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277

4.4 Betalains in Food: Occurrence, Stability, and Postharvest Modifications

Florian C. Stintzing and Reinhold Carle

CONTENTS

4.4.1 Definitions and Structures .........................................................................2774.4.2 Distribution, Natural Functions, and Actions............................................2784.4.3 Current and Potential Food Sources .........................................................278

4.4.3.1 Amaranth .....................................................................................2784.4.3.2 Red Beet ......................................................................................2784.4.3.3 Yellow Beet .................................................................................2844.4.3.4 Swiss Chard.................................................................................2844.4.3.5 Cactus Pear..................................................................................2854.4.3.6 Pitahaya .......................................................................................286

4.4.4 Stability and Postharvest Modifications....................................................2864.4.4.1 Parameters Affecting Stability ....................................................2864.4.4.2 Model Food Systems...................................................................2894.4.4.3 Real Food Systems......................................................................289

4.4.5 Current and Future Perspectives ...............................................................2894.4.5.1 Betalainic Crops ..........................................................................2894.4.5.2 Technological Aspects.................................................................2904.4.5.3 Nutritional Aspects......................................................................290

References..............................................................................................................290

4.4.1 DEFINITIONS AND STRUCTURES

Betalains are N-heterocyclic water-soluble pigments deposited in vacuoles. Theircommon precursor is betalamic acid consisting of a 1,7-diazaheptamethin system,an extended π-electron system exhibiting a canary yellow color. Betalamic acid maycondense with cyclo-dopa to yield betanidin, the common precursor aglycon of thered betacyanins. Betanidin in turn may be glycosylated and/or acylated, yielding 29

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278 Food Colorants: Chemical and Functional Properties

genuine structures known to date. Due to stereoisomerism at C15 their number isdoubled, except for neobetanin (14,15-dehydrobetanin) which is devoid of the chiralcenter at C-15.1

The yellow analogues, the betaxanthins, are composed of betalamic acid withamino acids or amines, respectively, amounting to 26 structures known to occurnaturally.2–4 Structures unambiguously assigned by NMR spectroscopy usually carrytrivial names derived from the plant material from which they have been first isolated.The substitution patterns of betalyanins and betaxanthins hitherto reported togetherwith their particular plant sources are listed in Table 4.4.1 and 4.4.2, respectively.

4.4.2 DISTRIBUTION, NATURAL FUNCTIONS, AND ACTIONS

Betalains are known to occur in 13 plant families of the Caryophyllales and havenever been found to co-occur with anthocyanins in the same plant.46–48 The presenceof betalains in some genera of higher fungi such as Amanita, Hygrocybe, andHygrophorus represents a paradigm of chemical convergence. While their occurrenceis quite limited as compared to the anthocyanins, betalain presence in edible plantmaterial is even more restricted.49–52 The structural diversity of beta-cyanins andbetaxanthins found in food plants i given in Figure 4.4.1A and 4.4.1B.

4.4.3 CURRENT AND POTENTIAL FOOD SOURCES

4.4.3.1 AMARANTH

The Amaranthaceae include 65 genera and about 1000 species of annual and perennialplants, shrubs, and trees from tropical, subtropical, and temperate zones of Africa,South America, and south east Asia.53 Traditionally used in folk medicine, some mem-bers are used for pigment exploitation.54

Consequently, studies of the betalain patternand the potential for processing amaranth into coloring preparations have been carriedout.55 Amaranth plants may be subdivided into grain and vegetable or leafy types. Theyare well known as ornamental plants and recent studies were dedicated to the coloredinflorescences of Celosia sp.56 However, their broader use for food coloring outsidetheir region of origin, where no traditional usage exists, remains doubtful.

4.4.3.2 RED BEET

Starting in the early 1970s, red beets have been applied commercially since thepioneering work by Von Elbe and co-workers.57,58 At that time, red beet betalainsconsisting of betanin, isobetanin, prebetanin, and smaller quantities of vulgaxanthinI and vulgaxanthin II, respectively, were proposed as viable alternatives to syntheticcolorants, especially in low-acid foods such as dairy and meat products.6,25,42,59–63 Inthe latter commodities, red beet was even considered as a replacement for nitrite toachieve reddening, but of course it lacks the antimicrobial activity of nitrite. More-over, red beet addition contributes to higher nitrate levels of the particular foodwhich represents a potential source of nitrite.

9357_C004.fm 78 Monday, August 27, 2007 10:13 AM


Betalain

s in Fo

od

: Occu

rrence, Stab

ility, and

Posth

arvest Mo

difi

cation

s279

TABLE 4.4.1Substitution Patterns of Genuine Betacyanins of Higher Plants

Betacyanin (Bc) Substituent Occurrence References

5-O-glycosidesAmaranthin Glucuronyl-glucose Amaranthus sp.; Bougainvillea sp. 5–8Betanin (syn. phytolaccanin) Glucose Amaranthus sp.; Beta sp., Hylocereus sp.,

Opuntia sp., Phytolacca sp., Portulaca sp.9–13

Bougainvillein r-I Sophorose Bougainvillea sp.; Hylocereus sp.; Mesembryanthemum sp.

14–16

Celosianin I Coumaroyl-glucuronyl-glucose Celosia sp. 10,11Celosianin II Feruloyl-glucuronyl-glucose Celosia sp. 10,112-Decarboxybetanin Glucose Beta sp. 17,18Malonyl-2-Descarboxybetanin Malonyl-glucose Schlumbergera sp. 17Hylocerenin 3-Hydroxy-3-methyl-glutaryl-glucose Hylocereus sp. 15,19Iresinin I Glucuronyl-3-hydroxy-3-methyl-glutaryl-glucose Iresine sp. 10,11,20Lampranthin I Coumaroyl-glucose Lampranthus sp. 21Lampranthin II Feruloyl-glucose Beta sp., Lampranthus sp.,

Mesembryanthemum sp., Phytolacca sp. 2,12,16,21

Neobetanin (=14,15-Dehydrobetanin) Glucose Beta sp., Opuntia sp. 4,22–24Phyllocactin Malonyl-glucose Beta sp., Hylocereus sp., Opuntia sp. 4,14,15,19Prebetanin 6-Sulfatyl-glucose Beta sp.; Phytolacca sp. 12,23,25Rivinianin 3-Sulfatyl-glucose Rivina sp. 26Phytolacca-Bca Feruloyl-apiose Phytolacca sp. Schlumbergera sp. 12,27

Continued.

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280Fo

od

Co

loran

ts: Ch

emical an

d Fu

nctio

nal Pro

perties

TABLE 4.4.1 (Continued)Substitution Patterns of Genuine Betacyanins of Higher Plants

Betacyanin (Bc) Substituent Occurrence References

Schlumbergera-Bc Ia Malonyl-apiose Schlumbergera sp. 27Schlumbergera-Bc IIa Feruloyl-malonyl-apiose Schlumbergera sp. 27Mesembryan themum-Bc Ia Feruloyl-di-glucose Mesembryanthemum sp., Beta sp. 4,16Mesembryan themum-Bc IIa Di-feruloyl-di-glucose Mesembryanthemum sp. 16Mesembryan themum-Bc IIIa Tri-feruloyl-di-glucose Mesembryanthemum sp. 16

6-O-glycosidesBougainvillein v Sophorose Bougainvillea sp. 5,28Gomphrenin I Glucose Basella sp., Bougainvillea sp., Gomphrena sp.,

Opuntia sp., Phytolacca sp. 4,5,10–13,29–31

Gomphrenin II Coumaroyl-glucose Gomphrena sp. 10,31Gomphrenin III Feruloyl-glucose Gomphrena sp. 10,31Bougainvillea-Bc Ia 6-Caffeoyl-sophoroside Bougainvillea sp. 5Bougainvillea-Bc IIa 6″-Coumaroyl-sophorose Bougainvillea sp. 5Bougainvillea-Bc IIIa 6-Coumaroyl-sophorose Bougainvillea sp. 5Bougainvillea-Bc IVa 2″-sophorosyl-6-caffeoyl-6″-coumaroyl-

sophoroseBougainvillea sp. 5

Bougainvillea-Bc Va 2″-glucosyl-6-caffeoyl-6″-coumaroyl-sophoroside

Bougainvillea sp. 5

Bougainvillea-Bc VIa 2″-glucosyl-6,6″-di-coumaroyl-sophoroside Bougainvillea sp. 5Bougainvillea-Bc VIIa 6,6″-di-coumaroyl-sophoroside Bougainvillea sp. 5

a No trivial name established.

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Betalains in Food: Occurrence, Stability, and Postharvest Modifications 281

A great number of further investigations addressed betalain stability related topH, oxygen, light, and temperature.64–66 Technological studies were also carried outto establish a production scheme for industrial processing of red beets into juiceconcentrate. To increase pigment contents in the final product, a fermentation stepwas proposed to remove sugars, thus achieving a greater color intensity of theresulting 65°Bx concentrate. Moreover, breeding strategies were early consideredto be successful for increasing the pigment content of the raw material.

The fortunate constellation of scientists at Wisconsin facilitated further achieve-ments toward increasing pigment content in red beets. In a recurrent selectionprogram, betalain contents of red beet crops were enhanced by 200%.67,68 Thisexample should encourage both food scientists and plant breeders to join forces andinvest in the improvement of promising color crops.

TABLE 4.4.2Substitution Patterns of Genuine Betaxanthins of Higher Plants

Betaxanthin (Bx) Substituent Occurrence References

Dopaxanthin Dopa Glottiphyllum sp., Lampranthus, sp.,Portulaca sp.

32–35

Humilixanthin 5-Hydroxynorvaline Rivina sp. 36Indicaxanthin Proline Opuntia sp., Beta sp. 2,4,34,37–39Miraxanthin I Methionine sulfoxide Mirabilis sp. 40,41Miraxanthin II Aspartic acid Mirabilis sp. 41,42Miraxanthin III Tyramine Beta sp., Mirabilis sp. 2,41,42Miraxanthin V Dopamine Beta sp., Mirabilis sp. 2,42Muscaaurin VII Histidine Beta sp., Opuntia sp. 2,4,39Portulacaxanthin I Hydroxyproline Portulaca sp. 43Portulacaxanthin II Tyrosine Beta sp., Portulaca sp. 2,44Portulacaxanthin III Glycine Beta sp., Portulaca sp. 2,44Vulgaxanthin I Glutamine Beta sp., Opuntia sp., Portulaca sp. 2,4,23,34,

39,40,45Vulgaxanthin II Glutamic acid Beta sp., Portulaca sp. 2,23,34,35,

40,45Vulgaxanthin III Asparagine Beta sp. 2Vulgaxanthin IV Leucine Beta sp., Opuntia sp. 2,39Ala-Bxa Alanine Beta sp. 2Gaba-Bxa γ-Amino butyric acid Beta sp., Opuntia sp. 2,39,45HisA-Bxa Histamine Beta sp. 2Ile-Bxa Isoleucine Beta sp., Opuntia sp. 2,39Met-Bxa Methionine Opuntia sp. 4MeTyr-Bxa 3-Methoxytyramine Beta sp., Celosia sp. 2,13Phe-Bxa Phenylalanine Beta sp., Celosia sp., Opuntia sp. 2,39Ser-Bxa Serine Beta sp., Opuntia sp. 2,39Trp-Bxa Tryptophan Beta sp., Celosia sp. 2,13,39Val-Bxa Valine Beta sp., Opuntia sp. 39

a No trivial name established.




FIGURE 4.4.1A Betaxanthins (bx) from food. 1. Alanine –bx. 2. Glycine bx. 3. γ-aminobu-tyric acid –bx. 4. Serine –bx. 5. Methionine –bx. 6. Leucine –bx. 7. Isoleucine –bx. 8. Valine–bx. 9. Proline bx. 10. Asparagine bx. 11. Glutamic acid bx. 12. Glutamine bx. 13. Methoxy-tyramine bx. 14. Phenylalanine –bx. 15. Tyrosine –bx. 16. Tyramine –bx. 17. Tryptophan–bx. 18. Dopamine –bx. 19. Histidine –bx. 20. Histamine –bx.

H2N O

+

H

COO−H

N

NH COOHHOOC

H 11

12

HO

11H

HOOC COOH

N

HCOO−

H

+

4

H3C CH3

20191817

16151413

1110

8765

9

321

S

11H

HOOC COOHNH

NH

COO−

H

+

H3C

HOOCH3

H +N

COOH11H

HOOC

HO

H +N

COOH11H

HOOC

COO−+HN

NHHOOC

H 11COOHCOOH

11H

HOOC NH

NH +

COO−

CH3

HN

HN

H +N

11H

HOOC NH

NH

NH

NN

HOOC

H 11COOH

N+H

COO−

HH

H +N

COOH11H

HOOC

COO−

H H

COO−

HO

11H

HOOC COOHNH

N

HCOO−

H

+

O

NH

NHHOOC

H 11COOH

N+H

HO

H

H2N

O

+

H

COO−H

N

NH COOHHOOC

H 11

H

N COO− COO−H +

N

COOH11H

HOOC

HH3C CH3

H3CCH3

H

HOOC

H 11COOH

N+H

COO−

HOOH

H +N

NHHOOC

H 11COOH

COO−+HN

COOH11H

HOOC

HN

COO−H +

N

NH

NH

NH

NH

HOOC

H 11COOH

COOH11H

HOOC

NH COOHHOOC

H 11

N+

COO−

H

NH COOH

NH N

H




The adverse properties of red beet preparations are without doubt, their highnitrate content, their unpleasant smell due to pyrazine derivatives and geosmin witha low odor threshold, and finally the risk of carry-over of earth-bound germs.66,69–74

Since nitrate has been associated with nitrite and nitrosamine formation, the latterexhibiting cancer-inducing properties, procedures have been developed to reducethe nitrate content of red beet preparations.75,76 This has been achieved by microbialdenitrification.77–79

FIGURE 4.4.1B Betacyanins from foods. I. Betanin. II. Neobetanin. III. Gomphrenin I. IV.Prebetanin. V. Bougainvillein r-I. VI. Phyllocactin. VII. Amaranthin. VIII. Hylocerenin. IX.Lampranthin II.

N

H

HOOC

+ COO−

HO

O 5

15

OHO

HO1'

6'C

HHO

2'

2'

O HH

C6'

1'HO

HOO

15

5O

HOCOO−+

COOHHOOC

H

N

H

HO

HOHO

6'C

HHO

2' O

OH

OH

NH COOH

H

NH

NH

OH

O2'

O C6'

HOHO

OH

H

N

H

HOOC COOH

+ COO−

HO

O 5

15

OHO

HO1'

6'C

HHO

2'

O

COOHNH

OH2'

O HH

C6'

1'HO

HOO

15

5O

HOCOO−+

HOOC

H

N

S

O

OHO

HO

O

C

O

C

N

H

HOOC COOH

+ COO−

HO

O 5

15

OHO

HO1'

6'CH

HO

2'

OH

14

N

H

HOOC COOH

+ COO−

HO

O 5

15

OHO

HO1'

6'C

HHO

2'

OH

H

NH

15

O

HO

COOH+

COOHHOOC

H

N

OH2'

O HH

C6'

1'HO

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6

H

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C6'

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15

5O

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COOHHOOC

H

NOH

C

O

NH

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H3CO

CH2 CHOOC CH2

OH

CH33''

NH

OH2'

O HH

C6'

1'HO

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15

5O

HOCOO−+H

N

HOOC COOH

NH

I II

III IV

V VI

VII VIII

IX

NH




Geosmin formation has been associated with Streptomyces species that generatethe smell of wet earth.80,81 It has therefore been intriguing to investigate whetherred beet alone has the potential to produce geosmin. Using cell cultures, Lu andco-workers unambiguously demonstrated that geosmin formation was an endoge-nous trait of beetroot.82,83 Geosmin removal may be desired and has been achievedby ultrafiltration patented by the Henkel Company.84 Due to culture conditions,earth-bound yeasts and Bacillaceae spores attached to the roots may be easilycarried over into the refined color preparation, thus representing a serious hazard.Therefore, special requirements apply, i.e., yeast counts must remain below 10/gfor dairy products.85

4.4.3.3 YELLOW BEET

Although the subjects of early analytical investigations in the late 1970s, yellowbeets, also known as golden beets, have not yet been widely introduced on themarket as vegetables or as sources of yellow pigments. The pigment profile respon-sible for the bright yellow tint was first characterised by Strack and Reznik34 andSavolainen and Kuusi86 and was found to consist of up to ten constituents, four ofwhich were assigned to vulgaxanthin I, vulgaxanthin II, indicaxanthin, and beta-lamic acid. A more recent study by Stintzing and co-workers revealed an evenmore complex pattern.39

Yellow beets may be valuable companions for red beets in establishing colorblends, thus allowing a broader color range than obtainable with red beets alone. Inaddition, food-derived yellow water-soluble pigments are scarce and golden beetsmay enlarge the candidate list of betalainic food sources. According to our obser-vations, yellow beets have considerable browning potential, which may have dis-couraged potential producers of food colorants (Stintzing and coworkers, unpub-lished data). While the same browning potential is also inherent to red beets, theyellow color of the golden variety is overpowered by oxidized phenolics, thusrendering its preparations visibly unattractive. In contrast, red beets exhibit thepotential to compensate for this brownish tint by their red-purple pigments.

High levels of polyphenol oxidase activities are held responsible for the unde-sirable appearance, but till now, the browning potentials of different red and goldenbeet varieties appeared to be unknown.87,88 Because of the apparent need for yellowwater-soluble color preparations, our current investigations are aimed at developinga line for processing yellow beets into juice and concentrate. To obtain a product ofbrilliant yellow appearance, efficient strategies for browning inhibition are elabo-rated. Together with CIE L*C*h° measurements, HPLC-DAD represents a powerfultool for accompanying and evaluating technological studies. Apart from being usedas sources for food coloring uses, yellow beets are delicious vegetables that exhibitless earthy aromas than red beets and add new color to everyday dishes.

4.4.3.4 SWISS CHARD

Swiss chard, also known as foliage beet, silver beet, or perpetual beet, is mostlyknown as a green-white vegetable; the colored cultivars are less widespread.




Zakharova and co-workers studied a red variety in 1997 to gain a closer insight intoits polyphenol oxidase activity.89 More recently, a study addressing the antioxidantproperties of a red colored Swiss chard was published.90 However, the pigmentswere erroneously addressed as anthocyanins.

Since it was reported that the Bright Lights cultivar presented a number of colors,a thorough investigation was dedicated to the pigment profiles of yellow, orange,red, and violet petioles.2,91 Applying a highly powerful HPLC separation, a total of19 betaxanthins could be characterized, some of which were new structures. Thecolor gamut ranging from yellow to orange to red and violet was found to be dueto a fine-tuned blend of betacyanins and betaxanthins. The yellow and the violetpetioles exclusively contained yellow and red betalains, respectively, while increas-ing proportions of betacyanins and constant betaxanthin concentrations were respon-sible for the orange and red tonalities.2

These principles are most important to obtain tailor-made coloring foodstuffsallowing a larger variety of shades. Whether colored Swiss chard petioles may gainimportance for food coloring in addition to their culinary value is doubtful. Due tothe low pigment contents of 4-8 mg/100g petiole, exploitation for food coloringpurposes does not appear to be economically viable. Reaching a hectare yield of 35to 40 tons annually, only 50% of the harvested material is colored suitable forextraction. Hence, breeding activities to enhance pigment contents are consideredprerequisite to rendering Swiss chard more economically feasible.

4.4.3.5 CACTUS PEAR

Cactus pear, previously known as prickly pear, has been mainly studied for its greenfleshed pads known as nopalitos.92 It originates from Mexico but is cultivated inChile and Peru, mainly for cochineal production, and also in the U.S. (Californiaand Texas), South Africa, India, Israel, and the Mediterranean, amounting to a totalof 100,000 hectares under cultivation.93 The cactus pear fruit is a berry with manyseeds and a mean weight of 160 g. Its fruit pulp covers a color range from deeppurple to green.94,95

The overall fruit composition was studied thoroughly starting in the early 1980s,when Askar and El-Samahy reported extraordinarily high amino acid contents.96

Later investigations characterized cactus pears as sweet fruits containing fructoseand glucose with a high sugar-to-acid ratio and a faint melon–cucumber-like taste.94,95

Most interestingly, the presence of high taurine levels in South African cactus pearsand lower levels in Italian cultivars were described.38,97 Since then, taurine has beenmainly believed to be of animal origin. As a result, cactus pears were considered asalternative sources for natural taurine, especially for functional drinks.94,98,99

Proline, however, is the predominant amino acid and most interestingly, itsbetalamic acid adduct indicaxanthin is the major betalain pigment in cactus pear.100

Proline functions as an osmolyte, accumulating in water- and heat-stressed planttissues, and in comparison to other amino acids, it exhibits an extraordinarily highsolubility of 1623 mg/L water at 25°C.101–108

Odoux and Domínguez-López were the first to propose cactus pears for foodcoloring purposes.109 In-depth research was carried out by Stintzing and co-workers




since 1999.94,110,111 Color blends with pitahaya have been systematically investigatedand demonstrate the viability of cactus fruits for color preparations.112 Until recently,high pectin content along with the concomitant presence of reducing sugars andhigh amino acid contents potentially leading to brown Maillard-type products havebeen the main hurdles related to cactus pear processing. Therefore, a line for theproduction of cactus pear juice at semi-industrial scale has been established andextended to coloring concentrates and fruit powders from the most common Euro-pean cactus pear, Opuntia ficus indica cv. Gialla.113,114 In a parallel investigation,purple cactus pears were used as starting materials for spray-dried powders.115

These studies demonstrate that the processing of cactus pears into coloringpreparations is feasible. It is now up to companies to exploit the enormous potentialof these fruits. The by-products from Opuntia processing such as seeds and peelscould be further processed into vegetable oil and pectin, respectively.

4.4.3.6 PITAHAYA

Originating from tropical regions of North and South America, pitahayas are nowgrown in Guatemala, Nicaragua, Vietnam, Taiwan, Thailand, Indonesia, Israel, theU.S. and Australia. Their fruits are oblongate, 8 to 10 cm long, with a mean weightof 350 to 450 g. The seeds are small and digestible, comparable to those from kiwi.116

In the strictest sense, pitayas originate from columnar cacti belonging to the Cereusand Stenocereus subfamilies. On the other hand, pitahayas are the fruits from theHylocereus and Selenicereus vine cacti.

Pitahayas, usually called pitayas, comprise all fruits belonging to the Hylocereusand Selenicereus species. While the latter produce white-fleshed fruits such asSelenicereus megalanthus, fruits derived from Hylocereus sp. may exhibit a white,red, or even red-purple fruit pulp. In parallel studies, the pigments responsible forthe red-purple color of H. polyrhizus were identified as betanin, phyllocactin, andthe 3-hydroxy-3-methyl-glutaric acid adduct of betanin, later named hylocerenin.15–19

Various clones have been investigated with respect to their pigment profiles exhib-iting varying patterns among clones.117

4.4.4 STABILITY AND POSTHARVEST MODIFICATIONS

4.4.4.1 PARAMETERS AFFECTING STABILITY

Since several previous reviews have treated betalain stability, the interested readeris referred to those for more detailed information.64–66,91,111,118 This chapter is intendedto present some general aspects of handling betalainic foodstuffs.

As soon as betalains are extracted from the vacuoles through plant tissue decom-partmentalization, they are prone to degradation. Factors supporting stabilization orrather destabilization are summarized in Figure 4.4.2.

Two main groups can be discerned (1) endogenous factors such as plant enzymeslike polyphenoloxidase, peroxidase, and β-glucosidase and (2) conditions prevailingin the extraction medium will decide the fates of betalain pigments among whichtemperature, oxygen, and pH are considered the most important conditions.




pH value — It is generally accepted that betalains are most stable in a broadpH range from 3 to 7.119 In contrast to the more common anthocyanins, they exhibitunchanged tint and retain their coloring power.111,120 The pH optima of betalainsonly differ slightly. Interestingly, color retention is optimum at the pH of the respec-tive plant source, i.e., red beet betalains are most stable at the pH of red beet around5.5 to 6.0.61,121–24 Shifting the pH to values below 3 will result in color loss, althoughthe exact mechanisms have not yet been elucidated.118 In alkaline media, the betalainswill be cleaved into their biosynthetic precursors, betalamic acid and cyclo-dopa(betacyanins) or amino compounds (betaxanthins), all of which may further degrade,thus ultimately leading to color fading.66,91,111 As a rule, betacyanins are consideredmore resistant to acidic media, while betaxanthins are most stable at neutral pH.These characteristics may change according to their respective chemical structures.In general, betalains are most suited for low-acid foods such as dairy and meatproducts as well as ice cream.

aw value — As with other natural colorants, a greater water activity is detrimentalto color stability through cleavage as is the case in alkaline solutions. Along withgreater water content, both the mobility of food components and oxygen solubilitywill be increased, adding up to betalain destabilization.125 As a result, betalains maybe applied successfully to fruit fillings, creams, instant products such as soups, andconfectionary items.64,66,126 On the other hand, color preparations are usually spray-dried or concentrated to at least 65°Bx to secure maximum stability during storageand transportation.127–129

Endogenous and exogenous enzymes — While the role of β-glucosidase affect-ing betalains is quite clear, the roles of polyphenoloxidase and peroxidase are lessobvious. Betacyanin glycosides may be cleaved into the corresponding aglycones, the

FIGURE 4.4.2 Factors influencing betalain stability.

Betalain stability

−

3 > pH > 7 ϑ > 20°C h ∗ν

aW > 0.6; rel. humidity > 5%

O2; H2 O2; SO2

metal ions peroxidase polyphenoloxidase β-glucosidaselow pigment concentration

low degree of glycosylation

low degree of acylation

3 < pH < 7

ϑ < 10°C

darkness

aW ≤ 0.6; rel. humidity ≤ 5%

N2 atmosphere

antioxidants

chelating agents

matrix constituents

high pigment concentration

high degree of glycosylation

high degree of acylation

+




latter being highly unstable and oxidized further. On the other hand, acylated betacy-anins will not be affected and remain stable toward both endogenous glucosidaseactivity and side activities that may stem from pectinolytic preparations during mashenzymation.15,20,29 Microorganisms intentionally applied for denitrification and alsofor sugar removal prior to concentration or lactic acid fermentation may be the thirdsource of glycoside cleaving activities.76–79,129–133 Polyphenoloxidases and peroxidasesreleased from the comminuted plant tissue will be inactivated only by heat.87 Hence,the period between mashing and heating of the filtered juice should be minimized.While peroxidases affect betalain stability by radical formation with concomitantpigment fading, polyphenoloxidases chiefly oxidize betalain aglycones and accompa-nying colorless phenolics, thereby unfavorably affecting color. Finally, acidificationpresents a feasible tool for reducing the activities of endogenous enzymes.134–136

Metal ions — Several investigations have proven that Sn2+, Al3+, Ni2+, Cr2+, Fe2+,Fe3+, and Cu2+ accelerate betalain color loss, the latter two being the most deleteri-ous.137–139 Metal ions may originate from the crop, from soil accompanying the crop,and from the processing machinery. The first source can hardly be influenced, butearth-borne metal contamination can be minimized by extensive washing. The poten-tial contamination should not pose a problem when suitable processing equipmentis used. To reduce oxidative damage by metal ions, the use of citric acid is recom-mended for acidification and as a chelating agent in the manufacture of pigmentpreparations.140

Oxygen — Oxygen is a substrate for all of the above-mentioned oxidativeenzymes; it catalyzes metal-induced oxidation and also directly affects betalainstability, especially beyond their optimum pH levels.121,141 Nitrogen flushing may berecommended, but will rarely be applied under normal processing conditions.Although the oxygen solubility in water is improved when water temperature islowered, oxidative events are assumed to proceed more slowly in cool environments,thus improving overall pigment retention.

Light — In plant tissues, betalains function as UV-screens, readily absorbinglight energy.16,142 Hence, as is true for all natural pigments, betalains will degradewhen exposed to light, especially in the presence of oxygen and at pH deviatingfrom the stability optimum.58,143,144 Generally, processing and storage should becarried out in the absence of deleterious light exposure. Consequently, products thatare sold in transparent packaging should be subjected to extensive testing under realillumination conditions. It should also be considered that light exposure will heatthe food products, thus aggravating color loss. Interestingly, the effect of light isonly measurable below 25°C; above that it is overpowered by thermal damage.143

Temperature — Temperature is the most critical factor related to betalainstability.64,65,91,118,144 However, thermal processing is required to inactivate enzymesand prevent microbial spoilage. To avoid the heat burden of sterilization conditionsfor preparations with pH values above 4.3 such as red beet or cactus pear, acidifi-cation to pH 4 by citric acid addition is advantageous. Thus, pasteurization below100°C is sufficient to achieve microbial stability. However, since betalain degradationis enforced from at temperatures higher than 30°C, color losses during processingare inevitable. In conclusion, strategies for maximizing pigment retention are com-plex and need to be scrutinized to assure optimum betalain protection and color yield.




4.4.4.2 MODEL FOOD SYSTEMS

Most published data deals with model solutions to assess the major factors influ-encing betalain stability, among which pH and temperature are most frequentlyaddressed. Until recently, total color loss was assessed by spectrophotometric mon-itoring of the decline at the wavelength of maximum absorption. To predict colorfading over time, kinetic data were derived therefrom, most often obeying first-orderdecay principles.

Only Schwartz and Von Elbe attempted to investigate the degradation productsrelated to thermal treatment and found 15-decarboxylated betanin in addition to beta-lamic acid and cyclo-dopa-glucoside.145 These findings were scrutinized recently whenred beet and purple pitahaya juice were exposed to heat in a varying range of holdingtimes and produced a great number of different dehydrogenated and decarboxylatedproducts.146,147 Subsequently, studies of both aqueous and ethanolic solutions yieldingsimilar degradation products were conducted.148,149 Very recently, the structural char-acterizations of the novel structures and color alterations thereby induced were inves-tigated in depth.146,147,150,151 While decarboxylation of betacyanins at C15 and C2 did notaffect visual appearance, 17-decarboxy structures exhibited an orange hue.8,151

In addition, dehydrogenated structures were found to cause a hypsochromicshift.146,147,151 Since, with the exception of neobetanin, these betacyanin degradationproducts were found not to be native to betalain crops, it is suggested that thesepeculiar structures may be used as heat indicators to assess the thermal loads of therespective products have undergone during production.

4.4.4.3 REAL FOOD SYSTEMS

Although a protective matrix effect has been assumed to be responsible for improvedbetalain stability, investigations on the tinctorial stability of foods colored withbetalain preparations are scarce.123,153,154 Hávlíková et al. performed a 10-day stabilitystudy of soft drinks at a betalain concentration of 2.5 g/100 g at pH 3.154 The additionof 50 mg ascorbic acid/100 g beverage solution was shown to improve overall colorstability. Temperatures below 5°C were recommended for optimum pigment reten-tion. Cai and Corke applied amaranth pigments to jelly, ice cream, and modelbeverages at different concentrations over a period of 20 weeks.155 While stabilitywas reasonable for ice creams at –18°C and jellies at 4 and 14°C, respectively, thelatter faded at 25°C, and especially at 37°C. In consideration of the little dataavailable, more extended studies are warranted for assessing betacyanin and betax-anthin retention and considering different matrices such as foods rich in starch,protein, or pectin to target novel betalain applications for food coloring.

4.4.5 CURRENT AND FUTURE PERSPECTIVES

4.4.5.1 BETALAINIC CROPS

Betalains have recently regained importance due to continuing interest in naturalfood colorants. Currently, red beet is the only food source commercially exploited,although amaranth, Swiss chard, yellow beet, and cactus fruit represent promising




future color crops. The processing of cactus fruit into colored preparations bearsgood prospects because (1) schemes for processing cactus fruits into juices andconcentrates are established and ready for industrial scale-up, (2) Opuntia plantsexhibit great genetic potential to be exploited for increasing pigment yield withoutthe necessity of GMO plants but rather by appropriate selection and breeding strat-egies, and (3) Opuntia is an ecologically viable crop showing an excellent climaticadaptability and high water-use efficiency, making it a suitable crop for cultivationin both arid and semi-arid regions.94,114–116,156–159

Enlarging the current acreages and thereby reducing the costs of the fruit isconsidered the last hurdle for cactus exploitation on an industrial scale. Due todropping market prices for cochineal, the red anthraquinone pigment from femalelice settling on cactus pear pads, plantations that are no longer needed for cochinealharvest could be converted into fruit production. On the other hand, screening ofcactus fruits for their coloring potential is still very rare and requires intensification.Joint research activities of food scientists and agriculturists to extend our knowledgeabout cactus therefore appears of utmost importance.

4.4.5.2 TECHNOLOGICAL ASPECTS

From a technological view, measures to stabilize betalains during the productionprocess need to be elaborated. Some approaches have been described in the earlierliterature, mainly for red beets and only recently for alternative crops such as cactuspears and red-violet pitahayas. 86,114,115,122,138,139,144, 160,161 Despite being indispensablefor practical application, systematic studies on the stability of betalain preparationsin real food systems during storage are lacking.

4.4.5.3 NUTRITIONAL ASPECTS

Besides their interesting color application values, betalainic plants are also worth-while from a nutritional standpoint. Research on this topic has recently been resumedwith great scientific vigor in both in vitro and in vivo studies on red beets, amaranth,red-colored Swiss chard, red-violet pitahayas, and especially cactuspears.90,92,97,162–182 In the future, betalainic color crops will be benchmarked becauseof their pigment structure and quantity and also because of the individual andsynergistic activities of their components such as colorless phenolics, amino com-pounds, peptides, proteins, and hydrocolloids.

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