microwaves in commercial meat processing is very limited and is exclusively used in the processing of ready-to-eat bacon.

Advantages of microwave heating include rapidity, wide degree of selectivity, ease of control, and lower energy usage.^3,4 On the contrary, there are limitations, which include limited capacity that depends on the quantity of the load, excessive steaming (produces sogginess in some products), focal heating in warmed areas of the product (affects uniformity), limitations in the containing materials of products (cannot utilize metal containers), and limited applicability with respect to browning.³ Modifications or combinations with other treatments have been used to address these issues.

The effects of microwave processing on the destruction of microorganisms, especially pathogenic bacteria, have been studied extensively.^4–6 While destruction TABLE 6.1

Condition 5 43 38 72 Auto

Predry 5 50 0 — Auto

Smoke 30 50 0 — Closed On

Color set 10 65 0 — Auto

Cook 15 75 60 47 Auto

Finish To 71°C core 80 72 70 Auto

Source: Courtesy of Robert Hanson, Alkar, Lodi, WI.

Thermal Processing of Meat Products 159 of food-borne pathogens by microwaves is believed to be primarily due to thermal effects, research indicates that nonthermal effectscan also cause inactivation of microorganisms. Doores⁴ provides a more thorough treatise on the destruction of microorganisms by microwaves.

6.2.4 COMBINATION THERMAL PROCESSING

Several commercial processes combine dry and moist thermal methodologies to achieve particular characteristics in meat products. For example, in the production of frankfurters, initial stages of cooking are characteristic of dry thermal process-ing, followed by steps where steam is injected to accelerate the cooking process.

Other products may use dry heat to develop particular flavors, followed by a moist stage that finishes the cooking to a desired final temperature to destroy microorganisms.

Thermal processing equipment used for meat product manufacture can be broadly divided into two basic categories: batch and continuous. In batch cooking systems, products are loaded into the oven, cooked, and unloaded as a single batch (Figure 6.1). Components of a batch oven are depicted in Figure 6.2 and basically consist of heating and cooling systems, air circulation systems, temperature and humidity control systems, and dampers to ensure proper distribution of the heat-ing/cooling medium or air within the oven when loaded (Figure 6.3). Most of the batch ovens have the capability to chill the product using chilled water or brine systems, and the product is subsequently moved to refrigerated rooms to further chill the products. Capacities of these ovens range widely from 150 to 25,000 kg,

FIGURE 6.1 Large-batch meat processing oven. (From Alkar-RapidPak, Inc. With permission.)

160 Thermal Food Processing: New Technologies and Quality Issues

based on the production capacity required and type of product processed. In con-tinuous cooking systems, the cooking and cooling functions are integrated into a single unit with multiple zones (Figure 6.4). The products are loaded onto conveying systems and are moved through either one or more cooking zones, and then through the cooling zone. The products are normally conveyed by chains, walking beams, or belt conveyors.

FIGURE 6.2 Typical components of a batch oven. (From Alkar-RapidPak, Inc. With permission.)

FIGURE 6.3 Oscillating airflow in a forced-air batch oven. (From Alkar-RapidPak, Inc.

With permission.)

Fresh Air Intake Dampers Gas Burner

Main Air Supply Fan

Humidity Valve Exhaust Fan

Exhaust Damper

Rotating Dampers

Cleaning Pump

Thermal Processing of Meat Products 161

Continuous cooking equipment is commercially available and widely used to achieve higher yields, uniformity, enhanced safety, and longer shelf life of products.³ The continuous ovens utilize different technologies to expose meat products to the heat. The four basic types are natural convection oven, forced-air convection oven, counterflow oven, and impingement oven. Natural convection ovens rely on expo-sure to a heat source with unforced air movement, very common in home appli-ances, but sparingly used in commercial operations. Forced-air convection ovens feature internal fans to accelerate the cooking rate and ensure uniformity of cooking.

Counterflow ovens are characterized by a continuous conveyor that moves the cold product toward a heat source at the opposite end, accompanied by a counterflow air movement that accelerates the heat transfer process. Impingement ovens feature high air velocities from nozzles directed at the surface of the product that accelerate the cooking process (Figure 6.5).³ The efficacy of continuous ovens has been studied extensively in microbiological challenge studies with meat and poultry products.^7–15 6.3 EFFECTS OF THERMAL PROCESSING

With very few exceptions (dried meat, fermented sausage, rare parts of steaks, etc.), most meat products are rethermalized at a certain point before consumption by either the processor or the consumer.² Thermal processing imparts several FIGURE 6.4 Straight-line chain conveyor — continuous frankfurter cook/chill. Linked sau-sages are looped onto sticks and then the sticks are manually placed on a dual-chain conveyor.

The sticks span the gap between the two chains. (From Alkar-RapidPak, Inc. With permission.)

162 Thermal Food Processing: New Technologies and Quality Issues

characteristic properties to meat products that will not be possible otherwise. Some of the advantages include palatability, color development, tenderization, and added value to the finished product.¹⁶ Due to the variability of processes, products, and regimes used in the industry, the effects caused by heat on meat are very diverse.

However, the common effects in meat products are discussed below.³ 6.3.1 EFFECTS IN PROTEINS

Meat muscles freed from adhering fat contain on average 21.5% nitrogenous material, the majority of which are proteins.¹⁷ Thermal processing of meat prod-ucts affects the structural characteristic of myofibrillar proteins and enzymes in the muscle. Several degrees of changes are observed, depending on the intensity of the heat treatment applied.

6.3.1.1 Denaturation

Thermal processing of meats causes coagulation of the proteins on the surface of the muscle, followed by protein denaturation that results in structural changes, thus affecting their solubility in the meat system. Proteins are usually unfolded from quaternary structures and lose their native conformation. Despite the dena-turation of proteins during heating, these changes have little detrimental effect on the nutritional value of the meat, with the exception of overheated products that may be affected by the degradation of the amino acid lysine.² Initially, meat juices are solidified due to the denaturation and solubility changes in soluble proteins. Subsequently, changes in surface muscle fibers result in surface meat color changes. These changes are observed in cured as well as noncured products.

Specific changes in meat proteins vary by the type of muscle fiber and temperature and are summarized in Table 6.2.

Structural changes are very important in comminuted products because during the formation of meat emulsions, fat is surrounded by proteins that are stabilized FIGURE 6.5 Air impingement steam cooking system. (From Alkar-RapidPak, Inc. With permission.)

Thermal Processing of Meat Products 163

by the heat treatment through coagulation (product binding), providing a homo-geneous product. Additionally, the denaturation/coagulation and reduced moisture on the surface are responsible for the skin formation of some of these products.

6.3.1.2 Color Changes

Heat treatment of muscles causes color changes in the protein characterized by a change from red to brown or gray in noncured products and stabilizes the characteristic red/pink color in cured meats. Cured products do not change to brown or gray as nitrite reacts with the muscle pigments to produce a stable pink

TABLE 6.2

Classification of Some Processed, Ready-to-Eat (RTE) Meat and Poultry Products

Dried Products

Basturma, pastirma, basturmi, beef sticks, carne seca, dried beef, dry duck breast, meat/poultry jerky Salt-Cured Products^a

Cappicola, coppa, country ham, dry cured duck, parma ham, prosciutto, prosciutti Fermented Products^b

Alessandri (dry sausage), apenino (dry sausage), Arles or D’Arles (dry sausage), blockwurst (semidry sausage), cacciatore/cacciatora (dry sausage), cervelat, soft cervelat, chorizo, Lebanon bologna, pepperoni, salami, soft salami (Genoa, Italian, German, summer sausage, thuringer, soft thuringer) Cooked or Otherwise Processed Whole or Comminuted Products

Meat: Berliner (cooked, smoked sausage), bologna, cooked bratwurst, Braunschweiger/liver sausage, breakfast link sausage or patties, brown-and-serve sausage, burritos, cheese smokies, cheesefurter, cheesewurst/cheddarwurst, chili, chorizo, cooked beef, cooked ham, cooked pork in BBQ sauce, cotto salami, Fleischkaese (cured, cooked sausage), frankfurters, gyros, meat loaf, meat salads, frozen meat soups, Nem-Chua (cooked, pickled ham with shredded pork skin), pasta with meat sauce, pastrami, pickled pig’s feet in vinegar, pickled sausages/meat in vinegar, piroshki, pork barbecue, pork sausage patties, ravioli, roast beef, roast pork, souse, stews, white hots, wieners

Poultry (includes products containing any amount of poultry)

Chicken burritos, chicken BBQ, chicken bologna, chicken breast, chicken franks, cooked poultry, cooked poultry rolls, corn chowder with chicken, poultry loaf, poultry patties, poultry rolls, frozen poultry soups, turkey BBQ, turkey franks

Thermally Processed, Commercially Sterile Products

Canned spaghetti with meatballs, canned corned beef hash, canned ham, canned chicken salad, canned soups with meat or poultry

aMajority of these products do not undergo any thermal process.

bMost of these products originated in Europe, and as processed in Europe, they do not receive any heat treatment. However, the U.S. versions of many of these products receive a mild heat treatment.

164 Thermal Food Processing: New Technologies and Quality Issues color.^2,3 Overheating causes the formation of dark colors due to dehydration. The amine groups of the amino acids (lysine and alanine) that make up muscle proteins react with the available reducing sugars, such as glucose, and undergo a Maillard browning reaction.¹⁸

6.3.1.3 Improved Palatability

Cooking of meat to temperatures exceeding 70°C intensifies the flavor of meat and changes the blood-like or serumy taste of fresh meat to the pronounced cooked flavor and aroma.³ Although flavor and aroma are dependent on the species, cooking method, spices used, meat aging, amount and kind of fat, as well as feeding regime, heat processing enhances these flavors, improving the product acceptability. Addi-tionally, meat is made more tender, especially nonprime meat cuts, due to the softening of connective tissue.²

6.3.1.4 Inactivation of Proteolytic Enzymes

Normally, enzymatic activity is relatively slow compared with microbial degra-dation due to bacteria. However, it has been noticed that in irradiated products, usually free, or with reduced microbial levels, proteolytic changes occur, causing flavor (bitterness) and color changes in meat products, accompanied by the formation of tyrosine crystals.³ A scalding process reaching 55 to 60°C may be sufficient to inactivate these enzymes and reduce this problem.

6.3.2 EFFECTS IN FATS

The fat content in meat is highly variable and dependent on the amount of fat removed from the muscle during preparation of the meat cut. On average, the fat contained in muscles freed from adhering fat is only 1.5%.¹⁷

6.3.2.1 Solubility

Thermal processing of meat products causes the fat to melt. While the melting temperatures generally are in the range of 37 to 40°C, the melting point of fats within each animal species depends on the feed type and the proportion of satu-rated:unsaturated fats in the animal feed. The released soluble fat escapes from the product mixture (muscle or comminuted product) at low temperatures unless held in an effective matrix. Therefore, in comminuted products, coagulation of the protein matrix is critical to retain the fat during thermal processing. Fatty tissues are heat tolerant up to 130 to 180°C; however, some adipose cells may burst in the process.²

6.3.2.2 Flavor Changes

The characteristic flavors of different meat species appear to reside mainly in the respective fats. Because of this, the older the animal, the stronger the flavor is due to changes in the oxidation levels of fats. Thermal processing triggers the development

Thermal Processing of Meat Products 165 of pleasant flavors and organoleptic enhancements in meat products. Juiciness is increased, allowing flavors from fat to be more readily perceived. The heating of fatty acids in the presence of air enhances oxidation, modifying flavor profiles of cooked products. Some meat components are degraded by hydrolysis, developing enhanced flavor compounds such as glutamic acid and its derivatives. Other flavors are produced by Maillard-type reactions on the surface of meat products (at 150°C).² Finally, the thermal process enhances the perception of other flavors associated with salt, spices, and curing agents added in the formulation.

6.3.2.3 Humidity Changes

Thermal processing causes free water in the muscle to be released and evaporated.

There is a decrease in humidity levels in the surface of the product, causing drying that reduces the water activity. Being exposed to a lower water activity environ-ment reduces bacterial growth of surviving and recontaminating bacteria.³ The extent of water loss depends mainly on the product temperature, cooking time, and environmental conditions of humidity and temperature. There is a large increase in water loss in the temperature range of 50 to 60°C, reaching 80 to 100% of total loss by the time the temperature reaches 80°C.²