ELEMENTS OF FOOD PROCESSING
METHODS AND EQUIPMENT
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There are over 18,000 different food items in today's supermarkets, which are processed to a greater or lesser degree, and thousands of new products are introduced each year.
Modern food processing has three major aims:
The following table summarizes the item to be controlled and comments on the major approaches involved in this control.
|To be Controlled||Heat||Cold||Chemicals||Aw*||Mechanical|
|Micro-organisms||Prevents growth||Reduces growth rate||Preserva-tives re-tard growth||Do not grow below Aw of 0.6||Reduces numbers|
|.Enzymes||Destroyed by heat activity||Decrease reaction rate||Modify activity||Alters rate of enzyme activity||increase ES complex formation|
|Chemical Reactions||Increases chemical rate, browning, oxidation||Reduces reaction rate||May inhibit or activate||Can alter rate of reaction, especially oxidation||Not applicable|
|Physical Structure||Increases effects||Decreases effects||May modifiy structure||High. Aw may cause caking||Can destroy structures|
*Aw = water activity
Insuring the safety of food involves careful control of the process from the farm gate to the consumer. Safety includes control of both chemical and microbiological characteristics of the product. Most processing places emphasis on microbial control, and often has as its objective the elimination of organisms or prevention of their growth.
Processes that are aimed at prevention of growth include:
Processes that are aimed at minimizing organisms include:
A further method of processing that is aimed at the control of undesirable microflora is the deliberated addition of microorganisms and the use of fermentation.
Safety from a chemical viewpoint generally relates to keeping undesirable chemicals, such as pesticides, insecticides and antibiotics out of the food supply.
Making sure that food products are free from extraneous mater (metal, glass, wood, etc.) is another facet of food safety.
Quality of a food product involves maintenance (or improvement) of the key attributes of the product - including color, flavor and texture.
To maintain quality it is important to control:
Today's consumers want food products that are convenient to use and still have all the qualities of a fresh product.
Most food processing is comprised as a series of physical processes that can be broken down into a number of basic operations. The "unit operations" can stand alone and depend upon coherent physical principles.
Unit operations include:
1. Fluid flow - moving a fluid product from one point to another with varying degrees of turbulence.
2. Heat transfer - in which heat is either removed or added and includes heating
3. Contact equilibrium processes or mass transfer, which may or may not require a change in state. Generally a molecular species is transferred to or from a product. Processes that use mass transfer include distillation, gas absorption, crystallization, membrane processes, drying, evaporation.
4. Mechanical separation, such as filtration, centrifugation, sedimentation, sieving
5. Size adjustment either through size reductions (slicing, dicing, cutting, grinding) or size increase (aggregation, agglomeration, gelation).
6. Mixing - to make homogenous blends or dry or liquid ingredients. This may include solubilizing solids, preparing emulsions or foams - or - dry blending of ingredients such as for dry cake mixes.
7. Fermentation, while not a physical operation, is for the purpose of this course included as a unit operation.
Some food processing operations may utilize a single unit operation, but most food processing includes a combination of unit operations to achieve the total process.
For example, the manufacture of a dried coffee whitener from a combination of fluid and dry ingredients includes the following unit operations in sequence:
-Size reduction (homogenization)
-Heat transfer (heating)
-Heat transfer (cooling)
-Mass transfer (conversion of water to vapor during drying)
Pasteurization of milk to destroy the most heat resistance vegetative pathogen includes the unit operations of fluid flow and heat transfer (both heating and cooling).
Some other examples of unit processes, and their associated unit operations include:
-freeze drying (heat transfer, mass transfer)
-extrusion (fluid flow, heat transfer, mass transfer, size reduction in the case of cereals and
-ice cream manufacture is comprised of two unit processes:
mix making (mixing, fluid flow, heat transfer, size reduction)
freezing (fluid flow, heat transfer, mass transfer of air into the ice cream)
FOOD PROCESSING EQUIPMENT
Transport of fluids is achieved either by gravity flow or through the use of pumps. In gravity flow the flow is "laminar", where the flow is transfer from the fluid to the wall between adjacent layers. Adjacent molecules do not mix. In most instances, however fluids are transported from one unit operation or process to another by pumps and in "turbulent" flow where there is mixing of adjacent particles. Two different types of pumps are commonly used for different purposes:
The centrifugal pump utilizes a rotating impeller to create a centrifugal force within the pump cavity, so that the fluid is accelerated until it attains its tangential velocity close to the impeller tip. The flow is controlled by the choice of impeller diameter and rotary speed of the pump drive. The capacity of a centrifugal pump is dependent upon the speed, impeller length and the inlet and outlet diameters. Product viscosity is an important factor affecting centrifugal pump performance. If the product is sufficiently viscous, the pump cavity will not fill with every revolution and the efficiency of the pump will be greatly reduced. Centrifugal pumps are used to transport fluid and for cleaning operations. Centrifugal pumps may be high speed or low speed, with high speed pumps being used for cleaning (CIP). .
A positive pump generally consists of a reciprocating or rotating cavity between two lobes or gears and a rotor. Fluid enters by gravity or a difference in pressure and the fluid forms the seals between the rotating parts. The rotating move of the rotor produces the pressure to cause the fluid to flow. Because there is no frictional loss, positive pumps are used where a constant rate of flow is required (timing pump), for high viscosity fluids or for transporting fragile solids suspended in a fluid (such as moving cottage cheese curd from a vat to a filler).
An agitation device may be placed in a tank for a number of purposes. The mixing device may need to produce bulk circulation in the tank, a controlled velocity at some surface of the tank or a controlled rate of shear. Two major purposes of mixing are either heat transfer or ingredient incorporation. Different mixer configurations will be used to achieve different purposes. The efficiency of mixing will depend upon: -design of impeller -diameter of impeller -speed -baffles
Heat is either transferred into a product (heating) or removed from a product (cooling).
Heating is used to destroy microorganisms to provide for a healthful food, prolong shelf life through the destruction of certain enzymes and to promote a product with acceptable taste odor and appearance.
The factors that influence the heat transfer into or out of the product include:
1. Heat exchanger design
2. Heat transfer properties of the product
3. Density (weight per unit volume)
4. Method of heat transfer
5. Viscosity (related to the amount of force required to move the fluid product)
A variety of heat exchanges are utilized in the food industry, which include:
-plate heat exchanges
-tubular heat exchanges
-swept surface heat excahngers.
Plate heat exchanges pass fluid over a plate where a heating or cooling medium is being passed up or down on the other side of the place. The thin film makes for rapid heat transfer and is the most efficient method of heat fluids of low viscosity.
Tubular heat exchanges general are compose of a tube within a tube, in which product and heating or cooling medium are flowing in opposite (counter current) directions. This a low cost method of heating or cooling and is applicable to fluids of higher viscosities that generally passed through a plate heat exchanger.
Swept surface heat exchanges have blades that scrape the surface of the heat exchanger and bring new product continuously to the heat or cooling surface. There are utilized for fluids of very high viscosity. An ice cream freezer is an example of a swept surface heat exchanger.
Common unit processes that include heat transfer as a unit operation include:
-Separations (based on density, size or shape)
Pasteurization: Pasteurization is a process of heating a product at a specific temperature for a controlled period of time to destroy the most heat resistant vegetative pathogenic organism. Originally developed for fluid dairy products, the process is also applied to other fluids, including fruit juices and juice products. In addition to destruction of microorganisms, pasteurization also achieves almost complete destruction of undesirable milk enzymes - such as lipase.
Pasteurization was originally conducted in jacket vats equipped with an agitator to give compete mixing and the ability to heat and cool. For milk vat pasteurization required a heat treatment of 145°F for 30 minutes, with the air space above the product being heated to a higher temperature to insure all molecules of the product were adequately pasteurized. The component parts of a vat pasteurizer are the tank body, heating/cooling channels; inlet port, outlet valve with leak detector; agitator and drive motor; product indicating thermometer; recording devices; air space heater and indicating thermometer; steam inlet and outlet, cooling water inlet and outlet.
Next plate heat exchanger were used with a higher temperature and shorter time (161.5/15 seconds)being used to achieve the same bacterial destruction. This is called HTST pasteurization. To improve efficiency the cold milk is used to partially cool the pasteurized milk and the hot milk partially heats the cold milk in a processed call regeneration. In this section of the pasteurizer cold milk is on one side of the plate and the hot milk on the other. The component parts of an HTST pasteurizer include the balance tank, cold side of regenerator, timing pump, heating section, holding tube, flow diversion valve, hot side of regenerator, cooling section, controls.
Ultra high temperature (UHT) pasteurization is achieved with plate heat exchanges operating at temperatures of at least 190 F for 2 seconds. The same principles are used as for HTST, except that higher temperatures are used and the equipment is designed for very rapid heating and cooling.
Sterilization: The primary object of sterilization is to destroy the most heat resistant pathogenic spore forming organism
Flame Sterilizer - conveyor, preheater, gas jets
Still Retort - retort body, product basket, steam inlet, water inlet, pressure gage, pressure relief valve
Hydraulic cooker = a continuous sterilizer, where the cans are rotating during the sterilization process
Still Retort = A type of batch sterilizer used for canning food, where the cans are not in motion during the sterilization process.
Baking: Baking is the process by which moisture is removed from wheat based (or other cereal grain, products. Batch or continuous oven are utilized to heat the products resulting in an increase in volume of the product, gelatinization or starch of protein "elation to set the final structure of the product. Baking is an essential process for bread, cakes, cookies and crackers.
A rotary oven is a fairly simple device which has a cabinet, oven trays, heating element, thermostat and temperature controls. Many commercial ovens are continuous in that the product moves though the oven on a continuous belt.
Concentration: Concentration can be achieved through evaporation and through reverse osmosis.
Evaporation generally involves heating the fluid in a vessel under vacuum to cause a change in state of water from liquid to vapor and then recovers of water by passing the vapor through a condenser. The component parts of the evaporator include an enclosed vessel with product inlet, manhole, heating rings, finished product outlet, condenser, steam inlet, vacuum and condenser pump. In some product evaporation causes the loss of flavor volatiles and this case, a low temperature unit is added to recover the flavor volatiles so that they can be added back to the product.
To reduce operating costs, multiple effect evaporators are utilized, which have 2 or more evaporators placed in series to provide a means for the continuous concentration of a fluid product. This increases the efficiency of the evaporation process.
Reverse osmosis (RO) is a process where the fluid is passed through a semi-permeable membrane with very small pores that permit only the transfer of water. Most systems consist a the membrane cast on a solid porous backing - usually in the form of a tube. High pressure is applied to force the water (called permeate) through the membrane the concentrated fluid (called retentate) is retained in the tubing. The rate of water removal decreases as the fluid is concentrate, until it is no longer economically feasible to remove more water.
Concentration is often used as a pre-step to drying to reduced the amount of water that needs to be removed in drying and reducing drying costs. Evaporation can achieve higher solids economically than can reserve osmosis. RO is preferred over evaporation for heat sensitive fluids.
Drying: Three common methods of drying are: (a) sun or tray drying, (b) spray drying and (c) freeze drying.
Sun or tray drying is least expensive, followed by spray drying and freeze drying. The drying method of choice is generally based on the characteristics of the product.
Products that are already solid lend themselves to sun or tray drying. These include fruits and vegetables. The products may be dried by exposure to sun or place in trays and dried in a current of warm or hot air.
Products that are very heat sensitive are freeze dried. Commercially only instant coffee is widely freeze dried. Some freeze dried fruits are beginning to reach the market, but these are in limited quantities. In freeze drying the moisture is removed without a phase change (sublimation). A freeze drying is comprised of a dryer cabinet; drying chamber with: heating/cooling shelves, trays and door; vacuum pump, condenser, controls and digital readout.
The most common drying method is spray drying, which is applicable to fluid products. The bulk density (weight per unit volume) is controlled to a large extent by the solids that are sent to the dryer. A spray drying has an air inlet, air heater, drying chamber, inlet atomizer, cyclone chamber, cyclone separator, dry product collection vessel, hot air inlet and outlet, drying fan and motor, controls. There are several different designs of spray nozzles through which the fluid is atomized into the heated air. These generally are either centrifugal nozzles or high pressure spray nozzles. The type of nozzles will vary with the product being dried.
For some products that are very hydroscopic, the dried product may be partially re-wetted and the redried. This produces agglomerated products that are easily dispsered in solution. Spray dried powders with a surfactant is also a method for improving dispersion.
Another method, no longer in common use, is roller drying -- where the product is allow to flow over a hot, rotating drum and the dried product is scraped off. This was a low cost method of drying, but created a lot of heat damage to the product.
Freezing: Lowering the temperature below the freezing point of the product stops microorganisms from growing and reduces the activity of enzymes. Vegetables and some fruits are heat treated (blanched) before freezing to eliminate enzymes.
One of the most common frozen foods is ice cream .An ice cream freezer can be operated either in batch or continuous mode. A batch ice cram freeze consists of a freezer cylinder; dasher assembly with scraper blades, counter rotating agitator; mix inlet, ice cream outlet gate; motor. A continuous ice cream freezer consists of an inlet pump; air injector assembly; freezer cylinder; dasher assemble with scraper blades; outlet pressure gage; ice cream back pressure regulating valve; refrigeration unit inside cabinet. The continuous freezer provides much better control of drawing temperature and overrun than batch machines and is the machine of choice for commercial ice cream operations. The product from these machines is place in low temperature freezers to freeze out additional water and produce the packaged products found in super markets.
Soft serve machines are generally some modification of the batch freezer and the product is served directly from the machine without further hardening.
For frozen fruits and vegetables, the products are generally frozen in the package and generally is blast freezers, where the product is moved through the freezer in a continuous manner.
Separations: Separations can be achieved on the basis of density or size and shape.
Separations that are based on density differences include the separation of cream from milk, recovery of solids from suspensions, and removal of bacterial from fluids.
Size reduction can be through the use of high shear forces, graters, cutters or slicers.
Emulsions with very small fat globule droplets are frequently made with a homogenizer which is a high shear positive pump that forces fluid though a very small orifice at very high pressure to form or reduce the size of an emulsion. The positive pump uses a reciprocating or rotating cavity between two lobes, gears or between a stationary cavity and a rotor. The fluid forms the seal between the rotating parts. The components of a homogenizer consist of a suction manifold; suction check valves, plungers and cylinders; discharge check valves, discharge manifold; discharge pressure gage; 1st and 2nd stage homogenizing valves; plunger seals;
Typical equipment for size reduction in meat products and their component parts include:
Powdered products are frequently ground into find particles through ball mills.
Useful microorganisms are used to preserve a number of food products, which include:
Lactic acid fermentations
All fermentations results from the action of microorganisms growing on carbohydrates in the product and producing some material (alcohol, or acid) which minimizes the growth of undesirable organisms and preserves the product.
The equipment used for fermentation varies as a function of the product. Generally, a fermentation vessel is used - which may be open or closed. Modern beer, wine and cheese operations utilize closed vessels for the initial fermentation, where careful sanitation practices can be employed to present contamination. Cultures used for the fermentations are prepared under close sanitary control.
Pictures of Processing euipment:
|drumdryer. jpg||Impingement dryer.jpg||spraydryer.jpeg|