Water is the most used natural resource in the production of canned food, for washing food, for preparing covering liquids, for rinsing cans, plant sanitization, steam generation and other activities.

Next we will detail some causes that provoke the deterioration of the metallic sanitary containers (ETP, TFS) in the productive processes of canned food, either by the bad quality of the water or by its contamination with aggressive chemical products to the tinplate.

Packaging deburring

The internal or external sanitary varnish applied to the containers is a protection against the chemical aggression of the covering liquid inside the can and a barrier against corrosion from the outside. Light or heavy deburring of the container leaves the sheet metal exposed to the corrosive action of the environment and especially of the process water with a high content of residual chlorine, chloramines, metal ions and dissolved oxygen. The following are some of the causes that can lead to the varnishing of containers with subsequent deterioration of the sheet metal:

• Deficient packing or palletizing in the dispatch of the containers that leave the cans or lids loose, in such a way that in their transfer to the processing plant it is easy to hit and friction between them, especially with the sharp flanges that are much more abrasive, this event leads to premature varnish removal. If to this scenario is added a sudden transport, it will be possible to verify even deformed containers or bent flanges that will finally end up in false closures, falls and spikes that compromise the hermeticity.
• Depalletizers, conveyors, aerial rod feeders and other unsuitable devices for transferring containers in production, which cause knocks between them, as well as scratches on the bottoms and bodies during transport to packaging, dosing, closing, etc.
• Poorly calibrated can seaming machine and worn seaming tooling, which results in external varnishing of the double seam due to varnish sticking to the profiles of the rollers, as well as loss of varnish on the side of the countersink due to slippage.
• Bulk or jumble arrangement of the cans in the autoclave cars, which causes some lids or bottoms of the containers to mark each other and lose varnish, this event is generated when the lids expand or dilate with the heating in the thermal process.
• The use of poorly designed or conditioned cooker&cooler rotary autoclaves, where the can, not being able to roll and remain static, drags and gets de-glossed.
• Incorrectly calibrated separator blades or cap feeder chain in the sealing machine, which causes scratches on the inside of the cap with loss of varnish. This event can leave the sheet metal exposed to the covering liquid inside the can causing dissolution of metal ions, in the case of iron the food can acquire a metallic taste, black iron sulphides, hydrogen production that shortens the shelf life of the can and in extreme cases with acidic or acidified foods perforate the can.

Contamination of the process water with condensate and boiler carry-over

Canning plants mostly use saturated steam for processes that require heating, such as cooking, blanching, blanching, blanching, preparation of packing media, hot packing, pasteurizing, sterilizing and other operations.

The saturated steam is generated in the boiler room and transported in carbon steel pipes to the equipment that require it, in this transfer pipes of several meters long are used, producing steam condensation on the way by pressure drop in the line, radiation, elbows, joints and other factors. Usually the condensates are recovered with thermodynamic traps and other devices to return them to the boiler room and reuse them by taking advantage of their high temperature or latent heat, but there are cases in which the condensates can contaminate with highly corrosive liquids the water used in the canning production processes and chemically damage the containers in areas where there is presence of varnish removal with exposed sheet metal.

The most extreme case of process water contamination is with alkaline aqueous foam from the boiler, also called boiler carryover. The following are some of the causes that allow condensate and boiler carryover to contaminate the process water:

• A common cause is that the pipes that transport the steam to the production areas do not have a condensate retention and return system, conditioned with a main manifold, accumulators, traps, bottles and pump that return it to the boiler room to take advantage of its remaining heat energy.
• Do not implement the “goose neck” assembly design in the steam connections to the equipment that require it, which consists of installing the outlet pipes at the top of the main or larger pipe and then down to the equipment with a “U”, so that the condensate remains at the bottom and at the end is retained in a bottle with a thermodynamic trap for its return or reuse.
• Heating water used in cooking food, preparing liquids, sauces and others with direct steam, which consists of injecting steam directly into the water with a perforated tube to bring it to a boil. The event is exacerbated when the equipment service lines come out of the bottom of the main or larger piping where the bulk of the condensate is located. In this case the use of boilers, steam jackets or heat exchangers that heat the water with indirect steam is recommended.
• One of the most aggressive drags is the one coming from the boiler, because it is saturated with chemical additives that keep it in a pH between 10 and 12, if this comes in direct contact with the can, in the autoclaves for example, it would detach the external varnish and oxidize the can. This type of carryover is caused by several factors, such as low pressure steam production in the boilers, excessive steam demand with sudden drop in line pressure, boilers producing with high water level, high foaming and few surface blowdowns.
• From the above it can be seen that there was not a good standardization of the plant in “peak production”, that is, when all the equipment that consumes steam is processing. The standardization considers; a thermodynamic balance that evaluates the balance between steam generation versus steam consumption, a good design in the steam piping distribution considering the cross-sectional areas and the line pressures in process.

Filling of the covering liquid in acid and acidified canned foods

In the case of acidic and acidified preserves, it is recommended to take great care of the integrity of the sanitary varnish on the cans and to use vacuum volumetric fillers for the dosage of the covering liquids in order to prevent the overflow from soaking the container bodies externally.

Inadequate can washing

The washing of the cans in canning plants is normally done after the sealing or placing of the lid, it is done continuously at the speed of the seaming machine and is carried out in washing machines equipped with nozzles or spray, which spray pressurized drinking water with a pump to the cans that circulate inside. Here are some recommendations to avoid external damage to the container during washing:

• Usually the transport of cylindrical cans inside the washing machines is done by rolling, on a rail made of stainless steel rods and in the case of irregular cans, such as flat, oval, club or those that cannot be rolled, conveyors with belts, volta or chains are used. In any of the two cases, it is necessary to avoid that the can is externally deburred, either by blow or rubbing on some welding residue, metal edge, badly tied rods, raised rivets of bands and other imperfections.
• There are many plants that use direct steam to heat the can wash water and install for this purpose, a pipe inside the water reservoir with several perforations to facilitate the exit of steam. In this case it is recommended to implement the gooseneck mounting design to the steam inlet, to avoid contamination of the wash water with condensate carryover and even worse with aqueous foam from the boiler.
• When filling cans with heavily pigmented sauces, such as tomato, mustard, curry and others, using the cascade or flute system that leave traces of sauce adhering to the body of the containers that quickly color the wash water, the can washer should be equipped with two water storage tanks connected to the recirculation pump, so that if the water in one reservoir becomes pigmented, the other reservoir tank can be enabled immediately without stopping the closure. It is important that the cans coming out of the washing machine are without any coloration and well washed, otherwise these color stains will be permanently fixed in the heat treatment.
• In the production of acidic and acidified canned food, the can wash water tends to acidify due to contamination with acidified brine (pH 2.2) that remains adhered to the body of the can. In this case it is recommended to monitor the turbidity and pH of the wash water, to change it when it becomes saturated with salts and acidified to pH 5 maximum.
• When oil preserves are produced, it is important to wash the excess that has been adhered externally in the body of the container, otherwise the fat will be strongly fixed in the heat treatment and will be very difficult to remove, also if the labeling is in line the label will be stained. In this case the washing water must contain a neutral soap that removes all traces of oil without being chemically aggressive to the container.

Heat treatment, sterilisation and cooling of preserves

Here are some recommendations to avoid the deterioration of the packaging at this stage of the process.

• In the assembly of autoclaves, pasteurizers or heaters, it should be considered to install the steam connection coming from the main pipe with the goose neck coupling, as we have already mentioned before, to avoid incorporating condensate drags and even worse, highly alkaline aqueous foam from the boiler into the equipment. Carryovers are critical in autoclaves that process with water spray and heat the process water stored in the base of the equipment with direct steam using a steam distributor tube or steam Spreader, carryovers contaminate the water with highly corrosive liquids and dissolved metal ions that oxidize the containers externally where there is no varnish.
• The design or standardization of conventional autoclaves and direct steam pasteurizers should consider installing traps, drains or bleeders in the lower part of the equipment that constantly eliminate the condensate that is generated during the entire thermal process. Under no circumstances should this condensate accumulate and reach the cans, since it will affect the container externally and the most critical thing is that the canned products involved will not meet the programmed commercial sterility, since the condensate is colder than the saturated steam.
• The water used both for the thermal process and for subsequent cooling must be treated to avoid the presence of organic matter and above all of microorganisms that can contaminate the canned food by infiltration. The Food and Drug Administration (FDA) recommends that this water be guaranteed to have trace amounts of measurable residual chlorine and usually 0.5 ppm is used to avoid the risk of external corrosion of the container.
• For the thermal treatment and cooling of canned food in metal containers, it is recommended to use soft water, with no more than 50 mg/L (50 ppm) CaCO3, the softer the better to avoid the typical hygroscopic calcareous white spots on the container lids, product of the precipitation of calcium and magnesium salts dissolved in the hard water, which is normally above 120 mg/L (120 ppm) CaCO3.
• The canned products that use water for pasteurized or sterilized, should use water as soft as possible to avoid galvanic corrosion, which is an electrochemical process where a metal corrodes when in contact with another type of metal, here galvanic current is produced by potential differential between the two metals that are in an electrolyte which is water, first the varnish comes off and then comes the corrosion. It should be noted that distilled water with zero hardness does not conduct electricity. Personally in this case, I recommend to use dual equipment, to process with direct steam ETP, TFS containers and with water aluminium, pouches, plastic, glass, etc.
• Cooling of canned foods after heat treatment should be carried out until the food reaches 40 °C (104 °F), firstly to avoid reactivation of thermophilic microorganisms such as Bacillus stearothermophilus and secondly so that the heat remaining in the can can evaporates or dissipates the remaining water after cooling.

Recirculation of the water used in the heat treatment and cooling of canned foods

The recirculated water after being used in the process ends up hot and is sent with a pump to a cooling tower, which is usually equipped with a forced air turbine that cools in countercurrent, the water finally falls into a pool where it is stored for treatment with germicides, such as hypochlorites or chlorine gas that eliminate microorganisms.

There are plants that install in the closed water recirculation circuit, filters to retain small pieces of organic matter coming from the packaged food, lids, rags and any foreign object that contaminates the water storage and treatment pools.