Entrevista semi-estructurada y grupos focales

The cleavage of starch polymer by γ-irradiation is accompanied by the production of free radi- cals that reduce the viscosity of starch by chain reaction. The inorganic peroxides used as addi- tives are easily decomposed by γ-irradiation to produce free radicals. Therefore, a combination of γ-irradiation and inorganic peroxides has a synergistic effect on the formation of free radicals within the starch molecules, which can play a decisive role in decreasing the initial viscosity and increasing the viscosity stability of starch. In one study, the effect of γ-irradiation (10 kGy) on vis- cosity stability of starches consequent to addition of 1–3% ammonium persulfate (APS) to starches has been investigated. It has been found that initial viscosity decreases with an increase in the concentration of added APS. Addition of more that 2% APS dramatically improves the viscosity stability of starch. These investigations suggest that production of modified starches with various levels of viscosity as well as with excellent stability are feasible by control of γ-irradiation dose levels and the addition of APS. Figure 8.2 shows the effects of APS on the viscosity stability of starch paste (Kang et al. 1999).

8.2.2 effecton proteinanD protein-baseD fooD matrices

Irradiation at 1 kGy has shown a decrease in gluten viscosity of commercial Mexican bread-making wheat flour (Arvizu et al. 2006). Studies are reported on the effects of gamma irradiation on the rheo- logical behavior of mixtures of proteins (soy, caseinates, and whey) and glycerol, wherein a decrease in viscosity is observed with irradiation dose. Similar behavior has also been observed for disper- sions containing calcium caseinates and glycerol. At a 2:1 ratio of proteins:glycerol, 21, 35, and 40% reductions in viscosity are observed at 5, 15, and 25 kGy, respectively; the corresponding values for

500 Commercial oxidized starch APS 1% + 10 kGy APS 1.5% + 10 kGy APS 2% + 10 kGy APS 3% + 10 kGy 400 300 Viscosity (cp) 200 100 0 0 1 2 3 Time (hr) 4 5 6

FIGURe 8.2 Effects of ammonium persulfate on the viscosity stability of starch paste. Starch paste (15%)

was made from 10 kGy irradiated starch after mixing 1–3% ammonium persulfate by soaking method. (From Kang, I. J., Byun, M. W., Yook, H. S., Bae, C. H., Lee, H. S., Kwon, J. H., and Chung, C. K., Radiation Physics and Chemistry, 54, 425–30, 1999. With permission.)

Effect of Irradiation on Food Texture and Rheology 111

a 1:1 ratio are 23, 33, and 37%. For these non-Newtonian dispersions, the apparent viscosities are, however, unaffected by irradiation. Sodium caseinate shows a trend toward formation of aggrega- tion of macromolecules at 5 kGy. The change in viscosities of solutions of soy protein isolate, whey protein concentrate, calcium caseinate, and sodium caseinate in glycerol with irradiation dose are illustrated in Figures 8.3a, b, c, and d respectively (Sabato and Lacroix 2002).

The viscosity of liquid egg white decreases dramatically on irradiation regardless of irradia- tion doses used. The egg white becomes watery even after irradiating the shell eggs at 1.0 kGy. Ovomucin is one of the major proteins in egg white, which plays an important role in its gel-like structure. Irradiation causes changes in carbohydrate and protein moieties involved in formation of ovomucin complex, resulting in a loss of gel-like structure. The dramatic decrease in the viscos- ity of egg white is an important physical change in egg by irradiation, which can be used in egg processing. Watery egg white will facilitate the separation of egg white and yolk and low viscosity can improve the flow of liquid egg white or liquid whole egg in plant facilities that break eggs (Min et al. 2005).

Several studies have suggested that irradiation below 3.5 kGy does not affect the gelation proper- ties of liquid egg white significantly. Hardness, springiness (elasticity), cohesiveness, gumminess, and chewiness of irradiated eggs are not very different from their nonirradiated counter parts. Sensory analysis is also unable to detect any texture differences between irradiated and nonirradi- ated hard-cooked egg whites. Therefore, it has been suggested that irradiation of shell eggs below

200 (a) (b) (c) (d) 180 160 140 120 100 Viscosity (cP) Viscosity (cP) Viscosity (cP) Viscosity (cP) 80 60 40 20 _{0 5 10}

Irradiation dose (kGy)15 20 1:1 SPI:Gly 2:1 SPI:Gly 1:1 CaCas:Gly 2:1 CaCas:Gly 1:1 NaCas:Gly 2:1 NaCas:Gly 1:1 WPC:Gly 2:1 WPC:Gly 25 0 5 10

Irradiation dose (kGy)15 20 25 0 5Irradiation dose (kGy)10 15 20 25

0 5 10

Irradiation dose (kGy)15 20 25 18 45 40 35 30 25 20 15 10 16 14 12 10 8 6 0 1 2 3 4 5 6 7 8 9

FIGURe 8.3 (a) Viscosity of soy protein isolate and glycerol solution vs irradiation dose. (b) Viscosity of

whey protein isolate and glycerol solution vs irradiation dose. (c) Viscosity of calcium caseinate and glycerol solution vs irradiation dose. (d) Viscosity of sodium caseinate and glycerol solution vs irradiation dose. (From Sabato, S. F., and Lacroix, M., Radiation Physics and Chemistry, 63, 357–59, 2002. With permission.)

2.0 kGy does not alter the thermal characteristics of egg white proteins. If used for pasteurizing liquid eggs, however, irradiation can improve the efficiency of egg processing steps such as adding or mixing for removal of sugars and for spray drying (Min et al. 2005).

Studies have also found that the storage modulus of the egg yolk as a function of frequency increases during frozen storage for both nonprocessed and irradiated samples. This may be due to the freezing-induced aggregation and gelation of lipoproteins in egg yolk. The storage modulus of the irradiated samples is a little higher than that of nonprocessed samples before storage. This sug- gests some coagulation of lipoproteins in irradiated egg yolk, thus causing the loss of soluble protein immediately after irradiation. Irradiated samples show aggregation during storage. A significant decrease in the storage modulus is observed in nonprocessed samples, which indicates structural breakdown in proteins and other polymers by enzyme or microbial activities. A corresponding decrease in the irradiated samples is delayed and is insignificant. Electron beam irradiation can therefore be an attractive alternative to other preservation methods for liquid egg proteins (Huang, Herald, and Mueller 1997).

8.2.3 effecton otHer fooDsanD fooD biopolymers

Irradiation may not always affect rheology of foods at certain doses. The best example is that of honey, when irradiated at 5 and 10 kGy, which does not show any significant change (p < 0.05) in viscosity (Table 8.3) and rheology compared to the control nonirradiated samples. In three differ- ent temperature regimes studied, both control and irradiated samples display Newtonian behavior. Equations of linearity obtained from plots of shear stress vs shear rate of irradiated honey samples show very high correlation coefficients (Table 8.4), as is true for Newtonian fluids (Sabato 2004).

tABLe 8.3

Averages and standard Deviation of Viscosity Values for Honey (Parana Region) as a Function of Irradiation Doses, Measured at three Different temperatures

Viscosity (cP)

temperature (°C) 0 kGy 5 kGy 10 kGy

30 6142 ± 510 5849 ± 1157 6939 ± 1815

35 3849 ± 239 3594 ± 397 4112 ± 579

40 2433 ± 211 2229 ± 526 2530 ± 428

Source: Sabato, S. F., Radiation Physics and Chemistry, 71, 99–102, 2004. With permission.

tABLe 8.4

equations of Plots of shear stress and shear Rate for Irradiated and Control Honey samples

equations temperature

(°C) 0 kGy 5 kGy 10 kGy

30 y = 69.822x–0.7803 (R2 _{= 0.9991) y = 51.430x–2.0321 (R}2 _{= 0.9975) y = 65.346x–1.2754 (R}2 _{= 0.9989)} 35 y = 39.833x–1.8628 (R2 _{= 0.9999) y = 31.094x–1.744 (R}2 _{= 0.9994) y = 35.868x–1.8315 (R}2 _{= 0.9970)} 40 y = 22.495x–3.1913 (R2 _{= 0.9944) y = 19.682x–1.5001 (R}2 _{= 0.9994) y = 21.921x –0.6642 (R}2 _{= 0.9996)} Source: Sabato, S. F., Radiation Physics and Chemistry, 71, 99–102, 2004. With permission.

Effect of Irradiation on Food Texture and Rheology 113

The rheological properties of hydrocolloids are particularly important when they are used in the formulation of any food, for their effects on their textural attributes. Many factors including the concentration of hydrocolloids, temperature, dissolution, electrical charge, previous thermal and mechanical treatments, and the presence of electrolytes may affect the rheology of the fluid food containing hydrocolloids. Irradiation has an important influence on the flow behavior of hydrocol- loid solutions. Viscosity and consistency of the same decreases with increasing radiation dose. Guar gum is very sensitive to irradiation and its solution apparently loses consistency after irradiation. There is a decrease in CI of salep (which principally contains the polysaccharide glucomannan) up to irradiation dose of 6 kGy, beyond which it increases. The change in CI with irradiation dose in salep occurs within a very narrow range (Dogan, Kayacier, and Ic 2007).

In a study on the effect of γ-irradiation (3 kGy) on water-soluble polysaccharides, chiefly pentosans, from hard red spring wheat, the viscosity of gel produced from the same was found to increase on irradiation. Water-soluble pentosans are highly branched polymers that form very viscous gels. Irradiation apparently changes the number and/or sequence of branching within the pentosan molecule, resulting in a very viscous gel. It is hypothesized that the irradiation dose is not high enough to effect hydrolysis of chemical bonds to reduce viscosity but is high enough to alter the structure. Nonirradiated pentosans have a high degree of branching wherein high numbers of L-arabinose units are present on the D-xylose chain. Postirradiation, the bran pentosans appear to be more linear with less arabinose side chains (Grant and D’Appolonia 1991). Carrageenans, agar, and alginate gels also decrease in viscosity with increasing dose of

5.0 (a) (b) (c) 4.5 4.0 3.5 3.0 Viscosity (cP) 2.5 2.0 1.5 1.0 _{0 2 4} Dose (kGy)6 8 10 Viscosity (cP) 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 Viscosity (cP) 100 90 80 70 60 50 40 30 Temp.60°C 0 2 4

Dose (kGy)6 8 10 0 2 4_{Dose (kGy)}6 8 10

Temp. 5°C

FIGURe 8.4 (a) Viscosity of carrageenan solution vs radiation dose, 250 rpm, 50°C. (b) Viscosity of agar solutions vs radiation dose, 250 rpm, 60°C. (c) Viscosity of alginate solution vs radiation dose, 60 rpm, 5°C. (From Aliste, A. J., Vieira, F. F., and Del Mastro, N. L., Radiation Physics and Chemistry, 57, 305–308, 2000. With permission.)

γ-irradiation (Figures 8.4a, b, and c respectively), principally due to similar reasons of shorten- ing of the polysaccharide chain consequent to radiolysis resulting in softer gels (Aliste, Vieira, and Del Mastro 2000).

The apparent viscosities of nonextruded and irradiated starch–xanthan gum samples are greater than that for the nonextruded irradiated starch in which xanthan gum is added after irradiation (Table 8.5). The water solubility index for starch–gum extrudates increases with an increase in the dose of irradiation. In general, water absorption index exhibits higher values in irradiated samples compared to nonirradiated samples. Similar trends are observed for overall extrudate expansion (Hanna et al. 1997).

8.3 eFFeCt oF IRRADIAtIon on FooD teXtURe

Firmness is an important quality factor for several horticultural products, especially to fetch good market value. Firmness is associated with cell morphology, turgor, and cell-wall middle lamella structure. Loss of firmness may be attributed to the cell injury caused by certain treatments. The texture of several fruits and vegetables has been reported to deteriorate both during and after irra- diation. Changes during irradiation are attributed to direct action on substances of cell wall and middle lamella responsible for the mechanical strength of tissues. Irradiation at high doses pro- motes softening of plant tissue caused by changes of pectic substances as well as degradation of celluloses.

8.3.1 postirraDiation textural cHangesin fruits

γ-Irradiation causes random hydrolytic breakage of polygalacturonide macromolecules, yielding frag- ments of lower molecular weight. The breakdown of protopectin to soluble pectin is linear with radia- tion dose and radiation scissors the 1,4-glycosidic bond. Pectin is degraded at a comparatively lower dose of irradiation. This degradative effect can be reduced by the addition of sugar and can be com- pletely eliminated up to 2.12 kGy when the pH and sugar concentration of the irradiated mixture is in the range suitable for jelly formation. Pasteurization doses of radiation result in leaching of calcium from the plant tissues which causes loss of texture. This radiation-induced softening can be mini- mized by dipping the fruits in calcium chloride solution after irradiation (Ahmad and Hussain 1973).

8.3.1.1 Mangoes

Exposure of mangoes (Tommy Atkins variety) to ionizing radiation of 1.0–3.1 kGy has been shown to induce a significant softening as a function of irradiation dose and storage time. The

tABLe 8.5

Apparent Viscosities (Mpa·s) of starch–Xanthan Gum Mixtures shear Rate (s–1₎ Xanthan Gum (%) 0.6 1.5 3 6 12 30 25 2628 1275 771 528 311 177 20 1656 877 570 386 263 136 15 1367 613 389 260 169 100 10 533 263 170 135 67 38 5 225 139 103 74 55 19 Control starch 68 50 41 29 20 11

Source: Hanna, M. A., Chinnaswamy, R., Gray, D. R., and Miladinov, V. D., Journal of Food Science, 62, 816–20, 1997. With permission.

Effect of Irradiation on Food Texture and Rheology 115

effect of dose and storage time on textural attributes of irradiated and nonirradiated mangoes stored at 12°C for 21 days is shown in Table 8.6 (Moreno et al. 2006). Similar results have been reported by Lacroix et al. (1990) and Lacroix, Jobin, and Gagnon (1992) for mango samples irra- diated at 0.50 and 0.95 kGy, and by El-Samahy et al. (2000) for mangoes exposed to γ-irradiation between 0.5 and 1.5 kGy. Irradiation affects stiffness of fruits, measured as Young’s modulus. Fruits become softer when exposed to the high dose of 3.1 kGy and show the lowest values of Young’s modulus at the end of the storage period. Microstructural studies show irradiated fruits to have more collapsed cells than nonirradiated controls. These changes in cell structure are consistent with measured texture characteristics wherein irradiation at high doses significantly reduces the stiffness and firmness of mangoes. Thus it can be rationally concluded that irradia- tion at 1.0 kGy can retain textural attributes, while that at 1.5 and 3.1 kGy induces undesirable texture or softening. However, exposure up to 3.1 kGy does not negatively affect juiciness of mangoes (Moreno et al. 2006). Investigations on texture of whole mangoes (Thai variety) and pulp after γ-irradiation showed it to be hard and slightly undesirable on the first day of storage but it became subsequently softer and more acceptable during storage with the progress of ripen- ing (Lacroix et al. 1993).

Mangoes, being a climacteric fruit, should be at a proper stage of ripeness at the time of irradia- tion. The use of radiation to offset senescence of mangoes has also been investigated. Alphanso and Desi mangoes irradiated at 0.25 kGy are firmer than the nonirradiated ones and there is a progressive deterioration of texture with increasing radiation dose. Irradiated Kent mangoes are relatively softer immediately after irradiation and contain higher water-soluble and lower insoluble pectins as compared to the control fruits. However, during storage, the increase in water-soluble and decrease in insoluble pectins of control mangoes proceeds at a faster rate than the correspond- ing irradiated ones. Similar changes are observed in the pectic substances of irradiated Dusehri mangoes and the fruits retain better texture when irradiated at 0.30–0.35 kGy, while higher doses have a deleterious effect. Irradiation of mangoes in their ripening stages at 0.64–0.92 kGy and storage at 18°C and 65% RH show a significant weakening of texture compared to the control (Ahmad and Hussain 1973).

tABLe 8.6

effect of Dose and storage time on texture Attributes of Irradiated and nonirradiated Mangoes stored up to 21 Days at 12°C

texture Parameter storage Day

Control (0 kGy) Low Dose (1 kGy) Medium Dose (1.5 kGy) High Dose (3.1 kGy) Force to rupture (N) 0 177.9 73.50 66.90 24.29 5 121.4 47.54 16.21 17.13 10 119.27 82.87 57.78 28.70 21 140.99 102.87 82.65 31.30 Toughness (J) 0 0.42 0.14 0.12 0.50 5 0.37 0.06 0.02 0.03 10 0.30 0.13 0.06 0.04 21 0.35 0.11 0.08 0.04 Young’s modulus or stiffness (MPa @ 3% strain) 0 0.78 0.32 0.29 0.11 5 0.53 0.21 0.07 0.08 10 0.53 0.37 0.25 0.13 21 0.63 0.45 0.36 0.14

Source: Moreno, M., Castell-Perez, E., Gomes, C., Da Silva, P. E., and Moreira, R. G., Journal of Food Science, 71, E80–86, 2006. With permission.

8.3.1.2 Apples

An immediate softening of apples has been observed on irradiation above 0.1 kGy. However, the dose threshold is different for different cultivars. The softening may be due to the decrease of pro- topectin and total pectin or to a change of insoluble pectic materials to soluble forms (Al-Bachir 1999). However, the fruits become firmer during storage compared to the controls. γ-Irradiation over 0.34 kGy causes significant softening of apple slices, which is related to an increase in the content of water-soluble pectin, but not the total pectin content. Total pectin content is unaffected by irradiation. Similarly, softening of minimally processed apple slices is also associated with increased water-soluble pectin and decreased oxalate-soluble pectin content. Both the water-soluble and the oxalate-soluble pectin fractions significantly correlate with the decrease in firmness upon irradiation. Dose lower than 0.4 kGy and higher than 2 kGy have a statistically significant effect on the firmness of irradiated apple slices during storage. However, these levels are far from practical since higher dose cause higher variation in absorbed dose within slices and a lower dose requires excessive treatment periods. Oxygen concentration and storage time do not affect firmness of irra- diated slices, suggesting that the softening of irradiated apple slices is probably a direct physical effect rather than that mediated by free radicals. As opposed to the observation on apple slices, pro- tective effects of anoxia against textural changes in irradiated nectarines, peaches, and pears have been reported. Heat treatment of whole fruits yield firmer products compared to nonheated fruits (Ahmad and Hussain 1973; Gunes, Hotchkiss, and Watkins 2001).

Calcium chloride treatment is carried out postradiation to retain texture of apple slices. However, irradiation at 2.5 and 5 kGy softens the slices stored under a controlled atmosphere for 4 weeks irrespective of calcium treatment. Dipping in 0.5% calcium chloride causes a limited but signifi- cant improvement on firmness during four-week storage under similar conditions. Slices treated with both calcium and irradiation are comparatively softer than nonirradiated control slices. The inability of calcium chloride to prevent irradiation-induced softening can be due to limited pen- etration into the tissues. Electron beam is reported to cause less softening, probably due to its lower penetration effect than γ-irradiation. Softening associated with electron beam irradiation may be confined to the surface of the slices and can be eliminated by normal calcium treatment (Gunes et al. 2001).

8.3.1.3 Pears

The optimum dose for radiation of pears has been found to be 2 kGy whereas 3 kGy is high and causes softening of the flesh. Irradiation seems to stimulate ripening in peaches and causes significant softening in some varieties, although in some varieties it shows no significant effect. Tissue softening in peaches and pears consequent to γ-irradiation corresponds to a decrease in protopectin and an increase in pectin and pectate. It has been postulated that increased polym- ethyl esterase activity may contribute to the initial pectin degradation in irradiated fruits (Ahmad and Hussain 1973).

8.3.1.4 Berries

For the berry family too, similar results as seen in apples are obtained for strawberries wherein 2 kGy is found to be optimum for radiopasteurization of the same. Higher doses cause disagree- able softening of tissues because of formation of a spongy water-soaked structure. For, raspberries, grapes, and black currants, irradiation has a similar adverse effect on their texture. Blueberries exposed to electron beam ionizing radiation endure significant softening throughout storage. Shear force values decrease significantly with irradiation dose. This softening effect induced by irradia- tion may be associated with the changes in the cell wall structure of the berries and the solubility of its pectin substances. A similar trend is observed with fruit toughness. Samples irradiated at 1.1, 1.6, and 3.2 kGy are reported to be 26, 34, and 49% softer than the control fruit respectively. The ones irradiated at 3.2 kGy are the softest and organoleptically totally unacceptable (Moreno et al. 2007).

Effect of Irradiation on Food Texture and Rheology 117

8.3.1.5 Cucumbers

The effects of irradiation on texture of cucumbers are contradictory. Khattak et al. (2005a) reported irradiation to have a significant effect on firmness of cucumbers. The firmness of cucumbers decreases gradually from 0–1 kGy, sharply at 1–2 kGy, and immediately after irradiation at 3 kGy. The texture, however, remains within acceptable limits up to a dose of 2.5 kGy after 14 days of storage. Hajare et al. (2006b) did not find any effect on both the central and peripheral regions of cucumber on radiation processing at 2 kGy. However, interestingly during storage, the hardness of the peripheral regions increased significantly until 16 days. This increase in firmness was attributed to the loss of moisture during storage at 8–10°C. The deviations in the studies can be accounted to the varietal differences of cucumbers studied across the Indian subcontinent.

8.3.1.6 Melons

Integrity in texture is the most important shelf-limiting quality factor in melons. Whole cantaloupes irradiated at 1.5 and 3.1 kGy are less firm (lower values of Young’s modulus) after the fourth day of storage and less tough after the eighth day of storage at 10°C. Fresh-cut samples of the same irradi- ated at 3.1 kGy have firmer texture (higher values of Young’s modulus) and are harder (higher value of rupture force) and tougher up to the eighth day of storage, after which values begin to decline steadily (Table 8.7). This apparent contradiction in texture improvement with increasing irradiation dose is attributed to the presence of air in polystyrene tray packages, which is believed to reduce the over- all density of irradiation target, thus improving penetration characteristics. However, no significant textural changes are obtained for the fresh-cut samples irradiated at lower doses (Castell-Perez et al. 2004). In a study conducted with low-dose electron beam irradiation on melons, “juiciness” was cho- sen as the texture attribute. This parameter showed a significant effect on irradiation at 0.5–1.0 kGy, with the least significant difference recorded at 0.5 kGy and the highest difference for the high-dose