Table 821

Stability of Betalains in Model Systems

Factor Model System Observation pH Betalain solutions Maximum color stability between pH 3.5 and 7

Xmax betacyanins: 537-538 nm Xmax betaxanthins: 475-478 nm pH < 3.5, Xmax goes toward lower wavelength, molar absorptivity decreases pH > 7, Xmax goes toward longer wavelength, molar absorptivity decreases Maximum color stability between pH 5.5 and 8

Maximum stability at pH 5.5 Maximum stability between pH 5 and 6 Heating reduces the red color but cooling may reverse the reaction; degradation follows a first-order reaction Rate of betalain degradation increases by 15.6% by daylight exposure at 15°C; A first-order degradation with pH dependence higher at pH 3 (k = 0.35/day) than at 5 (k = 0.11/day) under fluorescent light Total pigment destruction by UV radiation or gamma radiation

Water activity Betanin in different Low aw improves betalain stability; pigment stability model systems decreases one order of magnitude when aw is increased from 0.32 to 0.75

Oxygen Betanin solutions At pH 7.0 betanin degradation is 15% higher in air conditions than samples under nitrogen atmosphere

Temperature Light

Betanin solutions with oxygen Red beet solution Vulgaxanthin solution Betanin solution

Betanin solution

Source: Adapted from Delgado-Vargas et al. (2000).17

8. Processing and Stability a. In Model Systems

During processing, betalain stability is very important. They are pigments with high tinctoreal capacity, but are affected by different factors (Table 8.21).17 pH is highly important, and the effect will depend on the model or food system.127 128 Interestingly, the stability of red beet juice is higher than that of purified extracts, while optimal pigment stability in reconstituted powders has been noted at pH 5.7.129 The effect of temperature is clear on betalain stability and increased temperatures are associated with high degradation rates;130 if heating is not extreme or prolonged, the process of degradation is partially reversible.131 And betanin degradation produces betalamic acid and cyc/o-DOPA-5-O-glucoside.122132

The stability of betalains from Amaranthus species has been studied, where betalain degradation follows first-order kinetics; it is 100 times higher at 100°C

(t1/2 = 19 min) than at 40°C (t1/2 = 2571 min).133 Betanin degradation by temperature and/or pH effect is initiated by a nucleophilic attack (e.g., by water at the C11 position, which is the carbon atom adjacent to the quaternary amino nitrogen).4 Betalain pigments have low stability under light conditions; light excites the n electrons of the double bonds, which causes a higher reactivity (EA = 25 Kcalmol1 in darkness and 19.2 in illumination). Pigment stability in darkness (k = 0.07/day) is at least two times higher than in light conditions.131 The use of UV or gamma radiation imposes stressful conditions, and degradation is higher (Table 8.21).

Betalains show high stability at low water activities (Table 8.21); in fact, the reaction for its degradation involves water.134 135 This phenomenon has been corroborated by using water-alcohol systems where degradation is reduced by decreasing a^ which is associated with a reduced mobility of reactants or limited oxygen solubility.136 As has been mentioned before, dried betalains have improved stability, and spray drying is a feasible process for betalain production. During spray drying, betalains from Amaranthus are degraded by 3% at 150°C and up to 8% at 210°C; products obtained by spray drying below 180°C have similar characteristics to those produced by freeze-drying, and a high solid content favors productivity and stability.137

Coating agents are very important as well. The use of maltodextrins with a low dextrose equivalent (DE) might produce a very high degree of surface indentation and cracking, causing the wall system to become more permeable to oxygen; high DE maltodextrins could form a denser and more oxygen-impermeable wall system, providing better storage stability for pigments. Hence, 25 DE maltodextrin gives the highest pigment retention under the storage conditions; however, 25 DE maltodextrin also shows a high hygroscopicity, and long-term storage may produce high degradation. Thus, it has been found that the use of 25 DE and 10 DE (25:10 = 3:1) results in a good coating agent for betalain stability (63.6 weeks at 32% relative humidity (RH) and 25°C); additionally, the product has good solubility and has been graded as a suitable food-grade colorant.137

It is clear that light has a tremendous impact on increasing the degradation rate (Table 8.21). Moreover, it is also clear that dried Amaranthus pigments clearly have much higher storage stability than aqueous pigments: after 10 months of storage 93% at 4°C and 78% at 25°C of dried pigments are retained against 62 and 18%, respectively, of the aqueous pigments. Water activity is the most important factor for the storage stability of betacyanins in the dark and in the absence of air at any temperature. Consequently, dried Amaranthus pigments are stable enough for use as commercial colorants; indeed, their stability is higher than that of red radish betalains. The improved stability may be associated with acylation of betalains or with the presence of phenolics in the sample that act as antioxidants.133

Another important factor affecting stability is oxygen (Table 8.21), which causes product darkening and loss of color.131 Betanin reacts with molecular oxygen, producing pigment degradation in air-saturated solutions. Degradation kinetics under air atmosphere follows a first-order model.138 As previously mentioned, betanin degradation is a reversible process, but to favor this process it is necessary to have the samples under low levels of oxygen, which improves betalain retention from 54 to 92% (pH 4.75, 130 min, 15°C).128 Several procedures have been attempted to facilitate the regeneration process. In one, ascorbic, gluconic, isoascorbic, and metaphosphoric acids are used to

HOOC N COOH

HOOC N COOH

Betanidine

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