A Descriptive Study of Vitamin and Mineral Losses in Canned Vegetables

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2.1.3 Further processes

After blanching, the vegetables are peeled and cored, if required, and placed inside bottles or cans. The containers are vacuumed and hermetically sealed, ensuring the product remains sterile even after processing without needing to be refrigerated constantly, before they are sent to the next step to be cooked and packaged (Murano, 2003).

2.1.4 Cooking and Packaging

The hermetically sealed containers are then subjected to high temperatures. This process serves to cook the vegetables, as well as deactivate any remaining enzymes that will cause the vegetable to further ripen, as well as kill and remaining microorganisms that inhabit the plant matter. An additional response from the researchers’ interview states that because some microorganisms tend to be very hardy, the vegetables may be subjected to temperatures surmounting 120 °C. After this process is finished the containers are then cooled, and sealed completely air-tight.

The modern tin can is made of a ‘cold-reduced, low carbon steel,’ coated on both sides by a thin layer of commercial grade tin. “Plain internal tinplate cans are used for specific food types, including tomatoes and other tomato-based products, white fruits and some vegetables (e.g. mushrooms, asparagus). They are used, in preference to lacquered cans, in situations where a small lacquer discontinuity (e.g. scratch) would result in concentrated attack of the base steel (the small area of tin would quickly disappear) and could potentially lead to pinholing and microbial ingress.” The open-top can is made by interlocking the thin sheet metal about the base of the can, crimped, sealed using polymer sealants, and is soldered from the outside for reinforcement. After the produce is harvested, they are cleaned using high pressure sprays of water or submerged into tanks of water and cut, peeled, pureed, etc. before being blanched (submerged into very hot water or steamed for a very short time), this process is not intended to cook, but is usually done to create the desired texture in the produce or eliminate any unwanted flavours or smells (De Corcuera et al., n.d.). The cans are then filled with the food item along with its sauces or brine (saltwater) using filling machines to remove as much standing air in the can as possible. The lid is then placed on the top, sealed hermetically (usually by flattening it into place and then using a sealing compound) and the canned food is placed in pressure cookers and is finally subjected to very high temperatures of 116° C to cook the contents and to kill any undesirable microorganisms present. Most of the nutritional loss can be traced to the blanching process (De Corcuera et al., n.d.) .

2.2.0 Vitamin and mineral retention

2.2.1 Vitamin A Retention

According to Lešková et al. (2006), vitamin A is un-reactive under an inert atmosphere, but it loses its activity very quickly when heated in the presence of oxygen, especially at higher temperatures. Carotenoids are very prone to degradation. Their highly unsaturated structure makes them sensitive to heat, oxygen, and light; also, β-carotene is more prone to degradation than α-carotene. From the given heat treatments boiling seems to be the most damageable process. The loss of vitamin A can reach up to 67%. Meanwhile, frying could be said to be gentler as it was in the case of vitamin A. There were about 80% retention of β-carotene in vegetables. Similar findings can be observed during steaming, with losses ranging from 13-20% only. Results explicitly showed that the highest retention was obtained when the vegetables were cooked without any addition of water and that the lowest retention was related with the use of a large amount of water during cooking. The blanching of different vegetables before canning can diminish the level of β-carotene by around 20%

2.2.2 Vitamin B Retention

The vitamin B group consists of thiamin, riboflavin, niacin, vitamin B6, and folate. Among the vitamin B family, thiamin is the most sensitive to water, causing significant losses of up to 66% in canned spinach. With vitamin B2 (riboflavin), experiments show that lentils and asparagus can lose 50% or more of its original riboflavin content due to canning. Vitamin B3 (niacin) is mostly stable and resistant to degradation in canning, showing a retention percentage of 93% or higher in green peas, green beans, peaches and sweet potatoes. Vitamin B6 is a bit of an oddity as the amount of Vitamin B6 retained can range from 54% in mushrooms to 80% in lentils. Tomatoes, if under the proper condition, can even have its vitamin B6 increased by up to 38%. The study suggest that this is due to the structure of the tomato which “locks” away some of the vitamin B6. However, in general, vegetables contain 55-77% less vitamin B6 than their fresh counterparts.

Jirantanan and Liu (2014) found that canning beets caused a 30% loss of folate (vitamin B9) in canned beets, but none when in canned green beans. Because of the lack of information on folate retention in canned food, results are not conclusive as to what degree canning can affect folate. However, Jiratanan and Liu suggests that a packing medium in the can (in their case, water) may create a suitable environment for the retention of folate.

2.2.3 Vitamin C Retention

Vitamin C, commonly known as ascorbic acid, is soluble in water and sensitive to thermal conditions. As a consequence of its water solubility and thermal sensitivity, vital steps in canning gravely affect the Vitamin C content inside vegetables. Recent, as well as classical, studies have shown that vegetables that have undergone thermal processing, had their vitamin C content dropped by 85-90% on average in canned carrots and broccoli, with a 30-50% decrease in other vegetables like asparagus, lentils, and tomatoes (Rickman et al., 2007). While these studies only account for the “dry” weight of the vegetables—the canning fluids found alongside the vegetables drained off—USDA data shows that the canning liquid contains little to no amount of vitamin C anyway.

2.2.4 Vitamin E Retention

Among the common sources of vitamin E, spinach, potatoes, and tomatoes, tomato and tomato-based products have been the most studied produce in terms of nutritional retention (Rickman et al., 2008). α-tocopherol (Vitamin E) has been shown to degrade under a heated environment. α-tocopherolin tomato paste, have been shown to decrease by approximately 20.3%; however, α-tocopherol content in whole tomatoes have been