Natural Plant Colourants

Carotenoids

Carotenoids are compounds constituted by eight isoprenoid units (ip). The ip units are joined in a head-to-tail pattern, but the order is inverted at the molecule center (Delgaldo and Lopez, 2003).  Carotenoids are naturally occurring tetraterpene pigments widely distributed through out the living world (Zdzlaslaw, 2002). Lycopene (C₄₀ H₅₆) is considered the first colored carotenoid in the biosynthesis of many other natural carotenoids and it has a linear structure (Delgaldo and Lopez, 2003). Lycopene is the carotenoid that gives the red color to tomatoes and watermelons. Moreover, it is also common to find acyclic, cyclic, and shortened carotenoids, among others. Carotenoids have very important biological activities and their use as food and feed today is recommended largely due to their vitamin A and antioxidant activities important in maintaining life (Delgaldo and Lopez, 2003; Zdzislaw, 2002). Due to their ability to quench singlet oxygen and trap peroxyl radicals, carotenoids have been described as excellent antioxidants. In addition, an inverse association between the ingestion of carotenoid-containing fruits and vegetables and the risk of certain forms of chronic diseases has been suggested (Østerlie and Lerfall, 2005). Carotenoids of the higher plants are found in roots, stems, leaves, flowers and fruits giving them the red, yellow, and orange colors. In higher plants, carotenoids are found in plastids (Delgaldo and Lopez, 2003). They are in the chloroplast of photosynthetic tissues and they are found in chromoplasts. The majority of the carotenoids found in plant tissues is β-carotene (Zdzislaw, 2002). Zdzislaw (2002) defines β-carotene as an asymmetrical molecule of 40 carbon atoms, consisting of 8 isoprene units having 11 conjugated double bonds and 2 β-ionone rings. Animals derive carotenoid pigments by consumption of carotenoid-containing plant materials. The name carotenoid has been derived from the major pigment of the carrot (Zdzislaw, 2002).

 

Fig 7: Structural Formulae of different types of carotenes (Zdzislaw, 2002).

The color of carotenoid pigments is as a result of the presence of a system of conjugated double bonds. A minimum of seven conjugated double bonds is required for the yellow color to appear. The increase of double bonds results into a shift of the major adsorption bands to the longer wavelengths, and the hue of carotenoids become more red (Zdzislaw, 2002). Because of the highly conjugated double-bond system, carotenoids show ultraviolet and visible absorption spectrum characteristics. According to Zdzislaw (2002), for most carotenoids, three peaks or two peaks and a shoulder, are absorbed in the range of 400–500 nm. The absorption maxima and molecular extinction values are significantly affected by the solvent used.

In unprocessed plants, usually all-trans double-bond configurations occur, but cis isomers of each carotenoid are also possible. Processing and storage cause isomerization of carotenoids in foods and affect the color. The deepest color is due to compounds with all-trans configurations. So, increasing the number of cis bonds results into gradual lightening of the color. This is because cis isomers absorb less strongly than the all-trans isomer, and peak at 330–340 nm (Zdzislaw, 2002).

Due to the presence of asymmetric carbon atoms, many carotenoids have chiral centers. However, natural carotenoids exist only in one of the possible enantiomeric forms, because the biosynthesis is enantiomere selective.

Carotenoid classification.

By their structural elements

-Carotenes; carotenoids constituted by carbon and hydrogen e.g. α-carotene, β-carotene and β-cryptoxanthin.

-Xanthophylls; constituted by carbon, hydrogen and additionally oxygen. For example lutein, zeaxanthin, violaxanthin, neoxanthin, and flucoxanthin.

By their functionality.

-Primary; carotenoids required for the photosynthetic process e.g. β-carotene, lutein, zeaxanthin, violaxanthin, neoxanthin, and antheraxanthin.

-Secondary; their presence is not directly related with plant survival.

Carotenoids as food colors

Natural carotenoid extracts have been used as food colorants for centuries. The most commonly used natural carotenoid extracts for foodstuffs are annatto, paprika, and saffron. Many others sources, including alfalfa, carrot, and tomato, citrus peel, and palm oil, are also utilized (Delgado and Lopez, 2003). In food industries, carotenoids are utilized mainly to color alcoholic beverages, soft drinks, fresh meat, cheese, cereals, salad dressings, canned fruits and vegetables, chewing gum, yoghurt, ice-cream, milk, margarine, pasta, canned fish, bread, potato derived products, sauces, fruit juices, instant soups, oil, butter, etc.

Tomatoes (Lycopersicon esculentum)

Tomatoes have a high content of carotenoids with lycopene being the main compound making up 80 to 90% of the total carotenoid, followed by β-carotene (Delgado and Lopez, 2003). The lycopene content in tomatoes increases with ripeness. New tomato varieties with high and improved content have been developed and lycopene products have begun to be commercialized (Zdzislaw, 2002). However, according to Delgado and Lopez (2003), other modifications are required to use tomato extract as colorant because of its strong flavor. Adding lycopene from natural sources to minced meat could lead to a meat product with different taste, better color and with a well, documented health benefit and a new functional food would be developed and may be natural lycopene could replace nitrite added minced meat(Østerlie and Lerfall, 2005).

Processing and stability

In processing or storage of colored food, carotenoids are sensitive to treatments. High and short times are preferred conditions during processing of carotenoid-containing foods ( Fennema 1996; DeMan, 1999; Delgado and Lopez, 2003). Drying (uncontrolled) leads to carotenoid degradation due to the presence of free radicals. In tomatoes, the dehydration process leads to isomerization but it has been observed that osmotic treatment does not affect the isomeric profile of lycopene (Delgado, and Lopez, 2003). Freezing causes little changes in carotene content. Blanching increases carotenoid content relative to the raw tissue due to the inactivation of lipoxygenase which is known to catalyze oxidative decomposition of carotenoids. Carotenoids are extremely lipophilic compounds that are almost insoluble in water. In aqueous surroundings they tend to form aggregates or adhere to surfaces (Zdzislaw, 2002; Fennema 1996; DeMan, 1999; Delgado and Lopez, 2003)

Anthocyanins

Anthocyanins are flavonoids with a characteristic C₆ C₃C₆ carbon skeleton making them the most commonly distributed pigment group in plants kingdom (Deman, 1999; Fennema, 1996; Zdzislaw, 2002).

Anthocyanins occur in all higher plants, mostly in flowers and fruits but also in leaves, stems, and roots. In these parts they are found predominantly in outer cell layer and account for a wide range of colors including blue, purple, pink, orange, red, magenta and violet in the plant kingdom (http://www.food-info.net/uk/colour/anthocyanin.htm; DeMan, 1999). In food plants, the main sources of anthocyanins are berries, such as blackberries, grapes, blueberries, and some vegetables, such as egg-plants (aubergine), red cabbage and avocado. Other sources include oranges, elderberry, olives, red onion, fig, sweet potato, mango, Hibiscus spp (musayi plant) and purple corn. They play a definite role in attracting animals in pollination and seed dispersal. Accordng to Zdzislaw (2002), they may also have a role in the mechanism of plant resistance to insect attack.

The structure of anthocyanins is based on a C₁₅ skeleton consisting of a chromane ring bearing a second aromatic ring. The cyclic structures are arranged in the pattern C-6-C-3-C-6. Anthocyanins structure is complemented by one or more sugar molecules bonded at different hydroxylated position of the basic structure (Delgado and Lopez, 2003).

                                                                                   

All anthocyanins are based on a single basic core structure, the flavyllium ion (Zdzislaw, 2002). 

Fig 8; Flavylium cation (Zdzislaw, 2002).

These side groups can be a hydrogen atom (H), a hydroxide (OH) or a methoxy-group (OCH) depending on the considered pigment. According to Zdzislaw (2002), from about 20 known naturally occurring anthocyanidins, only 6 occur most frequently in plants and these include: pelargonidin, cyanidin, peonidin, delphinidin, petunidin, and malvidin.

Classification

Anthocyanins are glycosides of polyhydroxy and polymetoxy derivatives of 2-phenylbenzopyrylium or flavylium cation. They show high diversity in nature but all are based on a reduced number of basic anthocyanidin structure. Anthocyanin diversity is associated with the number of sugars found in nature but glycosylated anthocyanins are formed with glucose, rhamnose, xylose, galactose, arabinose, and fructose (Deman, 1999; Fennema, 1996; Zdzislaw, 2002).

The chemical combinations of these sugars with organic acids to produce acylated anthocyanins also increase diversity. Differences between individual anthocyanins are: the number of hydroxyl groups in the molecule; the degree of methylation of the hydroxyl groups; the nature, number, and position of glycosylation; and the nature and number of aromatic or aliphatic acids attached to the glucosyl residue.

Substitution of the hydroxyl and methoxyl groups affects the color of the anthocyanins. Color is also affected by the number of hydroxyl and methoxyl groups. If more hydroxyl groups are present, then the color goes towards bluish shade, and redness is increased if more methoxyl groups are present (http://en.wikipedia.org/wiki/Anthocyanin; Zdzislaw, 2002)

Chemically anthocyanins are subdivided into the; sugar-free anthocyanidine aglycons and anthocyanin glycosides.

Anthocyanins as food colors.

The anthocyanins constitute a large family of differently colored compounds and occur in countless mixtures in practically all parts of higher plants. They are of great economic importance as fruit pigments and thus are used to color fruit juices, jams, wines, some beverages, canned fruit, fruit syrups, yogurt, and other products. They are used as food additive with European Union number E163. In the USA, the grape color extracts were approved for use in non beverage foods, where as grape skin extract (enocyanin) is permitted in beverages. (DeMan, 1999; Delgado and Lopez, 2003)

Processing and stability

Anthocyanins are water soluble strong colors and have been used to color food since historical times (Delgado and Lopez 2003). Extracts of berries have been used to color drinks, pastries and other foods. However, some drawbacks in the use of anthocyanins as food colors exist.

Anthocyanins are water soluble and pH dependent which restricts their use. For example the color of red cabbage is enhanced with the addition of vinegar or other acid. On the other hand, when cooked in aluminum pans which cause a more alkaline environment, the color changes to purple and blue (http://www.food-info.net/uk/colour/anthocyanin.htm).

The color is also susceptible towards temperature, oxygen, UV-light and different co-factors. Temperatures destroy the flavylium ion, and thus cause loss of color. Temperature also causes maillard reactions, in which the sugar residues in the anthocyanins may be involved. Light may have a similar effect (Delgado and Lopez 2003; Zdzislaw 2002). Oxygen may destroy the anthocyanins, as do other oxidizing reagents, such as peroxides and vitamin C. Many other components in plants and foods may interact with the anthocyanins either destroying, changing or increasing the color. Quinones in apples for example, enhance the degradation of anthocyanins whereas the addition of sugar to strawberries stabilizes the color. (Delgado and Lopez, 2003; http://www.food-info.net/uk/colour/anthocyanin.htm)

All these factors limit the use of anthocyanins in foods. Some loss of color during storage may be prevented by storing at low temperatures, in dark containers or under oxygen-free packaging (Deman, 1999; Fennema, 1996; Zdzislaw, 2002; Delgado and Lopez 2003).

In practice the pure colors are very hard to obtain and most often (crude) extracts are used as food colors. Grape peel (E163 (i)), and black currant extract (E163 (iii)) are the most widely used anthocyanin mixtures in foods. (http://www.food-info.net/uk/colour/anthocyanin.htm)

Betalains

Betalains are immonium derivatives of betalamic acid, with a general formula based on the protonated 1, 2, 4, 7, 7-pentasubstituted 1, 7- diazaheptamethin system (Delgado and Lopez, 2003). Betalains occur in centrospermae, mainly in red beets, but also in some cactus fruits and mushrooms (Zdzislaw, 2002; Deman, 1996). Zdzislaw, (2002) reported that they consist of red-violet betacyanins (λmax ~ 540 nm) and yellow betaxanthins (λmax ~ 480 nm). The major betacyanin is betanin, glucoside of betanidin, which accounts for 75–95% of the total pigments of beets. The other red pigments are isobetanin (C-15 epimer of betanin), prebetanin, and isoprebetanin. According to Deman (1996), the latter two are sulfate monoesters of betanin and isobetanin, respectively. Unlike anthocyanins, betanins cannot be hydrolyzed to aglycone by acid hydrolysis without degradation (Zdzislaw, 2002). The major yellow pigments are vulgaxanthin I and II. High betalain content in beet root, on average 1% of the total solids, makes this vegetable a valuable source of the food colorant (Deman, 1996; Zdzislaw, 2002).

Fig 9: Molecular structures of betanin and isobetanin and the major yellow pigments, vulgaxanthin I and II (Zdzislaw, 2002).

Betalains are found in different plant organs, and are accumulated in cell vacuoles, mainly in epidermal and sub epidermal tissues (Delgado and Lopez, 2003). Some accumulate in plant stalks such as in the roots of red beet. Betalains are also present in the higher fungi Amanita, Hygrocybe, and Hygrosporus (Zdzislaw, 2002).

Classification:

Betalains are commonly classified into two based on their structural characteristics;

(i)                 Betacyanins (red-purple)

(ii)               Betaxanthins (yellow)

Each group of pigment is characterized by specific R₁-N-R₂ moieties.  R₁ and R₂ groups can be hydrogen, aromatic group or another substituent. Betalain color is attributable to the resonating double bonds. A large number of betaxanthins can be formed with the same dihydropyridine moiety, by conjugation with several amine compounds such as amino acids. The diversity of betacyanins is associated with the combination of the basic structures (betanidin and isobetanidin) with different glycosyl and acyl groups attached by the hydroxyl groups at positions 5 and 6. The most common glycosyl moiety is glucose and the most common acyl groups are sulfuric, malonic, citric and caffeic acids.

Betalains as food color.

According to Delgado and Lopez (2003), betalains have been in use as food colorants at least since the return of the 20^th century. Diaz et al., (2006) reported that there is a growing propensity to substitute synthetic colorants with natural pigments in the food industry. This is further confirmed by the red beet betacyanins being approved for use as a food additive in the United States of America (No. 1600), and in the European Union (E-162). Commercially, the red beet betacyanins are exempt from batch certification (http://www.food-info.net/uk/colour).

Processing and stability

The most-studied betalains are found in red beets (Beta vulgaris) in which the main betacyanins are betanin and isobetanin (Delgado and Lopez, 2003). Betalains stability is affected by temperature, pH, oxygen, light, and aqueous activity (Diaz et al., 2006). According to Delgado and Lopez (2003), enzyme degradation is also an important factor that must be considered when a betalain pigment product is to be processed.

Zdzislaw (2000), reviewed that color stability of betanin solution is strongly influenced by pH and heating. Betanins are stable at pH values of 4–6, but thermostability is greatest between pH values of 4 and 5. As a result of betanin degradation cyclo-DOPA and betalamic acid are formed. This reaction is reversible.

Fig 10: Degradation of betanin (Zdzislaw, 2002)

Light and air have a degrading effect on betanin according to Zdzislaw (2002). These effects are cumulative, but some protection may be offered by antioxidants such as ascorbic acid. Small amounts of metallic ions increase the rate of betanin degradation. Therefore a chelating agent can stabilize the color. Many protein systems present in food products also have some protective effect (Zdzislaw, 2002).

Freeze-drying is the best method to dry pigments which are sensitive to high temperatures (Diaz et al., 2006).

On market, Beetroot red (E 162) is available as liquid beetroot concentrate and as beetroot concentrate powders. According to Zdzislaw(2002) and Delgado and Lopez(2003), it is suitable for products of relatively short shelf life, which do not undergo as severe heat treatment such as meat and Soya protein products, ice cream, and gelatin desserts.

Note that all references used in all postings related to the topic of sausages, meat and meat product colorings will be posted in the last article about this topic

About the author
Mr. Sempiri Geoffery, the author of this article graduated from Makerere University with a Bsc In Food Science and Technology Degree in January, 2011.

Food Colors and Coloring of Foods.

Food Colors and Coloring of Foods.

Food color/colorant

This is any substance that is added to food or drink to change its color. (http://en.wikipedia.org/wiki/color).

Or

Food additive used to alter or improve the color of processed foods. (http://encyclopedia.farlex.com/additive). Food coloring is applied to both commercial food production and domestic cooking. The utilization of food colorants in foods is an important feature to the food industry (i.e. to both manufacturers and consumers) (Muntean, 2005).

Reasons to use color additives

Consumers recognize color, flavor, and texture as the main attributes of food with color being the most important of the three (Delgado and Lopez 2003, http://en.wikipedia.org/wiki/color). Today, food products are consumed far from where they are produced. As a result, processing and transportation of food are necessary to reduce degradation and loss of appearance. The use of color additives by the food industry is thus necessary to restore the original food appearance i.e. the added colorants, reinstate the novel look/color of foods after processing and storage treatments where natural colorant content has been reduced; ensure batch-to-batch color uniformity and masking natural variations in color; intensify color normally found in food i.e. the addition of colorants enhances naturally occurring colors which are in low intensity to consumer expectations (Muntean, 2005); protect other component e.g. flavors and vitamins from damage by light (Zdzlaslaw, 2002); obtain the best food appearance i.e. decorative or artistic purposes such as cake icing (http://en.wikipedia.org/wiki/color); preserve characteristics associated with food; help as a visual characteristics of food quality i.e. it influences acceptability of food, for example good quality fresh meat is expected to be bright red and any deviation from that is viewed as spoilt (Deman, 1999); and also adds visual delight and recognition/ identity to food products e.g. lime juice is expected to be green while sausages are expected to be pink in color (Fennema, 1996). Therefore, addition of food colorants has become a regular practice in the food industry to better or even alter the color of foods and drinks.

Food Colors/colorants used are either synthetic (artificial), such as tartrazine and amaranth, made from petrochemicals or natural colors such as chlorophyll, anthocyanins, caramel, and carotene (Fennema, 1996).

Choice and application of color

Color is a main quality parameter in foods (in particular meats) to be commercialized (Cornforth, 1994). According to Delgado and Lopez (2003), a number of factors must be considered when selecting the better color additive for specific applications. These include; color hue required, physical form (e.g. liquid, solid, emulsion), properties of the food stuff that will be colored e.g. oily or water- based product, content of tannins, pH and processing conditions (e.g. whether the process requires heating or cooling, storage conditions). In addition to the above, one factor of paramount importance is the relevant legislation (Zdzislaw, 2002).

Classification of colorants

Pigments can be classified in accordance with the different system. These systems are clearly defined, but all are closely related (Zdzislaw, 2002); the same type of colorants can be classified in different groups (e.g. carotenoids). Today, classification of colorants by their origin and legislation are the most important systems. This is in agreement with consumer preferences, which clearly favor natural pigments over synthetic pigments obtained from laboratories (Delgado and Lopez, 2003; Zdzislaw, 2002). Colors can be the natural ingredients of foodstuffs or other natural ingredients that are not normally used, such as a foodstuff or as a typical ingredient for a foodstuff.  Also considered as colors are products that are obtained by physical and/or chemical extraction from foodstuffs and other natural ingredients whose coloring ingredient has been extracted separately from nutritive and aromatic substances (Zdzislaw, 2002).

According to Zdzislaw, 2002, dried and/or concentrated ingredients or spices that are used in the production of foodstuffs and have, in addition to aromatic, flavoring or nutritional properties a secondary coloring effect, are not considered to be colors (for example paprika, curcuma and saffron). If the use of an ingredient is based exclusively on its coloring effect and it has no nutritional or aromatic properties, it is then considered to be a color.

Systems of classification of colorants.

According to Delgado and Lopez (2003), colorants are classified basing on either;

Origin:

As synthetic colorants that are organic compounds obtained by chemical synthesis e.g. Food Drug and Cosmetics (FD&C) colorants, natural colorants that are organic compounds obtained from living organisms. According to Østerlie and Lerfall (2005), the colorants are considered natural if they are from agricultural/biological sources, extracted without chemical reaction and have been in use for a long time e.g. carotenoids, anthocyanins, betalains and organic colorants that are found in nature or obtained by synthesis e.g. TiO₂.

Or

Legislation:

As certifiable; anthropogenic synthetics i.e. FD &C colorants and lakes. E.g. amaranth, allura red, sunset yellow, tartrazine, fast red E, and those exempted from certification; from natural origin (vegetable, mineral or animal) or synthetic counterparts e.g. grape juice, TiO₂, carmine and synthetic β-carotene.

Synthetic food colorants

The use of synthetic organic colors has been recognized for many years as the most reliable and economical method of restoring some of the food’s original shade to the processed product (Muntean, 2005). An even more important application of synthetic colorants is to improve and standardize the appearance of food products that have little or no natural color present, such as dessert powders, table jellies, ice and sugar confectioneries. The synthetic organic colors are superior to the natural pigments in tinctorial power, range and brilliance of shade, stability, ease of application, and cost-effectiveness (Muntean, 2005, Zdzislaw, 2002).

However, from a health and safety point of view, they are less acceptable to consumers. Over the past years increasing interest in natural food colorants has been observed (Zdzislaw, 2002). Synthetic food colorants are regulated by the government with seven synthetic colorants currently approved for use in food. These include 2 reds (#3 and #40), 2 blues (#1 and #2), 2 yellows (#5 and #6), 1 green (#3) (http://en.wikipedia.org/wiki/color). These seven colorants are grouped by the color-giving chemical functional group they contain. FD&C Red #40 and Yellow #6 both contain azo bonds (-N=N-) thus are referred to as azo colorants. FD&C Blue #1, Green #3 and Red #3 belong to the triphenylmethane group which contain three benzene rings attached to a central carbon atom (Delgaldo and Lopez, 2003; http://en.wikipedia.org/wiki/color). Just as with any substance, the chemical structure of these colorants determine its’ characteristics, for example if it is water soluble or not. Water-soluble colorants are useful in water-based foods, but not in fatty foods such as salad dressings and ice cream (Delgaldo and Lopez, 2003).

Natural Pigments

The naturally occurring colorants in food plants are the customary sources of color in food although the added colorants have assumed an extra vital role as the food processing industry is growing. Such colorants are viewed as accidental colorants as they are present only because they or their precursors are present (Mutean, 2000). 

Natural pigments are generally considered the pigments occurring in unprocessed food, as well as those that can be formed upon heating, processing, or storage (Zdzislaw, 2002). Chlorophylls and carotenoids are the most abundant pigments in nature. They are involved in fundamental processes and life on earth depends on them. Chlorophyll is not found in animals but carotenoids accumulate in some organs (e.g. eyes) and tissues e.g. skin of fish, bird plumage (Delgaldo and Lopez, 2003). In addition, flavonoids are scarce in fungi whereas riboflavin imparts the yellow color in the genera Russula and Lyophyllum. Betalains, melanin, a small number of carotenoids and certain anthroquiriones are common to fungi and plants (Delgaldo and Lopez, 2003).

More than 1000 pigments have been identified in fungi. Fungi are not photosynthetic and do not contain chlorophyll (Zdzislaw 2002). Carotenoid distribution in fungi is restricted to some orders (e.g. pharagmobasidiomycetidae, and Discomycetes). All natural pigments are unstable and participate in different reactions, so their color is strongly dependent on conditions (Zdzislaw, 2002).

Limitations of Natural Pigment use.

Delgaldo and Lopez (2003) reported that limitations to the use of natural pigments include; being produced by traditional methods, having a lower intensity in comparison to synthetic pigments and natural pigments require large quantities of raw material to obtain the same depth level like synthetic pigments i.e. they occur in small amounts in plants or plant part. Also natural pigments are highly sensitive to pH and temperature.

Note that all references used in all postings related to the topic of sausages, meat and meat product colorings will be posted in the last article about this topic

About the author
Mr. Sempiri Geoffery, the author of this article graduated from Makerere University with a Bsc In Food Science and Technology Degree in January, 2011. 

Nitrates, Nitrites, Meat and Meat Products

Nitrates, Nitrites, Meat and Meat Products

Nitrates and Nitrites are fundamental components of the global nitrogen cycle and are therefore found throughout the environment. Nitrates and nitrites are compounds that contain a nitrogen atom joined to oxygen atoms, with nitrate containing three oxygen atoms and nitrite containing two. In nature, nitrates are readily converted to nitrites and vice versa. Both are anions or ions with a negative charge. They tend to associate with cations, or ions with a positive charge to achieve a neutral charge balance. (Argonne National Laboratory, 2005).

Compound	Chemical Symbol	Molecular weight
Nitrate	NO3	62
Nitrite	NO2	46

Table 1: Chemical symbols of nitrate, nitrite and their molecular weights.

The role of Nitrates and Nitrites in Cured Meat Products

Potassium and sodium salts of the nitrate and nitrites are the most extensively used of all food additives (Stevanovic and Šentjurc, 2000). Nitrates and nitrites in cured meat and meat products play a multipurpose role; in addition to effectively inhibiting the growth and toxicogenic effect of Clostridium botulinum, nitrite is responsible for the development of typical cured-meat color and flavor, and also functions as an antioxidant (Rincón et al., 2008), retarding the development of rancidity, off-odors and off-flavors during storage, inhibiting the development of warmed-over flavor and preserving flavors of spices and smoke (Zdzlaslaw, 2002).

Adding nitrite to meat is only part of the curing process (Feiner, 2006). Ordinary table salt (sodium chloride) is added because of its effect on flavor. Sugar is added because of its contributions to flavor, browning during frying process and its ability to disguise high levels of salt in a meat product. Spices and other flavorings are often added to contribute to flavor, aroma and taste but not added for nutritional purposes (Feiner, 2006).

Sodium nitrite, rather than sodium nitrate, is the most commonly used for curing (although in some products, such as country ham, sodium nitrate is used because of the long aging period) (Stevanovic and Sentjuric, 2000). In a series of normal reactions, nitrite is converted to nitric oxide (Fennema, 1996). Nitric oxide combines with myoglobin, the pigment responsible for the natural red color of uncured meat forming nitric oxide myoglobin, which is a deep red color (as in uncooked dry sausage). This changes to the characteristic bright pink normally associated with cured and smoked meat (such as wieners and ham) when heated during the smoking process (Feiner, 2006).

When sodium nitrite is added with the salt, the meat develops a red, then pink color, which is associated with cured meats such as ham, bacon, hot dogs, and bologna. Nitrite reacts with the meat myoglobin to cause these color changes, first converting to the unstable nitrosomyoglobin (bright red), then on heating, to a more stable nitrosohemochrome, a pink pigment (Zdzlaslaw, 2002; Fennema, 1996).

Fig 5; Transformation of myoglobin in meat; dMMb and dMbNO, denatured forms of pigments (Zdzlaslaw, 2002)

Modification of the myoglobin molecule takes place in the meat curing process where nitric oxide (NO), which originates from the sodium nitrite or potassium nitrite curing agent, combines to form nitrosomyoglobin (Fennema, 1996).

According to Feiner (2006), this reaction takes place at pH value below 6.5 and in meat products a pH value of a round 4.7(salami) to 6.0 is present in the final product. When used, sodium nitrate (NaNO₃) does not contribute directly to the formation of the red curing color but rather reduced to sodium nitrite (NaNO₂) thus providing nitric oxide (NO) by the reactions above, which results into the formation of the characteristic pink cured meat color (Feiner 2006, Fidel, et al, 2009). Examples such products are; ham, corned beef, bacon, salami, and sausage.

Fig 6; Chemical structures of myoglobin and nitrosomyoglobin (http://nzic.org.nz/ChemProcesses/animal/5A.)

In the presence of thiol compounds as reducing agents in the reversible reaction, myoglobin may form a green sulfmyoglobin (Zdzlaslaw, 2002). Other reducing agents, for example ascorbate lead to formation of cholemyoglobin making the reaction irreversible (Fiener, 2006).

Animal blood and its dehydrated protein extracts, which are mainly hemoglobin, are a potential source of red and brown heme pigment which may be used as red and brown coloring to meat products. However in most countries their usage as food colorant is not permitted (Zdzlaslaw, 2002).

Meat, nitrates, nitrites and cancer
Colorectal cancer is the main cancer type that has been associated with high meat consumption. Based on a considerable number of studies a 12–17% increased risk of colorectal cancer was associated with a daily increase of 100 g of red meat and a 49% increased risk associated with a daily increase of 25 g of processed meat (ferguson, 2010). Processed meats include sausages, smoked beef and hams among others in which case nitrates and nitrites are used as additives.

Hill (1991) reported that the use of nitrites in cured meats experienced a serious drawback in the late 1960s in the usa due to the n-nitrosamines scare. The presence of some n-nitrosamines, as a consequence of the reaction of nitrites with secondary amines especially in thermally treated cured meats, caused a ban in the usa that was lifted after re-considering maximum amounts to be added. A number of n-nitrosamines are potential carcinogenic agents and are postulated to have several deleterious health effects, so their formation must be prevented. According to stevanivic and senjurc (2002) and hill (1991), the addition of ascorbic acid ensured the reaction of nitrite to nitric oxide and thus reduced the possibility for the formation of n-nitroso compounds in meat products. N-nitroso compounds are formed by the action of nitrous acid on a suitable secondary nitrogen group. If the parent nitrogen group is a secondary amine, then the product is an n-nitrosamine giving rise to n-nitrosamides, ureas to n-nitrosoureas, all of which are carcinogenic to the body. The n-nitrosamines are target organ specific and cause tumors at sites distant from that of their introduction into the body. N-nitrosamide and n-nitrosoureas are locally acting and cause tumors only at their sites of introduction. The n-nitrosamines are not directly acting mutagens but need activation by microsomal enzymes before they are mutagenic in the salmonella mutagenesis assay. They also need to be activated in the body, but this process which leads to the organotropism of these compounds, is little understood (hill 1991)

In reference to the above, it is therefore necessary to develop alternatives to nitrates and nitrites. Researchers have proposed different methods to inhibit the possibility of N-nitrosamine formation in cured meat products. These include a decrease in the level of added nitrite or the use of N-nitrosamine blocking agent such as ascorbate and α- tocopherol (Stevanivic and Senjurc, 2002). However, as far as N-nitrosamines are concerned, the most attractive and reliable method is total elimination of nitrites and nitrates from the curing process. The alternative of natural plant colorants extracted directly from plants or plant parts that totally eliminate the use of nitrite from the curing process can be a viable alternative for coloring comminuted meat as a substitute for nitrates, nitrites and synthetic colorants to give the products their characteristic pink color.

Note that all references used in all postings related to the topic of sausages, meat and meat product colorings will be posted in the last article about this topic.

About the author
Mr. Sempiri Geoffery, the author of this article graduated from Makerere University with a Bsc In Food Science and Technology Degree in January, 2011. 

Meat Pigment Chemistry

Meat Pigment Chemistry

The color is the first impression consumers have of any meat product and often is their basis for product selection or rejection (Deda et al., 2008). According to Cornforth (1998), it is the most universal quality gauge used by consumers to judge meat freshness.  Deterioration of meat color may indicate that the product is spoiled or has lost its nutritional valve.  Although meat color is not a good indicator of nutritional quality, it may indicate microbial spoilage. By maintaining meat and meat product color, we can maximize consumer quality perception (Zdzlaslaw, 2002).

The color of meat products is determined by a combination of different factors including moisture and fat content, but more important is the chemical form and concentration of the hemoproteins, especially that of myoglobin (Adamsen et al., 2006). Myoglobin (80%) and hemoglobin (20%) are the predominant meat pigments and accounts for the red color in meat (http://labs.ansci.uiuc.edu). The color pigment of the muscle tissue is myoglobin while haemoglobin is the color pigment of the blood (Feiner, 2006). Myoglobin is a complex protein, similar in function to the blood pigment hemoglobin, in that they both bind with the oxygen, which is required for metabolic activity of an animal. Although their functions are similar, their roles are different; hemoglobin acts as an oxygen carrier in the bloodstream, whereas myoglobin is essentially a storage vehicle for oxygen in muscle (Feiner, 2006).  In both pigments, the heme group is composed of the porphyrin ring system and the central iron atom bound with the globin; in myoglobin, the protein portion has a molecular weight of 17,000, and in hemoglobin about 67,000 (Zdzlaslaw, 2002). The color of meat from various species, such as poultry, pork, and beef, often differs in redness, and one cause of this difference is the amount of myoglobin in the meat (http://meat.tamu.edu/color).

Myoglobin Structure

Myoglobin is an oxygen-binding protein (globular protein of 153 amino acids) of the muscle (Fennema, 1996). This is the pigment chiefly responsible for the color of meat, though hemoglobin (the oxygen-binding protein in blood) is also present in small quantities (Feiner, 2006). Myoglobin is a monomeric protein consists of a single-chain globin protein and a color giving heme group in the centre (Feiner, 2006; Fennema, 1996; Deman 1999). The heme group consists of a flat prophyrin ring exhibiting a central iron atom (Fe²⁺). This iron atom has six coordination bonds, each representing an electron pair accepted by the iron from five nitrogen atoms; four from the porphyrin ring and one from a histidyl residue of the globin (Fennema, 1996; Cornforth, 1998). The sixth bond is available for binding with any atom that has an electron pair to donate, for example oxygen and nitric oxide. The oxidation state of the iron atom and the physical state of the globin play an important role in meat color formation (Feiner 2006; Zdzlaslaw, 2002). It has eight alpha helices and a hydrophobic core. It has a molecular weight of 16,700 Daltons (http://en.wikipedia.org/wiki/Myoglobin)

Fig 1; The structure of myoglobin (http://meat.tamu.edu/color.html)

According to Feiner (2006) and Cornforth (1998), myoglobin exists in three main forms, each producing a characteristic color; purple deoxymyoglobin (Mb), red oxymyoglobin (MbO₂), and brown metmyoglobin (metMb).

In living tissue, the physiologically active oxymyoglobin (MbO₂) and deoxymyoglobin (Mb) forms are maintained through the activity of metmyoglobin (metMb) reductase enzymes (Feiner, 2006). These processes decline postmortem, and storage conditions become more important in determining the proportion of each myoglobin form present (Warriss 2000).  In muscle immediately after slaughter, beef meat color is a deep purplish. As oxygen in the air comes in contact with exposed meat surfaces, it is absorbed and combines with myoglobin, turning the meat a brighter color (Cornforth, 1998). This brighter red pigment is called oxymyoglobin. Oxymyoglobin (MbO₂) is the pigment responsible for the preferential bright red color of raw meat, and is formed rapidly in the presence of oxygen at normal atmospheric pressure (Varnam and Sutherland, 1995). MbO₂ predominates at fresh meat surface (Cornforth, 1998).

Deoxymyoglobin (dMMb) exists in absence of oxygen, such as in the bulk of meat portions and in vacuum-packaged meats (Fennema, 1996). The ferrous iron becomes oxidized by free radicals when meat is stored for long periods of time, producing the brown pigment MetMb, which also forms where oxygen-dependent meat enzymes and aerobic microorganisms successfully compete with meat pigments for oxygen (Feiner, 2006). Myoglobin and oxymyoglobin lose electrons (oxidize), turning the pigment brown colored called metmyoglobin.

                               Fig 2; Basic transformation of myoglobin (Zdzlaslaw, 2002)

According to Zdzlaslaw (2002), oxymyoglobin and myoglobin exist in a state of equilibrium with oxygen; the ratio of these pigments depending on oxygen pressure. The heme pigment in meat is slowly oxidized to metmyoglobin and the formed metmyoglobin cannot bind oxygen (Cornforth, 1998).

      

Fig 3; Transformation of myoglobin to oxymyoglobin to metmyoglobin (http://meat.tamu.edu/color)

The Effect of Temperature on Meat Color.

Myoglobin (only about 0.5% of the wet weight of red meats), its response to heat largely determines the color of cooked meat (Nicola and Rosemary 2006). Heating causes denaturation of the globin, which then precipitates with other meat proteins. Denaturation of myoglobin and other proteins begins between 55⁰C and 65⁰C in meat, and most denaturation occurs at 75⁰C or 80⁰C (Varnam and Sutherland, 1995). The rate of myoglobin denaturation decreases with increasing meat temperature, and this is related to the simultaneous rise in meat pH with cooking (Nicola and Rosemary 2006).

The three forms of myoglobin differ in their sensitivity to heat. Deoxymyoglobin is the least sensitive to heat denaturation, followed by Oxymyoglobin, then by metmyoglobin, though the latter two (MbO₂ and metMb) have fairly similar heat sensitivity. As the globin is denatured, metMb forms the brown globin hemichromogen (ferrihemochrome) and the other myoglobins are denatured to the red globin hemochromogen, (ferrihemochrome) (Nicola and Rosemary, 2006). The latter is readily oxidized to the former, so ferrihemochrome is present in larger amounts in cooked meats (Varnam and Sutherland, 1995). Adequate cooking of meat produces a color change to off-white, grey, or brown hues, depending on the type of muscle (Nicola and Rosemary, 2006). The ultimate color depends on the extent of ferrihemochrome formation, which in turn is a product of the initial proportionality of the myoglobins, and the final concentration of undenatured oxymyoglobin (Gorgulho, 2009). Myoglobin, oxymyoglobin, and metmyoglobin can all be changed from one to the other when the appropriate conditions exist. (Nicola and Rosemary, 2006;  http://meat.tamu.edu/color).

A brown pigment, which is denatured metmyoglobin, is formed with cooking, which normally cannot be changed to form another pigment (Nicola and Rosemary, 2006).

Fig 4; Characteristics of the myoglobin pigment in meat, their dynamic relationships, and the denatured products formed during cooking (Source: Nicola and Rosemary, 2006).

The Influence of pH on Cooked Meat Color

Normal fresh meat has a pH ranging from 5.4 to 5.6 (Varnam and Sutherland, 1995). The amount of ferrihemochrome formation from myoglobin during cooking is affected by initial meat pH (Gorgulho, 2009). The muscle contains glycogen but with postmortem, glycogen is broken down to lactic acid, lowering the pH due to a reduction in oxygen supply. This acidification process continues until either the glycogen is consumed or the low pH inactivates glycolytic enzymes (Varnam and Sutherland, 1995).

Meat with a pH above 6.2 tends to have a tightly packed water-retaining fiber structure that impedes oxygen transfer and promote longer survival of oxygen-scavenging enzymes, favoring Mb rather than MbO₂ (Varnam and Sutherland, 1995). The purple-red myoglobin combines with the closed structure of the muscle to absorb rather than reflect light, making the meat dark, firm and dry (DFD) and for the pale, soft, exudative (PSE) meats, postmortem glycogen levels are reasonably high, and the acidification is accelerated so that the pH falls rapidly, while the muscle is still warm (Feiner, 2006; Adams and moss, 2000). The combination of high temperature and low pH causes protein denaturation, water loss and an open muscle structure. The low pH also tends to promote oxidation of MbO₂ and Mb to brown metMb, which combines with high light scattering from the meat surface, giving the meat its pale color (Adams and Moss, 2000).

The extent to which pH affects the cooked color of meat varies between species (high pH lowers myoglobin denaturation and meat becomes more red) (Nicola and Rosemary, 2006). Meat pH also influences other factors that affect cooked meat color. According to Nicola and Rosemary (2006), these factors include; fat content, freezing and rate of thawing, the initial form of the myoglobin (for example, Mb is less heat-sensitive and more stable at higher pH than other myoglobin forms), the condition and structure of the muscle fibers (for example, DFD vs. PSE), and the denaturation processes of other meat proteins, including enzymes.

Note that all references used in all postings related to the topic of sausages, meat and meat product colorings will be posted in the last article about this topic.

About the author
Mr. Sempiri Geoffery, the author of this article graduated from Makerere University with a Bsc In Food Science and Technology Degree in January, 2011.