The manufacture of canned fruit products involves filling, exhaustion, sealing, sterilization, and cooling operations (Fig. 8.3). Prepared fruit materials are filled into contain- ers, may be metal cans, glass jars, or retort pouches. The void space, called “headspace,” inside the filled container is usually limited to less than 10% of the volume. The process of “exhaustion” removes air from the filled container before sealing to prevent the damage or distortion of container by the thermal expansion of entrapped air and to minimize the oxidation of food and the inner surface of container. The her- metical seal on the container prevents the outside microor- ganisms from recontamination to the sterilized food. The
“sterilization” process is performed to inactivate the natural enzymes in the fruit, if a “blanching” process has not been practiced in advance, and, more importantly, to kill microor- ganisms inside the container. The microbial, chemical, and biochemical spoilages of products ought to be all prevented.
There are variations in the canning operations for different fruit products. The following description is common among the majority of these products.
Fresh fruit
Washing
Blanching
Preparation
Filling
Exhaustion
Sealing
Sterilization
Cooling
Product
Figure 8.3.Flow chart for the typical canning operation of fruit.
Selection of Fruit Materials
The canning season for a specific fruit is usually not year round. Good raw materials at the right ripeness are needed in the processing for high-quality products. Stored products may be used only if they meet the quality standard.
Many fruits in the “mellow-ripe” stage are pretreated or processed into the form of whole fruits, fruit halves, slices, dices, juices, purees, marmalades, or sauces for canning. Ba- nanas, pears, and some apples when harvested at the “green- mature” stage commonly produce a higher-quality product in canning. Plums, grapes, olives, gooseberries, and maraschino cherries are usually harvested and canned in the “firm-ripe”
stage instead (Prussia and Woodroof 1986).
Washing
Right after arriving at the cannery, the qualified fresh fruits are carefully washed to remove dust, dirt, and mold spores. This process should be carried out so thoroughly as to ensure the removal of heat-resistant molds, such asNeosartorya fischeri, which have been linked to mold growth in canned fruits (Jesenka et al. 1991). It is recommended that fruit materials be washed in clean chlorinated water. A common method of
chlorination is to add small quantities of household bleach to water (usually 50–100 ppm chlorine concentration). The fruits should be completely rinsed with clean water after the treatment. Washing may also be done by equipment in which fruits are subjected to high pressure water sprays or strong water streams while passing along a moving belt or while being tumbled on an agitating or revolving screen. However, caution should be paid to avoid injuring the fruits. Fruits to be peeled may not need washing beforehand.
Blanching
The blanching, or a “partial cook,” of fruits is an important operation in the canning process. Fresh fruits are immersed in water at 190–210◦F (88–99◦C) or exposed to live steam for a short period of time in order to inactivate oxidative enzymes, such as catalase, peroxidase, polyphenol oxidase, ascorbic acid oxidase, and lipoxygenase, that have the potential to cause flavor and textural changes of the product (Ramaswamy 2005). Adequacy of blanching is commonly confirmed based on the negative response of the heat-resistant enzyme perox- idase or polyphenol oxidase. In addition to the inactivation of enzymes, there are other benefits of blanching, such as expelling respiratory gases for increasing vacuum and pre- venting the oxidative deterioration of product and the internal corrosion of can; facilitating preparative operations, such as peeling, dicing, and cutting; removing undesirable flavors;
setting the natural color of certain products; helping clean- ing; and decreasing the microbial load (Downing 1996b). A disadvantage of blanching for example is the loss of water- soluble nutrients such as ascorbic acid. Therefore, blanching time should be kept as short as possible. The blanching of fruits is usually accomplished in equipment especially de- signed for individual applications. There are basically two types of blanchers, using hot water and steam, respectively.
Continuous hot-water immersion blanchers with conveyors are very common. Continuous steam blanchers that use a chain or belt conveyor to move fruits through a tunnel of live steam are also frequently used.
Preparation
Many fruits require some other pretreatments or preparative operations before filling into a container. Peeling, coring, slicing, and dicing are examples of these operations. Peeling is the most common one. Many methods are available for the peeling of fruits. Mechanical peeling and lye peeling are among the major ones.
Mechanical Peeling
Some fruits, for example, pears, apples and pineapples, may be knife-peeled by machines. The fruit is impaled and rotated against a stationary knife or vice versa that follows the con- tour of the fruit. The core may be removed at the same time.
Equipment is specially designed for each application. Abra- sion peeling is another type of mechanical peeling in which an agitating/tumbling action is utilized so that all surfaces of the fruit undergoing peeling are exposed to a rubbing action against an abrasive surface, thus loosening the peel, which is then removed by water sprays (Downing 1996b). Some machines fitted with cylinder brushes may be used to help detaching the loosened peel.
Lye Peeling
Apparatus for continuous lye peeling are very popular with fruits, such as peaches, nectarines, apricots, and pears. The lye peeling operation requires a generous water supply, lye (caustic soda or sodium hydroxide), and a source of heat.
The fruits are passed through a heated tank containing hot lye solution at a predetermined rate. The caustic lye solution dissolves the fruit skin. The degree of peeling can be adjusted by varying the concentration and temperature of lye solution, residence time, and agitation strength of the fruit in the so- lution. In practice, the temperature is maintained somewhere from 60◦C to close to the boiling point of solution. The res- idence time is usually 1–2 minutes in 2–10% caustic soda solution (Burrows 1996). After the treatment, fruits must be water washed with pressure sprays to remove the lye- disintegrated peel. Sometimes, an acid dip, most commonly citric acid, is used after washing to neutralize any remaining traces of caustic soda in peeled fruits.
After peeling and perhaps coring as well, fruits may be sliced or diced. In case there is delay between these treat- ments, fruits may be held in 2–3% salt solution to prevent browning.
Filling
Mechanical fillers are usually used for the filling of fruit products into cans. An accurate and consistent fill of the fruit and syrup/juice at a proper temperature is necessary to maintain a uniform headspace. The volume of headspace in a can is very critical. Insufficient headspace may slow down heat penetration into the can and result in under-sterilization of the product. It also allows less space for hydrogen gas to accumulate in the progress of internal can corrosion and renders a can more liable to “dome.” Excessive headspace may result in the underweight of can and hinder the successful operation of exhaustion as well.
Exhaustion
The exhaustion of a container is to remove air and entrapped gases from the container prior to sealing. The advantages of exhaustion include the alleviation of the internal pressure buildup in the container during sterilization, the prevention of the oxidative deterioration of the canned product, the re- duction in the growth and propagation of residual aerobic
microorganisms, and the mitigation of rusting on the inner surface of the can during storage. A sufficient degree of vac- uum in the sealed container also helps heat transfer in the sterilization process.
Common fruits, being of a highly corrosive nature because of their acidity, require a vacuum of 250 mm Hg or above.
The major exhausting methods in the canning of fruits are described below.
High-speed Mechanical Vacuum Sealing
A vacuum double-seaming machine is used. In the machine, cans filled with fruit and syrup in relatively cool condition are passed into a clincher that clinches the cans without forming an air-tight seal. The cans are then subjected to a vacuum for a short period of time for exhaustion right before the hermetic sealing in the same machine (Ramaswamy 2005).
This method is not good enough for cans requiring a very high vacuum or containing highly viscous products. There is another problem in applying high-speed mechanical vacuum sealing for fruits packed in syrup. The sudden suction of air may spill some syrup out of can and result in contaminating the equipment, soiling of the sealing surface, and insufficient filling in the can.
Hot-filling Exhaustion
The liquid food is preheated, hot-filled into a container, and then sealed as soon as possible. A spurt of steam into the headspace may be applied right before container sealing to purge away the residual air. The vapor pressure in headspace is approximately at atmospheric level upon the sealing of con- tainer. The condensation of vapor when the sealed container cools down generates a vacuum. The vacuum in hot-filled containers is usually weaker than that in cans exhausted by other methods.
Thermal Exhaustion
In thermal exhaustion, the fruit product is usually heated before and after filling. A filled open container is run through an exhaustion box that provides live steam to heat the product and to replace the air in the headspace. The container is sealed immediately after exhaustion. The disadvantages of thermal exhausting include being energy intensive due to high-steam consumption and the increased chance of contamination due to the dripping of condensate.
Sealing
After exhaustion, the containers should be sealed as soon as possible. A good sealing operation is necessary to render the success in canning. Faulty sealing usually results in leaking and recontamination of the canned product during and after sterilization.
First roll
Chuck Cover Can body
Second roll
Chuck Cover Can body Chuck
Cover Can body Figure 8.4.The formation of double seam.
The method of sealing varies with the type of container. A metal lid is placed on a filled metal can, and then fixed onto the can body by combined actions involving chuck, lifter, the 1st roll, and the 2nd roll in a typical double-seaming machine to form a double seam (Figs. 8.4 and 8.5). Double- seaming machines may operate at speeds as high as 600 cans per minute for particulate foods (Downing 1996b). Fluid products can be sealed at a speed up to 1600 cans per minute (Lopez 1987).
It is somewhat slower for sealing glass containers than metal cans because of the fragility of glass. Small glass containers may be filled and sealed at a speed up to 1300 containers per minute (Downing 1996b).
Flexible retort pouches may also be used in the packing of fruit products. The packaging material is basically composed of an aluminum foil between plastic layers in a laminated structure that is able to withstand common sterilization tem- peratures and to serve as the hermetic barrier for the product.
Pouches as containers are formed from roll stocks by fold- ing a single roll along its center or by bringing two separate rolls together heat-sealed side to side. The filling of product into a pouch has to be gentle enough as not to soil the top part of the inner surface where the seal will be formed. The
Countersink
Cover hook
Thickness of cover
Thickness of can body Body hook
Lining compound
Figure 8.5.Cross-section of a double seam.
filled retort pouches are usually heat-sealed using impulse bars that melt the plastic material on the top inner surface of the two opposite laminas to be fused together upon cooling.
It is advantageous to perform an effective exhaustion opera- tion that generates a high vacuum prior to sealing to prevent ballooning of the pouch during the subsequent heat steriliza- tion. The operation usually involves mechanical squeezing of the pouch, vacuuming by suction with a snorkel, or steam flushing. A pouch filler/sealer is commonly used for on-line high-speed filling, vacuuming, and sealing.
Sterilization
The sealed containers are heated in a sterilizer by contacting with steam, hot water, or atomized steam–air mixture. Ster- ilizers can be grouped into either batch type or continuous type. The conventional retorts are batch-type sterilizers. The continuous sterilizers being used in the industry include wa- ter bath sterilizer, cooker-cooler, hydrostatic cooker, flame sterilizer, etc.
In the sterilization operation of canned food, a faster rate of heat penetration, namely the transfer of heat from outside to the slowest heating point in the center part of a container, usually corresponds to a shorter processing time and a bet- ter quality product. The heat penetration rate in a specified container under a specific heating condition varies with the type of food, the volume of headspace, and the degree of vac- uum in the container. There are mainly two mechanisms of heat transfer in a container, namely, convection and conduc- tion. Heat penetration is comparatively faster in low-viscosity liquids such as fruit juice because convection prevails. For products composed of a high-viscosity liquid or a large por- tion of solids, such as juice concentrate and fruit cocktail, the heat penetration is primarily via conduction and is much slower. Agitation during the sterilization process may gen- erate forced convection and improve heat penetration in a conduction-heating container. A continuous sterilizer more or less provides agitation to the containers while they are being processed.
Conventional Retort
A conventional retort can be vertical or horizontal in layout.
It is subjected to batch-type operation. The retort is basically a steel tank capable of withstanding the high pressure of saturated steam at a temperature well above the boiling point of water, and equipped with steam, water and compressed air ducts, valves, vents, bleeders, drain, overflow, pressure gauge, water-level indicator, thermometer and temperature recorder-controller, etc. The low-acid fruit products in sealed containers are loaded in baskets or trays and placed inside the retort. The retort may or may not be filled with water before or after the loading of containers. In-water processing is preferred for high-vacuum large cans, glass containers, and retort pouches that are not tolerant to sudden temperature
and pressure changes. The lid or door of retort is closed, and then the inlet of steam, steam-heated water, or atomized steam–air mixture is opened to start the heating phase, or come-up, of the sterilization process. The heating phase ends when the retort reaches the preset processing temperature, usually no lower than 116◦C (240◦F), and then the holding phase follows. Cooling water is introduced into the retort to start the cooling phase when the expected holding time is up.
Partially cooled containers may be taken out of the retort and immersed in a trough of water for further cooling.
Most of the conventional retorts belong to the stationary type. However, some conventional retorts have a way of ro- tating or agitating the containers in the heating medium. Ag- itation improves the heat penetration in liquid or semi-liquid food cans, reduces the processing time, and achieves better quality retention.
Continuous Water-bath Sterilizer
A sterilization temperature below the boiling point of wa- ter may be enough for the production of acid and high-acid canned fruits. In such a circumstance, a water-bath steril- izer may be used to substitute for a retort. A modern con- tinuous water-bath sterilizer is composed of an elongated tank through which containers travel on a belt. The con- tainers may alternatively pass through a tunnel on a con- veyer belt subjected to continuous hot-water spray. Such a method is especially suitable for the sterilization of fruit products in glass jars that are sensitive to thermal shocks (Lund 1975).
Cooker-cooler
Cooker-cooler is a continuous agitating retort consisting of a cooking shell and a cooling shell in the basic structure. Cans are moved by a spiral and reel mechanism, and rolled by rubbing against the wall inside the shells. The agitating effect of this type of retort is excellent. It is equipped with specially designed ports or valves. Filled cans enter the cooker-cooler through a rotary transfer valve, which is designed to prevent the escape of steam from the cooking shell. Inside the cooking shell, cans are sterilized by steam supplied from the bottom of the shell. Uniform distribution of steam is ensured by a manifold steam supply system along the entire length of the shell. The “cooked” cans are then transferred through another rotary transfer valve into the next shell for cooling.
The cooling shell is approximately two-thirds full of water to provide flood cooling of cans as they proceed through the shell. Water enters at the discharge end and exits at the feed end of the cooling shell for a counter-flow cooling effect.
The reel in the cooling shell has a series of baffles. The combination of reel baffles and counter-flow movement of water ensures efficient usage of cooling water and controlled, uniform cooling of cans (Downing 1996c).
Hydrostatic Cooker
The principle of hydrostatic cookers is to balance or to “main- tain” the steam pressure by hydrostatic pressure. A hydro- static cooker is made up of four chambers: a hydrostatic
“come-up” or “in-feed” leg, a sterilizing steam chamber, a hydrostatic “come-down” or “discharge” leg, and an after- cooling system. Containers are conveyed through the ma- chine. They enter and move down the in-feed leg while being heated with the surrounding progressively hotter water in pro- gressively higher pressure. The containers continue to travel first horizontally in water and then upward through the water seal to the steam chamber. In the steam chamber, the contain- ers are exposed to a temperature ranged from 240◦F to 265◦F (116–129◦C) for sterilization. Upon leaving the steam cham- ber in the completion of holding phase, the containers pass another water seal where the cooling phase commences. The containers are conveyed through progressively cooler water to the top of the discharge water leg for cooling and pres- sure releasing (Downing 1996c). An after-cooling system is equipped downstream for further cooling of the containers to the desired final temperature.
Direct Flame Sterilization
The sterilization process that heats up containers to an in- ternal temperature above 100◦C is often performed in a special kind of equipment, commonly called “retort” as above-described, which maintains an environment at elevated pressure. The cost of the retort is bound to increase with the demand for a higher sterilizing temperature as its structure has to withstand a higher pressure. The direct flame steriliza- tion machine was developed in France to be a substitute for high pressure retorts. In the machine, cans are heated directly by rotating over gas flames. The agitation effect is strong. The sterilization can be accomplished in a short period of time.
The operation cost is very low. However, this method only works for cans with very strong structure to resist the high internal pressure built up in heating. The viscosity of the product is limited within a proper range as agitation will fail, and burn-on on the inner wall of the can will occur in the processing of highly viscous products.
Hot-fill (Hot-pack)
Another common sterilization method for high-acid fruit products, juice for example, is “hot-fill” or “hot-pack.” There is similarity between hot-fill and the previously described hot-filling exhaustion. A liquid or semi-liquid fruit product is heated, held for few min at a temperature sufficiently high although lower than the boiling point, and then filled into con- tainers while it is still hot (usually 85◦C and above). The filled container is sealed immediately after, allowed to stand for a while, and then turned upside to complete the sterilization on all sides in the container by the residual heat of the product.