The canning process is adjusted depending on types, varieties, sizes, and period of maturation of pulses. Two distinctly different types of canned peas are canned fresh peas and canned dry peas. The former is produced exclusively from peas harvested at an early stage of maturity. These peas, called vining peas, are extremely perishable. They must be processed in the cannery within a few hours from the time of harvesting to retain their excellent sensory properties and prevent preprocess microbial spoilage. Canned peas, on the other hand, are manufactured using dry peas that have been allowed to mature fully in the field before harvesting. In the dry state, these peas are perfectly stable and may be held for a long period until canning.
Manufacturers of canned peas enjoy certain production advantages not available to those producing canned fresh peas. The stability of dry peas, for example, permits manufacturers to can peas throughout the year. Doubling their weight during processing, dry peas, typically of the round-seeded green variety if grown in the U.S., also offer a comparatively inexpensive raw source of protein and dietary fiber. The manufacturing process for canned peas is similar to that used for fresh peas; however, there is a pre-soaking operation used to rehydrate the dry peas prior to blanching.
Dry peas are soaked for 10–15 hours in tanks constructed from stainless, galvanized steel or a suitable plastic material. Soak time is adjusted depending on the moisture content of dry peas. During the soaking period, the peas swell and absorb enough water to account for 95 - 110 % of the dry pea weight. The water temperature is ideally less than 68 ° Fahrenheit (20 ° Celsius). Since wet peas are an excellent medium for microbial growth, warm weather often makes it necessary to change the water once or twice to prevent a souring of the peas. The peas increase in volume as they absorb water and it is important that they continue to be covered with water during soaking. The soaking tanks should not be so large that the peas at the bottom are compressed, which will prevent them from swelling. About 4 tons is the maximum recommended tank size.
Excessive calcium levels in the soaking water may cause changes within the pea structure, resulting in excessive hardness of texture. Hard water may cause extended cook time or hard seed (Lopez, 1981). To avoid this, it is necessary to adjust the hardness of water. Water that is too soft can cause splitting of the peas and mushiness within the can. It is also important to remember that although the majority of water absorption take place during the soaking process, it does continue to a lesser extent during blanching and canning.
Blanching is an important operation within the manufacture of processed peas, accomplishing the following:
- Cleaning of the peas
- Reduction in microbial count on the peas
- Destruction of enzymes that can promote chemical deterioration of the peas
- Additional water uptake prior to filling
The blancher typically comprises a large steel vessel partially filled with hot water. Peas enter at one end and are contained behind a perforated screen running the length of the vessel. They are sent to the outlet end by a rotating helical screw. The speed at which the screw rotates controls the blanching time. Also important is control of the water temperature. The growth of thermophilic microorganisms can occur at blanch temperature, and give rise to souring. Typical blanching is done at water temperature of 190 – 199° Fahrenheit (88 – 93° Celsius) for 4 to 6 minutes. Though usually unnecessary, blanching water, as with the soak water, may be partially softened.
Brine Preparation and Filling
Brine is prepared by dissolving the required ingredients in water in an appropriate, preferably stainless steel, steam-jacketed kettle. The holding time of the brine before filling should not exceed 45 minutes or color degradation will result. Brine is generally filled into the cans and the weight should be adjusted accordingly based on trials. It is important to keep in mind that soaking and blanching time affects filling weight as water absorbed during canning varies depending on the moisture of peas before canning. After filling with brine, the can is tilted to provide a specified headspace before double seaming. The headspace and filling temperature should be sufficient to produce an internal vacuum within the can. The overflowed brine is screened and recirculated.
Sugar and salt are used in the brine covering for canned peas. They should be a food-grade quality. International CODEX Standard for white sugar and salt are specified in CODEX STAN 4-1981 and 150-1985, respectively. These provide detailed quality factors and permissible levels of contaminants.
Synthetic and natural coloring choices are important in the production process as well. Blue and yellow are common as a means for producing the appropriate green color. Dye companies generally provide a pre-blended mixture to meet individual requirements. Natural colors invariably do not have the heat stability to provide a suitable green color within the finished product thus synthetic dye may be preferred. Most countries have legislation controlling such colors.
Cans of peas are packed into crates, which are placed inside a suitable pressure-sterilized retort (i.e., a sterilizer used for canned food). Sterilization is a crucial step. For all low acid foods (pH >4.6), it is essential that a thermal process that is sufficient to achieve commercial sterility is used (i.e., destruction of all pathogens and all other microorganisms capable of metabolism at the intended product storage temperature and conditions). All commercial processes should be validated by heat penetration tests. Also, if it becomes necessary to reprocess a batch of cans due to steam failure or some other reason, the original process times may no longer be sufficient as the material viscosity within the cans will have markedly changed.
After sterilization has been achieved, cooling water is introduced into the retort so that the cans are cooled as rapidly as possible, thereby preventing undue product degradation. Because it is vital that the cooling water is of good microbiological quality, it is normal that the water should be disinfected by the addition of chlorine gas or another suitable chlorine compound. Immediately after water cooling, the temperature of the cans and their contents should be 104 to 122 ° Fahrenheit (40 to 50 ° Celsius). The temperature should be cool enough to inhibit the growth of any surviving thermophilic organisms but warm enough for the cans to dry. Once the crates of cans are removed from the retort, they should be tipped—while still wet—to remove water from the can end. Under no conditions should wet cans be manually handled due to a heightened risk of contamination of can surface.