Lesson- 14 Retort Pouch
The retort pouch is a rectangular, flexible, laminated plastic, four-side hermetically sealed pouch in which food is thermally processed. It is a lightweight, high-quality, durable, convenient and shelf stable packs. Foods packed and processed in retort pouches are in successful commercial use for a wide variety of foodstuffs in several countries, particularly Japan. The materials from which retort pouches are made are either aluminum foil bearing/plastic laminates or foil-free plastic laminate films. Retortable pouches can be either preformed or in-line formed using form/fill/seal equipment. Common pouch structures are PET/nylon/foil/PP and PET/nylon/EVOH or PVDC/PP. Retortable pouches are used by hotels, restaurants, and other institutions.
Retortable pouches must be inert, heat sealable, dimensionally stable and heat resistant to at least 121°C for typical process times. They should have low oxygen and water vapor permeability, be physically strong and have good ageing properties. Retail consumer products such as tuna, salmon, chicken patties, chipped beef, chili, and ground beef in retortable pouches have become available. Pouches are reverse printed in a wide range of graphics on the PET film before lamination, so that the print cannot come into contact with the food. All laminates are required to meet very stringent requirements to ensure no undesirable substances can be extracted into the packaged food.
14.1. Manufacturing of pouches
Pouches can either be formed from reels of laminated material either on in-line form/fill/seal machines in the packer’s plant or they may be obtained as preformed individual pouches sealed on three sides, cut and notched. Forming consists of folding the laminate material in the middle, polyester (or PA) side out, heat sealing the bottom and side seals and cutting to present a completed pouch. Alternatively two webs can be joined, heat seal surfaces face to face, sealed, cut and separated. Hot bar sealing is the most common practice. Notches are made in the side seal at the top or bottom to facilitate opening by the consumer. Modern pouches have cut rounded corners which reduce the possibility of perforation caused by pouch to pouch contact. Rounded corner seals can also be incorporated.
The four-seal flat shape and thin cross section of the pouch is designed to take advantage of rapid heat penetration during sterilization and on reheating, prior to consumption, saving energy and providing convenience. The flat shape also enables ease of heat sealing and promotes high seal integrity. Fin seal design and certain gusset features permit the design of upright standing pouches although they create multiple seal junctions with increased possibility of seal defects. Several of these upstanding pouches are, however, available commercially. A wide range is possible in the size and capacity of pouches.
14.2. Filling and sealing
The premade pouches are filled vertically in-line. Vertical form/fill/seal machines can be used for liquid products. Another method employs a web of pouch material which is formed on a horizontal bed into several adjacent cavities. The cavities are filled whilst the seal areas are shielded. This method is especially useful for filling placeable products. Thereafter the filled cavities are simultaneously sealed from the top using a second web fed from the reel. The essential requirements for filling are:
- Pouch should be cleaned, fully opened to the filling station, solids are filled first followed by the liquid food at a second station
- Matching fill-nozzle design and filler proportioning to the product
- Non-drip nozzles are used for filling
- Shielding of the sealing surfaces
- Bottom to top filling
- Specification and control of weight consistent with the maximum pouch thickness requirement
- Product consistency in formulation, temperature and viscosity
- Deaeration prior to filling.
Sealing machines like fillers are constantly being refined and speed has improved from 30 to 60 pouches per minute to the current production rate of 120–150 pouches per minute. Sealers incorporate either one of two common satisfactory sealing methods namely hot bar and impulse sealing. Both methods create a fused seal whilst the pouch material is clamped between opposing jaws, thereby welding the opposing seal surfaces by applying heat and pressure. Exact pouch-sealing conditions depend on the materials and machinery used.
14.3. Quality assurance
A successful pouch packaging quality system requires:
- Selection and continued monitoring of the most suitable laminate materials.
- Regular testing of formed pouches for seal strength, product resistance and freedom from taint.
- Careful selection, maintenance and control of filling, sealing, processing and handling machinery.
- Specifications for the control of product formulation, preparation (viscosity, aeration, fill temperature etc.) and filling (ingoing mass and absence of seal contamination).
- Post sealing inspection and testing of closure seals to confirm fusion, absence of defects and contamination.
- Control of critical parameters influencing processing lethality such as maximum pouch thickness and residual air content.
- Standardized retorting procedures applying only recommended process times and temperatures confirmed to achieve adequate lethality.
- Regular inspection and testing of retort equipment and controls to ensure uniform heat distribution.
- Visual inspection of all pouches to check sealing after processing.
- Handling only of dry pouches and packing into collective or individual outer packaging specially tested to provide adequate, subsequent, abuse resistance.
- It should be routine that all stocks are held 10–14 days prior to distribution and these should be free of blown spoilage on dispatch.
- Careful staff selection and training at all levels.
14.4. Shelf life
Whilst shelf life is determined by many factors such as storage temperature and the barrier properties of the particular film used, in general, satisfactory shelf stability in excess of two years is easily obtained for a wide range of products in foil bearing pouches. US military rations tested over two years at 20°C showed no significant change in product quality ratings. Some products have been successfully stored for as long as seven years and found to be safe and edible.
Foil-free laminates will demonstrate shelf stability commensurate with oxygen permeability of the particular laminate used and the sensitivity of the product. Commercial experience confirms, however, that product stability from four weeks to six months is obtainable. Nitrogen flushing of the outer container has been successful in extending the shelf life of product in foil-free pouches.