Do vitamin enriched foods preserve their value when exposed to higher temperatures?

Do vitamin enriched foods preserve their value when exposed to higher temperatures?

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At what temperatures do different kinds of vitamins are destroyed or lose their nutritional value?

Imagine you went to the store and bought vitamin enriched cacao powder. Then you made yourself a hot drink by pouring boiling water. How much in general(approximately) vitamins were destroyed by that heat.

"How to prepare/cook" usually doesn't have this kind of information. Thus any benefits of vitamins are lost(?).

Here's one estimation about how much vitamins or minerals can get lost due to various "food processing" (cooking, drying, freezing) methods:

According to NutritionData, cooking can result in a loss of vitamins:

  • Vitamin A: 25%
  • Vitamin C: 50 %
  • Vitamin B complex vitamins: 25-70%
  • Minerals: 25-70%; most important: potassium loss in cooking water. In this study with potatoes:

Leaching alone did not significantly reduce levels of potassium or other minerals in tubers. Boiling tuber cubes and shredded tubers decreased potassium levels by 50% and 75%, respectively.

According to USDA retention factors, the following vitamins are most affected by food processing:

  • Vitamin C
  • Vitamin B1, B6, B12 and folate

Vitamin and mineral loss is related to both the temperature and time of cooking.

Mineral loss can be prevented by cooking in vapour (steaming) instead of cooking in water.

According to this source, the following can be loses after heat treatment at 70 °C and (90 °C):

  • Vitamin A: 10% (30-40%)
  • Vitamin D: 15% (35%)
  • Vitamin B1: 15% (50%)
  • Folic acid: 5-20% (45%)
  • Vitamin C: 40% (85%)

Food Fortification: Technological Aspects

What is Food Fortification?

The Codex Alimentarius defines food fortification or enrichment as the addition of micronutrients to foods, whether or not they are normally contained in the food, for the purposes of preventing or correcting a demonstrated deficiency. The Codex also mentions that the amount of micronutrients to add should be sufficient to correct or prevent the deficiency when the food is consumed in normal amounts by the population at risk, but not likely to result in excessive intakes by individuals with a high intake of the fortified food. As stated, these recommendations are applicable to single foods.

The WHO/FAO proposed a more appropriate definition in the Guidelines on Food Fortification with Micronutrients, which focuses on the diet rather than on single foods. A single food may contribute toward improving the nutritional quality of the food supply but may not necessarily be sufficient as the only solution to prevent a micronutrient deficiency. This is the concept adopted in this chapter.

In food technology, food fortification and food enrichment have different meanings: fortification is reserved for the addition of micronutrients to a food that does not contain those compounds naturally, whereas enrichment is applicable when the natural contents of some micronutrients normally available in the food are intentionally increased. Two related terms are frequently used: restoration, when micronutrients are added to recover the original levels in a food that has partially or totally lost them during processing, for example, adding vitamin A and D to defatted milk to reproduce the content of those vitamins in whole milk, and nutritional equivalence, when the content of micronutrients of a manufactured food is modified to imitate the content of a natural food that is intended to be replaced, as, for example, adding vitamin A and D to margarine to achieve their natural contents in butter.

Micronutrients are vitamins and minerals that are required by humans in very small amounts most of them cannot be synthesized by the human body and therefore they should be obtained directly from the diet. The chemical sources of micronutrients used in food fortification are called fortificants. Thus, for example, ferrous sulfate, ferrous fumarate, and NaFeEDTA are fortificants used to increase the content of iron in foods. Fortificants are generally added to foods as part of premixes, which constitute the main ingredients in the fortification process.


Additive — A chemical compound that is added to foods to give them some desirable quality, such as preventing them from spoiling.

Antioxidant — A chemical compound that has the ability to prevent the oxidation of substances with which it is associated.

Curing — A term used for various methods of preserving foods, most commonly by treating them with salt or sugar.

Dehydration — The removal of water from a material.

Fermentation — A chemical reaction in which sugars are converted to organic acids.

Irradiation — The process by which some substance, such as a food, is exposed to some form of radiation, such as gamma rays or x rays.

Oxidation — A chemical reaction in which oxygen reacts with some other substance.

Pasteurization — A method for treating milk and other liquids by heating them to a high enough temperature for a long enough period of time to kill or inactivate any pathogens present in the liquid.

Pathogen — A disease causing microorganism such as a mold or a bacterium.

are capable of moving a minimum of 1,000 cans per minute through the sealing operation.

The majority of food preservation operations used today also employ some kind of chemical additive to reduce spoilage. Of the many dozens of chemical additives available, all are designed either to kill or retard the growth of pathogens or to prevent or retard chemical reactions that result in the oxidation of foods. Some familiar examples of the former class of food additives are sodium benzoate and benzoic acid calcium, sodium propionate, and propionic acid calcium, potassium, sodium sorbate, and sorbic acid and sodium and potassium sulfite. Examples of the latter class of additives include calcium, sodium ascorbate, and ascorbic acid (vitamin C) butylated hydroxyanisole (BHA) and buty-lated hydroxytoluene (BHT) lecithin and sodium and potassium sulfite and sulfur dioxide.

A special class of additives that reduce oxidation is known as the sequestrants. Sequestrants are compounds that “ capture ” metallic ions, such as those of copper, iron, and nickel, and remove them from contact with foods. The removal of these ions helps preserve foods because in their free state they increase the rate at which oxidation of foods takes place. Some examples of sequestrants used as food preservatives are ethylenediamine-tetraacetic acid (EDTA), citric acid, sorbitol, and tartaric acid.

Food Safety in the Food Industry

The Hazard Analysis Critical Control Points (HACCP) is a program within the food industry designed to promote food safety and prevent contamination by identifying all areas in food production and retail where contamination could occur. Companies and retailers determine the points during processing, packaging, shipping, or shelving where potential contamination may occur.. Those companies or retailers must then establish critical control points to prevent, control, or eliminate the potential for food contamination. The Canadian Food Inspection Agency supports the food industry to follow HACCP to ensure the safety of food in various sectors.

Everyday Connection

  1. Conduct a hazard analysis: The manufacturer must first determine any food safety hazards (ex. biological, chemicals, or physical) and identify preventative measures to control the hazards.
  2. Identify the critical control points: Critical control point (CCP) is a point or procedure in food manufacturing where control can be applied to prevent or eliminate food hazards that may cause the food to be unsafe.
  3. Establish critical limits: A critical limit is the maximum or minimum value that a food hazard must be controlled at a CCP to prevent, eliminate or reduce it to an acceptable level.
  4. Establish monitoring requirements: The manufacture must establish procedures to monitor the control points to ensure the process is under control and not above the CCP.
  5. Establish corrective actions: Corrective actions are needed when monitoring indicates a deviation from the established critical limit to ensure that no produce injurious to health has occurred as a result of the deviation.
  6. Establish verification procedures: Verification ensures that the HACCP plan is adequate with CCP records, critical limits and microbial sampling and analysis.
  7. Record keeping procedure: The manufacturer must maintain certain documents including its hazard analysis, HACCP plan, and records monitoring the CCP, critical limits, and the verification of handling processed deviations.

The Effect of Storage Method on the Vitamin C Content in Some Tropical Fruit Juices

Loss in vitamin C contents of some fruit juices namely, orange, lemon, lime, pineapple, paw-paw and carrot stored under different conditions was investigated. The juice from the fruit samples were extracted, stored at room temperature (29±1°C) in plastic bottles and in the refrigerator (4±1°C) for 4 weeks. The juices were all analysed for their vitamin C content by oxidation and reduction method. Results revealed that the rate at which vitamin C is lost during storage depends on the type of fruit and the storage method employed. The citrus fruits were found to follow a similar pattern of loss, while other fruits differ from this and among themselves. Loss of vitamin C correlated with pH only for pineapple, pawpaw and carrot, however, this cannot be said to be the controlling factor. Bacillus subtilis and Candida sp. were isolated from all the juices under both storage conditions, except for orange juice.

How to cite this article:

V.O. Ajibola, O.A. Babatunde and S. Suleiman, 2009. The Effect of Storage Method on the Vitamin C Content in Some Tropical Fruit Juices. Trends in Applied Sciences Research, 4: 79-84.

Vitamin C (also referred to as L-ascorbic acid) is the lactone 2,3-dienol-L-gluconic acid and it belongs to the water-soluble class of vitamins. Ascorbic acid is an odourless, white solid having the chemical formula C 6 H 8 O 6 . Vitamin C is mainly found in fruits and vegetables. In the nutritional content, vitamin C is the L-enantiomic form of ascorbic acid which also encompasses the oxidation product of dehydroascorbic acid with different oxidizing agent. It participates in numerous biochemical reactions, suggesting that vitamin C is important for every body process from bone formation to scar tissue repair (Rickman et al ., 2007). The only established role of the vitamin C appears to be in curing or preventing scurvy and it is the major water-soluble antioxidant within the body.

Factors that affect the vitamin C contents of citrus fruits include, production factors and climate conditions, maturity state and position on the tree, type of fruits (species and variety), handling and storage, type of container (Naggy, 1980). Immature fruit has the highest levels and decreases during the ripening process. Early maturing varieties have higher levels than late maturing types. High nitrogen fertilizer rates can lower vitamin C levels in citrus fruits. Proper potassium levels are also needed for good vitamin C level (Padayatty et al ., 2003).

Pasteur identified the growth micro organisms such as bacteria and fungi as the scientific cause of spoilage and decay in the 1860s, other causes include chemical changes from ripening and senescence (aging) processes occurring in the fruit. Bacteria and fungi are everywhere in our environment and most foods provide an excellent substrate ( for their growth (Manso et al ., 2001). Vitamin C bears an obvious structural similarity with hexose sugars hence, it is conceivable that the molecule might serve as a carbon source for respiration or bacterial growth that it might be fermented (Eddy and Ingram, 1953). Storage conditions of low temperature and humidity have been found to retard microbial growth chemical and biological processes are also slowed down (Manso et al ., 2001 However, once these protective barriers are breached, microbial growth is often unchecked and rapidly destroys the commodity. The flavour, texture and nutrition of many fruits and vegetables are reduced before visual appearance of spoilage (María Gil et al ., 2006).

Oxygen is the most destructive ingredient in juice causing degradation of vitamin C. However, one of the major sugar found in orange juice, fructose, can also cause vitamin C breakdown. The higher the fructose content, the greater the loss of vitamin C. Conversely, higher acid level of citric acid and malic acid stabilize vitamin C (Padayatty et al ., 2003). Canned juices are often regarded as less nutritious than fresh or frozen products therefore the preference for fresh/preserved fruits in this country. This necessitated this study on the effects of storage on the quality of some common fruits using vitamin C as the reference.

Sample Collection and Preparation
Fresh fruits of Citrus sinensis (orange), Citrus limon (lemon), Citrus aurantifolia (lime), Ananus comosus (pineapple), Asimina triloba (pawpaw) and carrot were purchased from retail outlets in Zaria, a Northern Nigerian city. The study was carried out in Ahmadu Bello Univeristy, Zaria-Nigeria between March and June 2007. These fruits were washed thoroughly with water and the juices were extracted by mechanical pressure. Each type of juice samples was filtered to remove pulp and seeds and stored in already labelled plastic containers.

All chemicals used were obtained from BDH London, unless otherwise stated were of analytical grade purity and double distilled water was used.

One percent starch indicator solution was prepared by adding 0.50 g of soluble starch in 50 mL of near-boiling water.

Iodine solution was prepared by dissolving 5.0 g of potassium iodide (KI) and 0.268 g of potassium iodate (KIO 3 ) in 200 mL of water followed by addition of 3 M sulphuric acid. The solution was made up to 500 mL in a graduated cylinder and then transferred to a beaker.

Vitamin C standard solution was prepared by dissolving 0.250 g of vitamin C in 100 mL of water and then diluted to 250 mL with water in a volumetric flask.

Vitamin C Determination by Iodine Titration
Oxidation-reduction method described by Helmenstine (2008)( was used.

Standardizing Solutions and Titration of Juice Samples
vitamin C solution (25 mL) was transferred into 100 mL conical flask and 10 drops of starch solution was added. This was titrated with the iodine solution until the first blue colour which persisted for about 20 sec was observed. Juice samples (25 mL) were titrated exactly the same way as the standard. The initial and final volume of iodine solution required to produce the colour change at the endpoint was recorded. Titration was performed in triplicate in all cases.

Microbial Test
The samples were cultured on blood agar medium, incubated at 37°C for 24 h, the colonies of the organisms were gram stained, biochemical tests were carried out to identify the bacteria, according to the method described by Singleton (1999). The yeast identification was performed with fluoroplate candida agar according to the method of Manafi and Willinger (1991).

The retention of vitamin C is often used as an estimate for the overall nutrient retention of food products because it is by far the least stable nutrient it is highly sensitive to oxidation and leaching into water-soluble media during storage (Davey et al ., 2000 Franke et al ., 2004). It begins to degrade immediately after harvest and degrades steadily during prolonged storage (Murcia et al ., 2000) and also continues to degrade during prolonged storage of frozen products (Rickman et al ., 2007). Results for the freshly squeezed fruits shows that the oranges had the highest vitamin C content, followed by lemons, limes, pineapple, pawpaw and carrot. The values obtained for citrus fruits are quite lower than values obtained elsewhere ( This is consistent with reports that, climate, especially temperature affect vitamin C level. Areas with cool nights produce citrus fruits with higher vitamin C levels. Hot tropical areas produce fruit with lower levels of vitamin C (Padayatty et al ., 2003). Also, environmental conditions that increase the acidity of citrus fruits also increase vitamin C levels.

The results have shown that the environment in which juice is stored can affect its vitamin C content significantly (Fig. 1). The pattern of loss in vitamin C showed an initial increase in the first two weeks followed by decrease in orange samples RT. The RC samples decreased initially, followed by an increase and then a decrease. The concentration of vitamin C decreased faster in RC than in RT samples, however, the same pattern was observed throughout the four weeks of storage. The reason for the initial increases is not understood, but Rickman et al . (2007) attributed this to a change in moisture content during the storage of frozen peas.

The trend in the concentration of vitamin C for the lemon samples, over the period of investigation is similar to that observed for oranges. There was an initial decrease, then an increase at two weeks and then a decrease. The difference in the concentration of vitamin C between RT and RC at any particular time is not much. The result also showed that more vitamin C is lost in lemon over this period than in oranges. For the lime sample the pattern of decrease differ slightly for the RC samples. The initial increase in the vitamin C content was not observed for RC samples. However, like the orange and lemon the RC samples lost more vitamin C than the RT samples.

Light exposure was found to promote browning in pineapple juice. Ten percent losses in vitamin C have been reported after 6 days at 5°C in pineapple pieces by María Gil et al . (2006). Pineapple samples showed a different pattern of decrease in vitamin C content compared to the citrus fruits. Here, the RC samples retained more vitamin C than the RT samples after four weeks of storage. The initial increase in the vitamin C content observed in the citrus fruits was not observed with the pineapple sample. This suggests that variation in moisture content cannot be the sole controlling factor leading to the initial increase observed in the citrus fruits. Again no reason can be proffered from this investigation why the retention of vitamin C is more in the RC samples than in the RT samples. Since, vitamin C is unstable in neutral and alkaline environments therefore the longer the exposure, the greater the loss of vitamin C. The increase in pH (Table 1) was related to deterioration of fruit characteristics (María Gil et al ., 2006).

The RT pawpaw sample showed a rapid initial decrease in vitamin C content within the first two weeks. At this period the RC sample showed a steady decrease with vitamin C content higher than RT. By the third week Vitamin C content in RT increased above RC after which, both RT and RC decreased very rapidly, with RC tending towards zero vitamin C content. Finally, in the carrot sample, RT and RC decreased rapidly at first with RC retaining more vitamin C up to the second week. After the second week, difference in vitamin C content between RT and RC became very small, both decreasing till the fourth week.

Variation of vitamin C content of fruits with time and mode of storage (a) Orange, (b) Lemon, (c) Lime, (d) Pineapple, (e) Paw paw and (f) Carrot

Many chemical reactions contribute to the loss of storage life of vitamin C and hence chemical deterioration of fruits. The majority of these reactions are enzymatically driven while others are chemical reactions that occur because of the senescence (aging) processes. This involves colour, flavour, and odour changes that result from a chemical reaction between the constituents of the fruit. Fruit can be a vector and provide a growth medium for many pathogenic microbes which can produce potent toxins. In this study Bacillus subtilis and Candida sp. were isolated from both RT and RC of all the fruits used in this investigation, except in orange where only Candida sp. was isolated. Bacillus subtilis is not considered a human pathogen it produces the proteolytic enzyme subtilisin (a protein-digesting enzyme) and has been implicated in food poisoning and spoilage (Ryan and Sherris, 1994). Candida albicans sp. (yeast) has been reported as the causative agent of spoilage of sugary foods, such as condensed milk, fruit juices and concentrates (Stratford et al ., 2002). The biochemical reactions occurring over the storage period together with microbial action in all fruit juices resulted in pH changes observed (Table 1). Two-tailed Spearman’s correlation showed that there is a significant correlation between pH and vitamin C at 95% confidence level for RT samples of pineapple (r 2 = 0.74), pawpaw (r 2 = 0.84) and carrot (r 2 = 0.75). For RC samples only pineapple (r 2 = 0.77) and pawpaw (r 2 = 0.70) showed a significant correlation. This result shows that pH is also not the sole controlling factor in the deterioration of vitamin C in fruit juice with storage life.

This study supports the common perception that fresh is often best for optimal vitamin C content, as long as the fresh product undergoes minimal storage at either room or refrigerated temperatures. Loss of vitamin with time differs from one fruit to the other under similar storage environments. While the refrigerated samples cause significant loss of ascorbic acid in the citrus fruits, this is not so in pineapple, pawpaw and carrot samples. Though pH is significant in the stability of vitamin C, it cannot be said to be the sole controlling factor leading to losses observed in all the fruits investigated.

We express the gratitude to Mallam Mikailu Abdullahi of the Department of Microbiology, National research Institute for Chemical Technology, Zaria-Nigeria, for identifying the microbes.


Davey, M.W., M. Van Montagu, D. Inze, M. Sanmartin and A. Kanellis et al., 2000. Plant L-ascorbic acid: Chemistry, function, metabolism, bioavailability and effects of processing. J. Sci. Food Agric., 80: 825-860.
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Eddy, B.P. and M. Ingram, 1953. Interactions between ascorbic acid and bacteria.htm Bacteriological Reviews provided courtesy of American Society for Microbiology (ASM). Bacteriol. Rev., 17: 93-107.

Franke, A.A., L.J. Custer, C. Arakaki and S.P. Murphy, 2004. Vitamin C and flavonoid levels of fruits and vegetables consumed in Hawaii. J. Food. Compos. Anal., 17: 1-35.
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Gil, M.I., E. Aguayo and A.A. Kader, 2006. Quality changes and nutrient retention in fresh-cut versus whole fruits during storage. J. Agric. Food Chem., 54: 4284-4296.
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Helmenstine, A.M., 2008. Vitamin C Determination by Iodine Titration. (Viewed 26/02/08).

Manafi, M. and B. Willinger, 1991. Rapid identification of Candida albicans by fluoroplate candida agar. J. Microbiol. Methods, 14: 103-107.
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Manso, M.C., F.A.R. Oliveira and J.M. Frias, 2001. Effect of ascorbic acid supplementation on orange juice shelf life. Acta Hortic., 566: 499-504.
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Murcia, M.A., B. Lopez-Ayerra, M. Martinez-Tom´e, A.M. Vera and F. Garc´ıa-Carmona, 2000. Evolution of ascorbic acid and peroxidase during industrial processing of broccoli. J. Sci. Food Agric., 80: 1882-1886.
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Nagy, S., 1980. Vitamin C contents of citrus fruit and their products: A review. J. Agric. Food Chem., 28: 8-18.
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Padayatty, S.J., A. Katz, Y. Wang, P. Eck and O. Kwon et al., 2003. Vitamin C as an antioxidant: Evaluation of its role in disease prevention. J. Am. Coll. Nutr., 22: 18-35.
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Rickman, J.C., D.M. Barrett and C.M. Bruhn, 2007. Nutritional comparison of fresh, frozen and canned fruits and vegetables. Part 1. Vitamins C and B and phenolic compounds. J. Sci. Food Agric., 87: 930-944.
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Ryan, K.J. and J.C. Sherris, 1994. Sherris Medical Microbiology: An Introduction to Infectious Diseases. 4th Edn., McGraw Hill, New York, ISBN: 0-8385-8541-8, pp: 917.

Singleton, P., 1999. Bacteria in Biology, Biotechnology and Medicine. 5th Edn., John Wiley and Sons Ltd., West Sussex, ISBN: 0471988774, pp: 334-454.


Three pathways for rancidification are recognized: [5]

Hydrolytic Edit

Hydrolytic rancidity refers to the odor that developed when triglycerides are hydrolyzed and free fatty acids are released. This reaction of lipid with water may require a catalyst (such as a lipase, [6] or acidic or alkaline conditions) leading to the formation of free fatty acids and glycerol. In particular, short-chain fatty acids, such as butyric acid, are malodorous. [7] When short-chain fatty acids are produced, they serve as catalysts themselves, further accelerating the reaction, a form of autocatalysis. [7]

Oxidative Edit

Oxidative rancidity is associated with the degradation by oxygen in the air.

Free-radical oxidation Edit

The double bonds of an unsaturated fatty acid can be cleaved by free-radical reactions involving molecular oxygen. This reaction causes the release of malodorous and highly volatile aldehydes and ketones. Because of the nature of free-radical reactions, the reaction is catalyzed by sunlight. [7] Oxidation primarily occurs with unsaturated fats. For example, even though meat is held under refrigeration or in a frozen state, the poly-unsaturated fat will continue to oxidize and slowly become rancid. The fat oxidation process, potentially resulting in rancidity, begins immediately after the animal is slaughtered and the muscle, intra-muscular, inter-muscular and surface fat becomes exposed to oxygen of the air. This chemical process continues during frozen storage, though more slowly at lower temperature. Oxidative rancidity can be prevented by light-proof packaging, oxygen-free atmosphere (air-tight containers) and by the addition of antioxidants. [7]

Enzyme-catalysed oxidation Edit

A double bond of an unsaturated fatty acid can be oxidised by oxygen from the air in reactions catalysed by plant or animal lipoxygenase enzymes, [6] producing a hydroperoxide as a reactive intermediate, as in free-radical peroxidation. The final products depend on conditions: the lypoxygenase article shows that if a hydroperoxide lyase enzyme is present, it can cleave the hydroperoxide to yield short-chain fatty acids and dicarboxylic acids (several of which were first discovered in rancid fats).

Microbial Edit

Microbial rancidity refers to a water-dependent process in which microorganisms, such as bacteria or molds, use their enzymes such as lipases to break down fat. [6] Pasteurization and/or addition of antioxidant ingredients such as vitamin E, can reduce this process by destroying or inhibiting microorganisms. [6]

Despite concerns among the scientific community, there is little data on the health effects of rancidity or lipid oxidation in humans. [8] [9] Animal studies show evidence of organ damage, inflammation, carcinogenesis, and advanced atherosclerosis, although typically the dose of oxidized lipids is larger than what would be consumed by humans. [10] [11] [12]

Antioxidants are often used as preservatives in fat-containing foods to delay the onset or slow the development of rancidity due to oxidation. Natural antioxidants include ascorbic acid (vitamin C) and tocopherols (vitamin E). Synthetic antioxidants include butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), TBHQ, propyl gallate and ethoxyquin. The natural antioxidants tend to be short-lived, [13] so synthetic antioxidants are used when a longer shelf-life is preferred. The effectiveness of water-soluble antioxidants is limited in preventing direct oxidation within fats, but is valuable in intercepting free radicals that travel through the aqueous parts of foods. A combination of water-soluble and fat-soluble antioxidants is ideal, usually in the ratio of fat to water.

In addition, rancidification can be decreased by storing fats and oils in a cool, dark place with little exposure to oxygen or free radicals, since heat and light accelerate the rate of reaction of fats with oxygen. Antimicrobial agents can also delay or prevent rancidification by inhibiting the growth of bacteria or other micro-organisms that affect the process. [1]

Oxygen scavenging technology can be used to remove oxygen from food packaging and therefore prevent oxidative rancidification.

Oxidative stability is a measure of oil or fat resistance to oxidation. Because the process takes place through a chain reaction, the oxidation reaction has a period when it is relatively slow, before it suddenly speeds up. The time for this to happen is called the "induction time", and it is repeatable under identical conditions (temperature, air flow, etc.). There are a number of ways to measure the progress of the oxidation reaction. One of the most popular methods currently in use is the Rancimat method.

The Rancimat method is carried out using an air current at temperatures between 50 and 220 °C. The volatile oxidation products (largely formic acid [14] ) are carried by the air current into the measuring vessel, where they are absorbed (dissolve) in the measuring fluid (distilled water). By continuous measurement of the conductivity of this solution, oxidation curves can be generated. The cusp point of the oxidation curve (the point where a rapid rise in the conductivity starts) gives the induction time of the rancidification reaction, [15] and can be taken as an indication of the oxidative stability of the sample.

The Rancimat method, the oxidative stability instrument (OSI) and the oxidograph were all developed as automatic versions of the more complicated AOM (active oxygen method), which is based on measuring peroxide values, [15] for determining the induction time of fats and oils. Over time, the Rancimat method has become established, and it has been accepted into a number of national and international standards, for example AOCS Cd 12b-92 and ISO 6886.

What effect does freezing have on the nutrient content of foods?

Freezing has very little effect on the nutrient content of foods. Some fruits and vegetables are blanched (immersed in boiling water for a short period) before freezing to inactivate enzymes and yeasts that would continue to cause food spoilage, even in the freezer. This process can cause some of the vitamin C (15 to 20%) to be lost. In spite of these losses, vegetables and fruits are frozen in peak condition soon after harvesting and are often higher in nutrients than their "fresh" counterparts. Harvested produce can sometimes take many days to be sorted, transported and distributed to stores. During this time, vitamins and minerals can be slowly lost from the food. Fresh soft fruits and green vegetables can lose as much as 15% of their vitamin C content daily when kept at room temperature.

There are almost no vitamin and mineral loses from frozen meats, fish and poultry because protein, vitamins A and D and minerals are not affected by freezing. During the defrosting process, there is a loss of liquid containing water-soluble vitamins and mineral salts, which will be lost in the cooking process if this liquid is not recovered.

Do vitamin enriched foods preserve their value when exposed to higher temperatures? - Biology

While flaxseed oil should not be heated because it can easily oxidize and lose too many of its valuable nutrients, it appears that heat does not have the same effect on whole flaxseeds. Flaxseeds contain a high concentration of alpha-linolenic acid (ALA). Our website profile shows them to contain over 3 grams of ALA in 2 tablespoons, and this amount of ALA represents 54% of their total fat content. Flaxseeds contain not only ALA, however, but other important nutrients as well, including vitamins, minerals, fiber, and lignan phytonutrients such as secoisolariciresinol diglucoside (SDG).

Research studies have shown that the ALA in flaxseeds and the lignan phytonutrients in this food are surprisingly heat stable. For this reason, we believe that it safe to use flaxseeds in baking and still receive substantial amounts of ALA and other nutrients when consuming the flax-containing cooked foods.

Studies testing the amount of omega-3 fat in baked goods indicate no significant breakdown or loss of beneficial fats occurs in baking. For example, in one study, the ALA content of muffins containing 25 grams of flaxseeds was not significantly reduced after baking. Researchers speculate that the omega-3 fats in flaxseed are resistant to heat because they are not isolated but rather are present in a matrix of other compounds that the flaxseeds contain, including the lignan phytonutrients that have antioxidant properties.

It's also worth pointing out that the temperatures used for baking were normal baking temperatures of 350°F (177°C) and higher&mdashnot specially lowered temperatures to see if the seeds needed lower heat to keep their ALA intact. Baking times were also normal&mdashfalling in the one to two hour range. In one study, the seeds were even exposed to a heat level of 660°F (349°C), apparently without damaging their ALA content.

The lignan phytonutrient SDG has also be found to be stable in its chemical structure when exposed to normal baking conditions. In one study, consumption of SDG-enriched muffins was found to enhance the production of mammalian lignans in women, reflecting their stability and bioavailability. In another study, women who ate raw, ground flaxseed daily for four weeks had similar plasma fatty acid profiles as those who ate milled flaxseed that had been baked in bread. Both groups of women showed a lowering of total cholesterol and "bad" LDL cholesterol, further reflecting that flaxseeds still have benefits when used in baked goods.

A study on incorporation of flaxseeds into pasta - involving overnight drying of the flax-containing pasta at temperatures of either 104F(40C) or 178F (80C) plus boiling of the dried pasta - also showed a reduction in ALA of 8% or less. And a study on the boiling of flax bolls (the seed-containing portion of the plant) showed a reduction in ALA of 4-5%. All of these studies are consistent in demonstrating the relatively stable nature of ALA in flaxseeds to heat.

Based upon these research studies (all cited in the References section below), it appears that the ALA in flaxseeds is relatively stable to heat, and that flaxseeds can provide substantial ALA benefits even after processing, incorporation into cooked foods.


Cunnane SC, Ganguli S, et al. High alpha-linolenic acid flaxseed (Linum usitatissimum): some nutritional properties in humans. Br J Nutr. 1993 Mar69(2):443-53.

Cunnane SC, Hamadeh MJ, Liede AC, et al. Nutritional attributes of traditional flaxseed in healthy young adults. Am J Clin Nutr. 1995 Jan61(1):62-8.

Fofana B, Cloutier S, Kirby CW, et al. A well balanced omega-6/omega-3 ratio in developing flax bolls after heating and its implications for use as a fresh vegetable by humans. Food Research International, Volume 44, Issue 8, October 2011, Pages 2459-2464.

Hallund J, Ravn-Haren G, et al. A lignan complex isolated from flaxseed does not affect plasma lipid concentrations or antioxidant capacity in healthy postmenopausal women. J Nutr. 2006 Jan136(1):112-6.

Hyvarinen HK, Pihlava JM, et al. Effect of processing and storage on the stability of flaxseed lignan added to bakery products. J Agric Food Chem. 2006 Jan 1154(1):48-53.

Manthey FA, Lee RE, Hall CA 3rd. Processing and cooking effects on lipid content and stability of alpha-linolenic acid in spaghetti containing ground flaxseed. J Agric Food Chem. 2002 Mar 1350(6):1668-71.

Villeneuve S, Des Marchais LP, Gauvreau V, et al. Effect of flaxseed processing on engineering properties and fatty acids profiles of pasta. Food and Bioproducts Processing, Volume 91, Issue 3, July 2013, Pages 183-191.

Enzymes are proteins that act as catalysts in a biochemical reaction to increase the rate of reaction without being used up in the reaction. Thousands of types of enzymes are at work in your body to carry out vital functions such as digestion and energy production. Biological and chemical reactions can happen very slowly and living organisms use enzymes to bump reaction rates up to a more favorable speed. Enzymes have multiple regions that can be activated by co-factors to turn them on and off. The co-factors are usually vitamins consumed through various food sources and open up the active site on the enzyme. Active sites are where reactions take place on an enzyme and can only act upon one substrate, which can be other proteins or sugars. A good way to think about this is a lock-and-key model. Only one key can open a lock correctly. Similarly, only one enzyme can attach to a substrate and make the reaction happen faster.

Your body contains around 3,000 unique enzymes, each speeding up the reaction for one specific protein product. Enzymes can make your brain cells work faster and help make energy to move your muscles. They also play a large role in the digestive system, including amylases that break down sugar, proteases that break down protein, and lipases that break down fat. All enzymes work on contact, so when one of these enzymes comes in contact with the right substrate, it starts to work immediately.

Pan-frying, roasting, and searing do not involve water. Although the food may still lose some vitamins, it is typically less than you would lose with a method that uses water. These methods can also enhance the flavor of your food, depending upon which one you choose. For example, roasting vegetables tend to give them a sweeter taste while softening their skins. Stir-frying retains more of the crispness and imparts a flavor that more closely resembles what you enjoy from eating raw vegetables.

For recipes that require the use of water, you can try using a method that reduces the contact that the water has with the ingredients.

For instance, blanching requires the food to sit in the water for less time, and this method is ideal for softening ingredients such as bell peppers. Steaming allows the heated water to gradually soften the produce without removing all of the Vitamin C. Microwaving, while not seen as particularly sophisticated, can also help to retain the Vitamin C content in vegetables.

Overall, it’s probably best not to rely on cooked vegetables to meet your Vitamin C intake needs, as most kitchens are not equipped with the necessary equipment to measure Vitamin C content. Since time, water, and heat all contribute to the destruction of Vitamin C, you cannot depend on the nutrition labels that only indicate the vitamin content of the food in its raw form. If you are concerned that you are not getting adequate Vitamin C from foods, consider Lypo-Spheric ® Vitamin C supplements. They are resistant to digestive juices that destroy Vitamin C before it reaches the bloodstream. Just don’t heat them as heat destroys not only Vitamin C in this case, but liposomes as well.

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