3.6. Retort Pouch Processing (RPP)

Sterilization using heat is one of the most efficient methods of food preservation. The main objective of thermal sterilization is to kill all viable microorganisms including spores present in the food in order to achieve long-term shelf stability without the need for refrigeration. The application, however, of such severe heat treatments adversely affects the nutritional value of food including losses in vitamins and essential fatty acids and protein denaturation, particularly for products processed in metal or glass containers. Optimization of thermal processing conditions for minimizing nutrient loss without compromising the quality and safety of foods is a major issue for the food industry. One of the suitable options to overcome this problem is retort pouch processing [235,236,237].

The idea of retort pouches was promoted by the US army in the early 1950s and the research continued through the 1960s. It was finally invented by the United States Army Natick R & D Command, Reynolds Metals Company and Continental Flexible Packaging. These companies received the Food Technology Industrial Achievement Award for their invention in 1978.

Retortable flexible containers, usually in the form of pouches, are laminate structures that are thermally processed like a can or glass bottle. They are shelf-stable and can be stored at room temperature for a period of more than one year without the need for refrigeration. The most common form of pouch consists of a three-layer laminated material made (from outside to inside) of polyester/aluminum foil/cast polypropylene. Pouches made of polyester/aluminum foil/nylon/cast polypropylene are also available (Figure 4).

The polyester layer provides excellent strength and printability.

The aluminum protects from exposure to light, gases, moisture and odors and prolongs product shelf life.

The nylon layer protects from abrasion.

The polypropylene layer acts as a heat seal surface and provides strength and flexibility.

(a) Retort pouch structural construction (image courtesy: flairpackaging.com), (b) commercial retort pouch packaging for smoked salmon.

Based on the above characteristics, it is noteworthy to mention that retort pouches possess good mechanical and heat transfer properties, high gas barrier and efficient sealing properties [238]. The materials that go into the packaging of retort pouches are FDA-approved and undergo sterilization processes which increase the durability of the packaging. Retort pouches and their types such as stand-up pouches, spout pouches and zip-lock pouches are also commercially available. The food to be thermally processed is first prepared, either raw or cooked, and then sealed into the retort pouch. The pouch is then heated to 116–121 °C for several minutes under high pressure inside a retort or autoclave. The food inside is cooked in a similar way to pressure cooking. The processing of foods in a retort pouch involves a series of operations including food product preparation, weighing, automatic transport to pouch, pouch opening, filling of product, pouch sealing, retort loading, retorting, retort unloading, drying and cartoning. The process is, in many respects, analogous to canning with the tin can being replaced by a cheaper heat-resistant flexible pouch. In comparison to frozen foods, the retort pouch provides a longer shelf life and does not require refrigeration, energy and expensive methods of distribution and storage. Major advantages of the retort pouch include [239]:

The specific construction of the pouch provides rapid heat transfer for sterilization during processing. A 30–40% reduction in processing time is possible, with energy savings.

Reduced heat exposure maintains product taste, color and flavor while resulting in fewer nutrient losses.

Preparation of products that need to be heated to serving temperature can be accomplished in 3–5 min by immersing the pouch in boiling water or placing the plastic container in a microwave oven.

Shelf life of retort pouch products is equivalent to that of foods in metal cans.

Refrigeration or freezing is not required by packers, retailers or consumers.

Pouches and containers do not corrode externally and there is a minimum of product–container interaction.

Easy opening of the pouch.

Empty retort pouches and nesting containers offer processors a reduction in storage space and lighter weight. Compared to empty cans, an equal number of retort pouches use 85% less space and are significantly lighter.

Production of pouches uses less energy compared to metal containers.

Simpson et al. [240] developed and optimized a mathematical model for thermal processing of conduction-heated foods in retortable pouches. The model was validated utilizing jack mackerel. The prediction errors obtained in the validation study were under 5%. Non-significant differences were found between the experimental and predicted values. Simulations showed that a significant reduction in process time (20–30%) could be attained utilizing variable retort temperature profiles while maintaining product quality. Manju et al. [241] conducted a study on seer fish moilee processed in retort pouches. Air inside the pouch was exhausted by steam injection, heat-sealed and heat-processed at 121.1 °C to a sterilizing effect (Fo 8.15 and total process time of 48.3 min. The shelf life of samples stored at ambient temperature (27 °C) was 18 months, whereas that of samples stored at 37 °C was only 10 months. Work carried out at the Central Institute of Fisheries Technology in India showed that sardines packaged in retort pouches made of polyester/aluminum foil/cast polypropylene had a shelf life of 3 years [242]. Kuda et al. [243] compared the quality of several retorted fish products treated with (i) the common retort (CR) process (using 115 °C, for 1.5 h) or (ii) with the high-temperature (125 °C) short-time (9 min) process (HTST). Analysis of the ATP-related compounds in raw fishes and retorted fish models showed that inosine monophosphate (IMP) was higher in HTST fishes than in CR fishes. In contrast, inosine (HxR), hypoxanthine (Hx) and K-value, an index of fish freshness, were higher in CR fishes. Sensory analysis showed that product umami and sweetness in the HTST fish were stronger than those of the CR fish. The bitterness was stronger in the CR fish compared to that of the HTST fish. The authors concluded that HTST is a favorable process for retorted fish products. The effect of thin metal oxide-coated barrier materials on the quality of shelf-stable salmon was investigated by Byun et al. [244]. Four different retort pouch structures were used: cast polypropylene (CPP); polyethylene terephthalate (PET)/silicon oxide-coated nylon/CPP (SiOx); aluminum oxide-coated PET/nylon/CPP (AlOx); PET/aluminum foil/CPP (FOIL). TBARS was measured during storage. Salmon packaged in SiO_X pouches had a higher TBARS value than salmon packaged in FOIL pouches after 8 weeks of storage. In sensory testing, salmon packaged in SiO_X pouches were less acceptable than salmon packaged in FOIL pouches after the same period. In contrast, salmon packaged in AlO_X and FOIL pouches had similar sensory and TBARS values. Overall, shelf-stable salmon packaged in AlO_X and FOIL had comparable shelf lives, while salmon packaged in SiO_X had a significantly lower shelf life compared to AlO_X or FOIL. Bindu et al. [245] prepared and processed fish peera, a traditional product from anchovies, in a retort pouch in an overpressure autoclave to an F_o value of 7 and a cooking time of 66.02 min. Analysis of organoleptic, chemical and microbiological parameters showed that this method resulted in a shelf life of 1 year at a storage temperature of 28 °C. “Kalia”, a popular Rohu fish dish, was packaged in a four-layer laminated retort pouch and processed in a steam/air mixture overpressure retort at 121.1 °C to three different F_o values of 7, 8 and 9 min [246]. Based on organoleptic and textural properties and the absence of viable microorganisms during storage, an F_o value of 8 min with a total process time of 41.7 min at 121.1 °C was reported to be satisfactory for the preparation of Rohu fish curry in retort pouches. In a similar study, Majumdar et al. [247] packaged boneless fish balls from Rohu fish in retort pouches and processed them in an overpressure retort at 121.1 °C to three different F_o values of 6, 7 and 9 min. Based on commercial sterility, sensory evaluation, color and texture profile analysis, an F_o = 7 min and a total process time of 42.21 min at 121.1 °C were found to be satisfactory for maintaining product quality.

Bindu et al. [248] worked on the preservation of a ready-to-eat mussel meat product with the aim to retain its desirable sensory properties (natural texture and succulence). The product was vacuum-packaged and processed in a retort pouch in an overpressure retort. The total process time was 35 min to an F_{o =} 9.8 and a cooking value of 90.3 min. Stored at room temperature, the samples exhibited a 1-year shelf life and achieved a high sensory score. Mohan et al. [249] prepared prawn “kuruma”, a dish based on Indian white shrimp. The product was packaged in conventional aluminum cans and in retort pouches. The retort pouch resulted in a 35.7% reduction in process time compared to aluminum cans of equal pack weight. Product sensory and textural attributes (color, firmness, chewiness and overall acceptability) in retort pouches were superior to those in aluminum cans. Mallick et al. [250] prepared shrimp in curry medium (SICM) and thermally processed the product in retort pouches to three different F_o values, i.e., 5, 7 and 9. The respective cooking values obtained during the thermal processing of SICM were 59.20, 67.45 and 69.73 min. The sensory textural and color parameter values determined were in good correlation with those of the instrumental parameter values. The authors concluded that the study will help to standardize the F_o value in order to achieve optimum sensory characteristics for the retort pouch-processed product.

Tribuzi et al. [251] processed chopped mussel meat packaged in retort pouches using a water immersion retort (F_{o =} 7 min, retort temperature of 118 °C). Pretreated samples (salted and marinated) exhibited increased yield during 1-year storage at room temperature. There was no effect on the other physicochemical parameters. Freshwater prawn in curry was thermally processed by Majumdar et al. [252] to three F_o values of 6, 8 and 9 at 116 °C. Respective process times were 53, 57 and 63 min, and the cooking value (CV) was 87.53, 107.93 and 117.55 min. Texture profile analysis showed that most textural parameters decreased with increasing F_o values. On the other hand, color parameter values increased with increasing F_o values. Optimum sensory scores were obtained when the product was processed to F_o 7 min. Sreelakshmi et al. [253] developed a ready-to-eat sandwich spread from the meat of mud crab. The product was thermally processed in retortable pouches in an overpressure retort at temperatures of 111.1, 116.1 and 121.1 °C, to F_o values of 5, 6 and 7 min. The process was optimized by evaluating the samples for texture, color, commercial sterility, TBA value and sensory testing. All samples were found to be acceptable based on these tests. The sample processed at 116.1 °C for 6 min scored the highest, with a cooking value of 84.29 and a total process time of 42.59 min. A ready-to-eat thermally processed black clam product was developed by Bindu et al. [254], retaining its desirable natural texture and succulence. The product was vacuum-packaged in an in-house developed retortable pouch and processed in a still overpressure retort. The total process time was 44 min with an F_o value of 9 and a cooking value of 99 min. The product was rated as excellent by a sensory panel and had a shelf life of 12 months at ambient temperature (28 °C).

In conclusion, it should be noted that all above innovative processing technologies are applied in combination with innovative packaging technologies such as modified atmosphere packaging, vacuum packaging, active packaging, intelligent packaging and biodegradable packaging, both at the bulk and retail packaging levels. This final stage of processing will be dealt with, in detail, in a separate review article.