Retort Processing

INTRODUCTION  

The demand for ready-to-eat processed food is skyrocketing in emerging nations like India due to changing lifestyles, rising job demands, and nuclear families.
Thermal processing is one of the most important methods for producing packaged, shelf-stable food goods.
Food preservation through canning has advanced to include the retort processing of foods in semi-rigid, flexible laminates. It is a thermal procedure that gives products a longer shelf life while maintaining appropriate nutritional and sensory properties [1].

Types of Retort  

The three fundamental retort processes are

Steam
Total Immersion.
Falling Water
Each category has further subcategories for steam/air, steam spray, water spray, and partial immersion.
When choosing a particular packaging medium, it is evident that some principles of heat transmission to sealed containers have benefits over others [2].

Steam Retort  

The first technique for in-container sterilisation is the saturated steam process.
The technique requires the retort vessel to be saturated with steam since air is considered an insulating medium.
It is necessary for the procedure that all air be let out of the retort by filling the vessel with steam and opening vent valves.
Since air is not allowed to enter the vessel at any point during any sterilisation stage, there is no overpressure during the sterilisation steps of this procedure.
Air overpressure may be supplied throughout the cooling process to avoid container deformation [3].

Saturated steam retorts

Water Immersion Retort  

The water immersion technology is the overpressure technology most frequently used to sterilise products.
The product is completely shielded from any impact of cooling air during the water immersion process, similar to a saturated steam process in which the product is completely submerged in water.
In contrast to saturated steam, air can be injected into the vessel during sterilisation, adding air (or steam) on top of the water creates overpressure.
In some cases, the steam is heated by the addition of air; the hot air pressurises the processing load and stirs the water as it rises to the surface [4].

Water-immersion-retort

Water Spray Retort    

Like water immersion, the water spray method likewise uses overpressure, but the product is subjected to the effects of overpressure air.
In water spray retorts, steam is the driving force for getting to the centre of the load, and it is comparable to the saturated steam process.
Air can enter the vessel during sterilisation with the Water Spray method, unlike saturated steam, which cannot.
By injecting air (or steam) into the retort, overpressure is created. The spray nozzles vaporise the steam and combine it with the air to overcome the insulating properties of the air [5].

Water-spray-retorts

Retort Pouches    

The Reynolds Metals Company, Continental Flexible Packaging, and the United States Army Natick R&D Command developed the retort pouch.
A retort pouch is made of laminated plastic and metal foil with three or four wide seals typically produced through aseptic processing. This makes it possible to package a wide range of beverages in a sterile manner, including everything from water to fully prepared, thermostabilised meals like ready-to-eat meals.
Using this method, cooked or raw food is first prepared and then sealed in a retort pouch.
The pouch is heated inside retort or autoclave machines to 240–250°F (116–121°C) for several minutes under high pressure. This method reliably eliminates all typically present microorganisms, especially Clostridium botulinum, preventing spoilage [6].
Propylene, the first layer, is a heat-seal surface and offers strength and flexibility.
The nylon layer, which is the following layer, provides abrasion protection.
The aluminium coating enhances the product’s shelf life and shields it from light, gases, and odour.
Excellent strength is provided by the last polyester layer, which is also extremely simple to print on. In addition, FDA-approved materials are used for packaging retort pouches, and sterilisation procedures boost the packaging’s tensile strength [7].

Schematic diagram of a retort pouch

Advantages    

Disadvantages 

Not as durable as cans,
Higher packing costs lower processing line speed;
Higher investment cost;
The possibility of rodent and insect assault and challenging leakage detection [7].