face. The final ash is discharged into a cold-water stream and carried to an ash collector.

In the smoldering smoke generator, smoking materials—sawdust or wood chips—are ignited by an electric heating element. The ignition time can be preset. When the set time is expired, the ignition automatically switches off. Air is injected at the bottom by a blower and is heated by the burning sawdusts or wood chips. The heated air is moving upward, and sawdust is supplied from the top, functioning as a countercurrent flow. Because of the heat of the burning sawdust beneath, smoke is generated as fresh sawdust travels from top to bottom. Smoke produced with the smoldering principle often has high air velocity, leading to excessively high smoldering temperature. The smoke-generation temperature can be lowered by reducing the air supply and slightly moistening the sawdust or wood chips to a moisture content of 30%.

The friction smoke generator produces smoke by pressing a piece of wood log tightly against a spinning disc. Heat is produced by fierce friction. This results in pyrolysis of the wood. Friction smoke generators produce less amounts of smoke than does the sawdust burning type of smoke generator. Another method used for smoke generation is the superheated steam method. The low-pressure steam is mixed with air and conducted to the superheater. The temperature of the steam/air mixture can be adjusted within the prescribed range of 243-400°C (470-750°F). The amount of air can be varied as required and can thus have a varying effect on smoke production. A screw conveyor brings sawdust and superheated steam/air mixture into a smoke-generating chamber to induce pyrolysis. Among these smoke-generation methods, the hot-plate smoke generator and the smoldering smoke generators are most popular in the United States.

Smoke Deposition

Smoke deposition on a product involves several physical and chemical processes linked closely with one another. An important chemical process is the carbonyl-amino reaction, which produces a desired golden brown color. As for the physical process, the dominant mechanism of the smoking process has been identified as vapor absorption. During the normal smoking process, vapor smoke and the liquified smoke particulates coexist in an equilibrium condition. The smoke particulates function as a reservoir of smoke constituents. When the equilibrium between smoke vapor and particulates is changed by dilution of air because of absorption from the product or as a result of increased smoke temperature, the reservoir releases a part of its contents to maintain the new equilibrium condition. Because the smoking process is an absorption, the major physical parameters effecting the rate of smoking deposition are smoke density, air velocity, relative humidity, and the surface condition of products. It is clear that a wet surface absorbs smoke faster than a dry surface and that smoke absorption should follow the first order of kinetics. The higher the concentration gradient between the ambient and surface, the faster the absorption rate. The rate of smoke deposition is also faster at high air velocities, although it is difficult to maintain high smoke density and high air velocity simultaneously. Smoke density and air velocity should be optimized according to smoke-house pressure as well as the exhaust system.

The smoking process based on air temperature can be classified as cold smoking, warm smoking, and hot smoking. The temperature ranges are below 27°C (80°F), between 22 and 54°C (80-130°F), and above 54°C (130°F), respectively. The relative humidity during cold smoking is not as critical as during hot smoking because of the low drying rate. In the hot-smoking process, the relative humidity should be precisely controlled to keep the product surface damp. The optimum relative humidity (RH) in a smokehouse for hot smoking is 60% RH and 71.1°C (160°F) (21).

Smoking of meat products results in altering the protein composition, such as lowering myofibrillar and sarcoplasmic protein nitrogens while increasing stromal protein nitrogen. These protein changes result in a cross-linking of surface protein or tough skin (22).

Liquid Smoke

To find a way of better controlling smoke reaction, decrease the chance of carcinogenesis, reduce smoke effluent, and make the application easier, liquid smoke has emerged. Liquid smoke is generally produced from natural smoke by condensation or water scrubbing. The most prevalent method used in the United States is the water-scrubbing method. This method includes the smoldering of sawdust, controlling oxidation condition, and absorption of smoke in water. In this process, the smoke solution is recycled until a given concentration is developed. The other method is the pyrolysis of hardwood sawdust, where the generated smoke is condensed to a liquid-form solution through a series of condensers. Liquid smoke can be applied directly into raw products, thereby producing smoke flavor and color simultaneously. Liquid smoke is often applied to a product surface. The application technique can be classified as dipping, spraying, atomization, and regeneration. Dipping or spraying is implemented immediately after stuffing or before thermal processing. Atomization is using high liquid pressure to form a cloud of liquid smoke to which products are then exposed. Atomization can be applied either before or during heat processing. Regeneration of liquid smoke is converting the liquid smoke from a liquid phase to a gas phase by heat. The regenerated smoke can have the same application as for natural smoke.

Liquid smoke has been available for a while. However, it cannot replace natural smoking because of an unsatisfactory flavor profile. In order to impact sufficient smoke color on the product surface, liquid-smoked products often develop bitter smoke flavor. Recently, liquid smoke flavor has been improved by adjusting the phenol and carbonyl ratio.

Pollution Control

The natural wood smoke that can cause irritation and odors is discharged to the atmosphere with the exhausted air that is necessary for humidity control inside the smokehouse. To eliminate or reduce air pollution, afterburners, closed-system smokehouses, water-scrubbing systems, or electrostatic precipitators have been employed. The afterburner uses heat, above 1,000°F, to burn the contaminants. It is a high-efficiency air-pollution control unit. High energy cost has prohibited its application. The basic principle of the closed-system smokehouse is to circulate humid air through a dehumidifier for humidity control without discharging the contaminated air into the atmosphere. However, a substantial amount of smoke must be carried into the smokehouse. Some air must be released in order to attain the material and pressure balance. Therefore, a complete closed-system house cannot be achieved for a natural smoke system (23). For a water-scrubbing system, air with smoke contaminants passes through a venturi. In the venturi, air with contaminants come in close contact with the scrubbing water. The collection of particulates on scrubbing is accomplished by inertia impaction, interception, and diffusion. The water droplets are collected from the stream by a baffle and a mist eliminator before the scrubbed air is discharged to the atmosphere. The water scrubber can achieve up to 80% efficiency. The electrostatic precipitator draws the contaminated air through prefilters that collect the larger particles and then the air passes through ionizers that electrically charge all the particles in the airstream. Ionized particles enter the collecting cells, where ground plates remove the particles. The electrostatic precipitator can effectively remove the particulate material in the air-stream but cannot eliminate smoke odors in the gas phase. To improve efficiency, a chemical absorption tower is installed after the electrostatic filter. The alkaline solution is circulated inside the tower to dissolve smoke odors as well as to keep the tower clean. The new system can provide up to 97% efficiency at a flow rate of 1,500 ft3/min.

Drying and Heating

The smokehouse is the major heat-processing equipment for producing processed meats. The modern smokehouse is equipped with instruments for programming air temperature and humidity during the process cycle. The air can be heated with direct gas-firing, steam coil, or electric heating elements. The humidity of air in a smokehouse is con trolled by the amount of air intake and exhaust and the injection of steam when necessary. The conditions of returned air (dry-bulb and wet-bulb temperatures) are usually used as the process control references. The conditioned air is then introduced into the smokehouse by a blower. The air circulation rate is normally greater than 10 air changes a minute. One air change is equivalent to one smokehouse volume in cubic feet. The airflow coming into the smokehouse is alternated from one side to the other to provide more uniform heat processing. Heat-processing schedules are specifically designed according to product quality requirement as well as microbial kill. Because there are many different products and quality specifications, heat-processing schedules also differ substantially. In designing a smokehouse, the production rate, product drying rate, and product heating rate with a specific set of heat-processing schedules should be predetermined. By setting up material and energy balance, the equipment capacity for air-handling, heating, and cooling can be determined.

The quality of processed meats is significantly affected by the heat-processing schedule (24-26). High humidity in a smokehouse can cause surface grease and poor color problems. These problems are common for batch-type smokehouses. The batch-type smokehouse usually starts at low temperatures because the cold-water shower of the previous lot. A wet-bulb temperature setting higher than the house temperature at the beginning of heat processing often calls an excessive amount of steam into the smokehouse. It is common practice to set the wet bulb at zero degrees in order to prevent the excessive humidity. Because the wet-bulb temperature is set at zero position, which is way below smokehouse temperature, the intake and exhaust dampers are controlled at a wide-open position, disregarding the humidity condition in the smokehouse. There is substantial variation in year-round weather conditions. If the exhaust capacity is determined under hot, humid summer conditions and a good quality product is produced, it is not necessary to run the exhaust at maximum capacity in cold, dry winter weather. Therefore, the exhaust dampers should be controlled according to the dryness of intake air (23). Microprocessors have increasingly been used to control the smokehouse system. The exhaust and intake air, steam injection, and exhaust fan can be controlled and operated independently without interlocking problem as are seen in current control systems. Microprocessor control provides much more flexibility for heat-processing optimization and energy conservation.


From microbiological research, it has been illustrated that product shelf life strongly depends on product temperature. By lowering product temperature, the onset of microbial growth can be substantially delayed. To reduce product temperature after smoking and cooking, air cooling and liquid chilling are two of the more commonly used methods in the meat industry. Salt brine and propylene glycol are two common cooling mediums. However, glycol solution can only be used for products with impermeable casing. There are several disadvantages with air cooling such as slow cooling rate, nonuniform cooling, and moisture loss. Air cooling is normally used to chill products in impermeable casing or metal molds. Because a continuous-processing system requires more rapid cooling, liquid cooling is used in almost every system. In addition, to compensate for the disadvantages of air cooling, liquid cooling with high chemical concentration can also retard the microbial growth. In general practice, the economical way to apply liquid cooling can be achieved by two steps:

1. Precool products with regular water.

2. Further cool the product with a refrigerated cooling medium such as brine.

Qualities that can be affected by brine chilling are shrink, flavor, and color. When moisture is extracted by brine, product weight loss and darker color occur. As long as brine concentration is higher than the salt concentrations in the product, salt migrates into the product and develops a salty flavor. Therefore, salt concentrations is the most critical condition ?n the brine-chilling process. The optimum salt concentration should be determined for each product. The best operation conditions should be at the equilibrium conditions of salt migration.

For the last decade, poultry products have taken a significant market share of processed meats. According to USDA regulation, frozen whole turkey must be chilled down to 0°F core temperature before it can be shipped. Freezing whole-bird turkey is a time-consuming process. The poultry industry is looking for a rapid-chilling method to increase productivity. Cryogenic-freezing can provide a fast freezing rate. However, it is a high-cost operation. Blast-air chilling is an economical process but takes a long time to achieve 0°F core temperature. Liquid freezing is an effective and also economical method. The common chemicals used to depress the freezing point below 0°F are calcium chloride, ethanol, and propylene glycol. However, calcium chloride contributes to bitterness at as low as 1% of concentration and ethanol causes medicinal aftertaste (27). Propylene glycol usually does not develop objectionable flavor. The high concentration of glycol at low temperature causes excessive viscosity of fluid and develops a pumping problem. Hence, the glycol solution is limited in the immersion system. Because of flavor and viscosity problems, liquid freezing has not been widely accepted. The most common practice in the poultry industry is blast-air freezing or salt brine prechilling followed by blast-air freezing. It usually takes 24-48 hours to reach 0°F core temperature.

Homemade Pet Food Secrets

Homemade Pet Food Secrets

It is a well known fact that homemade food is always a healthier option for pets when compared to the market packed food. The increasing hazards to the health of the pets have made pet owners stick to containment of commercial pet food. The basic fundamentals of health for human beings are applicable for pets also.

Get My Free Ebook

Post a comment