Rehydration of Crispy Cereal Products: A Tentative Explanation of

Chapter 17

Rehydration of Crispy Cereal Products: A Tentative Explanation of Texture Changes 1

Gaëlle Roudaut1, Catherine Dacremont , Baltasar Valles Pamies2, John R Mitchell , and Martine Le Meste1 Downloaded by CORNELL UNIV on August 26, 2016 | http://pubs.acs.org Publication Date: October 10, 2002 | doi: 10.1021/bk-2003-0836.ch017

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1Ecole Nationale Supérieure de Biologie Appliquée à la Nutrition et à l'Alimentation. 1, Esplanade Erasme, F21000 Dijon, France P T C Kemptthal, Nestlé, CH 8310 Kemptthal, Switzerland University of Nottingham, Sutton Bonington Campus, L E 5 R D Loughborough, United Kingdom 2

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Crispness has been seen to be a source of pleasant textural contrasts and to be associated with a sensation of both freshness and quality. The texture deterioration of crispy products upon rehydration is thus known to cause the rejection of many cereal products by consumers. For many years, this evolution of texture was attributed to the glass transition of the material becoming lower than ambient temperature. For this work, a double approach of the texture evolution was chosen: on the one hand, sensory properties of cereal products at various hydrations were studied, while on the other hand, the physical basis of the phenomena controlling the texture were considered. Crispness loss was shown to coincide with a hardness increase of the products. Localized motions preceding the glass transition could be responsible for the texture changes, moreover, sugars could contribute to the stability of the products versus hydration.

© 2003 American Chemical Society

Cadwallader and Weenen; Freshness and Shelf Life of Foods ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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224 Introduction

Downloaded by CORNELL UNIV on August 26, 2016 | http://pubs.acs.org Publication Date: October 10, 2002 | doi: 10.1021/bk-2003-0836.ch017

Freshness is defined as the state of "being as i f newly made". When freshness of food products is concerned, different scales of shelflife are available to the consumers : that of the fresh foods bought from producers and the more sophisticated, packaged manufactured foods. Whereas the first ones have a very limited shelflife, the manufactured products may be regarded as fresh as long as they remain protected from the outside environment in their original sealed packaging. Freshness covers a wide range of attributes from taste to flavor and texture, and shelflife relates to their stability over time. Among the different components of food quality, the importance given to texture by the consumer has considerably increased in the last decades. In early consumers studies ' on food texture, crispness was revealed to be not only seen as a source of pleasant textural contrasts but to be associated with a sensation of both freshness and quality. For products such as breakfast cereal, chips, wafers, the crispy texture is the key attribute for the consumer appreciation. The importance of crispness in the acceptability of a food has rapidly made the industry consider the necessity to control this characteristic as much at production level as at the storage or consumption one. Such an objective (controlling this texture attribute) implies to understand not only the sensory meaning of "crispy", but the physico-chemical parameters responsible for it. For the present work, the interest will be focused on the possible effects of storage on crispy products rather than the different parameters that would possibly affect the texture during the products manufacture (processing or formulation parameters). 1 2

Influence of rehydration upon storage on texture of crispy cereal food products : an instrumental and sensory approach Most low moisture fried, baked or extruded products such as breakfast cereals, wafers, biscuits, and snacks have a crispy texture resulting from a cellular structure. This attribute can be altered upon storage. A s an example, when studying the effect of storage on potato and tortilla chips, Lee et ah described fresh samples crispier than the stale ones. Upon storage, and due to water sorption from the atmosphere or by mass transport from neighboring components (in absence of efficient moisture barriers), the moisture content of these products increases (Figure 1), and crispness is l o s t ' ' . For this type of products, it is a major cause of consumer rejection. In spite of the economical impact of this loss, and although many 3

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works have been dedicated to it, there has been yet no agreement on the molecular basis of these textural changes.


The texture changes upon rehydration can be characterized by both sensory and instrumental analysis. For this purpose, crispness of cereal products equilibrated at different humidities is evaluated at first bite by a trained sensory panel, whereas a puncture test is used to assess the fracture properties of the materials.

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water content (%) Figure 1: Hydration effect on sensory crispness of extrudedflat bread. (Reproduced with permission from reference 7. Copyright 1998.) The influence of water on the texture properties is characterized (Figure 1) by an important decrease of the mark (between 0 and 10) given to crispness, the evolution can be described by a sigmoid curve with a critical value (corresponding to half of the maximum value) at 9 % of water. Similarly, the fracture properties exhibit important changes characterized by an attenuation of the jaggedness of the force-deformation curves (Figure 2). These patterns can be analyzed throughout the determination of the number of spatial ruptures (peak number per puncture distance). The logarithm of the latter is correlated to crispness : the higher the number of peaks (corresponding to fracture of individual cells) the greater the crispness. It suggests that the panel evaluates crispness on the basis of the number of ruptures detected at first bite. Among the different sensory attributes evaluated by the panel, the interest was also called on the evolution of the hardness. Indeed, as shown on Figure 3, hardness is increasing upon hydration up to 9 % of water. 7

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Figure 2 : Force-deformation curve obtained with extruded bread at 6% (lower one) and 10% moisture (higher).

It is interesting to compare it with the evolution of crispness: indeed while the latter decreases, hardness increases up to 9 % of water content, which is identical to the previously described critical water content for crispness. Similar features were obtained with instrumental measurements. A analysis on a wider set of data revealed that such effects could be illustrated by a texture continuum describing the evolution of texture versus hydration from crispy, crackle to hard. Further study aiming at the understanding of the molecular basis of texture changes requires the use of a physico-chemical approach based on techniques and concepts developed for material science. In material science, the loss of crispness upon rehydration could be described in terms of change in fracture mechanisms. Extensive studies performed on synthetic polymers have demonstrated that mechanical behavior is highly dependent on the physical state and organization of the polymer network . Thus for further study, the physical state of the cereal products has to be considered. 8, 9

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0 -I 5

,

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,

i

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water content (%) Figure 3: Influence of water content on the "hardness" descriptor for extruded flat bread), and the line is a modified sigmoid equation (Reproduced with permission from reference 7. Copyright 1998.)

Physical state of cereal-based products Baked or extruded cereal products are generally in the glassy state since cooking is accompanied by the disappearance of most crystalline structures of native starch . The sensitivity of cereal products to the water content contributed to the proposal of Tg as a predictive parameter for the stability of low moisture products ' ' . Indeed, stored above their glass transition temperature, amorphous samples exhibit mechanical changes; moreover due to collapse phenomena, cellular (a common feature to crispy foods) products could show densification which is also known to control mechanical/ texture properties ' ' . Finally, above their glass transition temperature starch-based products are known to be subjected to rétrogradation, this crystallization being able to affect the texture of the products . Differential Scanning Calorimetry (DSC) is classically used for the determination of the glass transition of simple systems (e.g. sugars). However, no event assignable to a glass transition was detected for the extruded bread described above; this is probably due to the heterogeneity of the system which causes a spreading of the transition over a very broad temperature range and thus an undetectable progressive increase of heat capacity. 11

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For complex systems, thermo-mechanical methods are more sensitive , they have been applied successfully to cereal-based products Thermo-Mechanical Analysis ( T M A ) , or Dynamic Mechanical Thermal Analysis ( D M T A ) ' are more accurate/more sensitive for Tg measurement. The glass transition temperature of extruded bread was determined by D M T A at several frequencies (from 5 to 40 Hz) from the drop of the storage modulus F , this feature was characterized by a frequency shift towards higher temperature with increasing frequency . The plasticizing effect of water on extruded bread is shown on Figure 4 throughout the decreasing Tg plotted as a function of the water content of the material. 21

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23f

1 1


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180 130 " u ο oc H

80 30 -20 -70 -0

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water content (% w/w wb) Figure 4 : Influence of water on the glass transition temperature (Tg) of extruded bread (determined at the onset of E' drop of DMTA at 5Hz It is important to notice that according to these data, the glass transition takes place at room temperature when extruded bread contains a minimum of 15 % of moisture. Therefore, the texture changes observed around 9 % could not be attributed to the glass transition of the samples, but possibly to more localized motions occurring in the glassy state . No marked macroscopic density changes were detected in this hydration range. No crystallinity could be detected with either D S C nor D M T A or X-rays diffraction, and therefore the hardening observed in the samples could not be attributed to rétrogradation or to crystals remaining from the balking process. 24

The hardening observed upon hydration has been described as an "antiplasticizing" effect. Indeed, although the increasing water content still decreases the glass transition temperature, the effect observed for limited


229 hydration (< 9%) is opposite to the usual consequences of plasticization on mechanical properties, whereas the softening one only becomes dominant for higher hydration values. This antiplasticizing process has been observed in several systems but its molecular origin has no yet been clearly identified. It has been attributed to a densification at a molecular level through a decreasing amount of defects (lower density regions) present in the glass ' , or to a strengthening of the interactions with die polymeric m a t r i x or to a structural relaxation . Finally, Fontanet et al suggest that the stiffening of the samples could result from a short range reorganization throughout Η-bonding (at such a limited scale that undetected by DSC or X-rays) allowed by a greater mobility induced by the dilution. 25

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Influence of solutes on the stability of cereal systems upon rehydration For a further understanding, model systems were prepared, they consisted of expanded extradâtes of waxy maize at different sucrose concentration. The effect of hydration was studied with the same techniques. Whereas the 0 % sucrose samples exhibit an important loss of crispness around 8 % of water, the critical water content for the 20 % sucrose samples is greater than 12 % (Figure 5 ). Moreover, it can be observed that although the crispness of sucrose-rich samples is affected at higher hydration, the effect is then more rapid than for the 0 % sucrose ones. The 10 % sucrose samples were rather heterogeneous which caused the irregular evolution of crispness versus hydration. However, the trend they follow places them in an intermediate position between the 0 % and the 20 % sucrose samples. The 0 % sucrose samples exhibit a sensitivity to hydration comparable to the 0.1 % aspartame ones, which confirms that the panel was not biased in its judgement by the sweetness of the tasted products. The influence of sugar content on sensory hardness is shown Figure 6. Similarly to crispness, the influence of rehydration on the sensory hardness of extruded waxy starch depends also on the sucrose content of the samples. The lower the sucrose content the lower the hydration at which the stiffening starts, (due to possible effects of sugar on the structure of the extradates, the values of hardness are not compared). Such results were confirmed by instrumental measurements where for a given moisture content the forces developed at fracture were greater at low sucrose concentration than at 20 % . 8


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10

0 -| 0

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water content (% wb) Figure 5: Influence of water content on crispness for extruded waxy maize with (·) 0 %, (+) 10 % and (x) 20 % sucrose and (o) 0.1% aspartame.

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water content (%wb)

Figure 6 : Influence of water content on hardness for extruded waxy maize with (·) 0% (+) 10% and (X) 20 % sucrose and (o) 0.1 % aspartame. t


231 Physical state of the starch-sucrose mixtures When homogeneous ternary mixtures of sucrose-starch are prepared, the resulting Tg is decreased compared to that of starch alone ; in the absence of phase separation the greater the sucrose concentration the lower the Tg. The plasticizing effect of both sucrose and water can be predicted using mathematical models such as the ten Brinke modified Karasz-Couchman model : 30

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χ

^ W ÀCp Tg + W À C p T g + W À C p T g 1

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gmixture

1

2

W ACp l

l

2

2

3

+ W ÀCp + W ACp 2

2

3

3

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where for the component x, W , is the weight fraction, A C p the heat capacity change at Tg and T g (K) the glass transition temperature in Kelvin. The values used for the prediction are given in Table I, and the calculated Tg as a function of water in Figure 7. x

x

x

Table I: Parameters used for the prediction of T g in ternary mixtures by K a r a z Couchman model Component Starch Sucrose Water 31

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ACp (J/g/K) 0.47 0.76 1.11

Tg(K) 502 343 134

The applicability of the model assumes both a good miscibility of the 2 components and no crystallisation. These conditions were verified for the considered samples. The prediction suggests that the sucrose-starch mixtures remain glassy at room temperature at any water content or sucrose of the range considered. Therefore, as previously observed for the commercial extruded bread, the textures changes observed in the samples cannot be attributed to the glass transition.

Hydration of glassy material : suggested mechanism The results obtained with the model products strengthen some of the hypothesis previously brought up. Glassy systems consist of both dense regions and lower density ones. The latter are characterized by the presence of voids


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0.05

0.1

0.15

0.2

water fraction (w/w wb) Figure 7 : Predicted Tg values for waxy maize starch as a function of water fraction for 0 % (dotted), 10 % (plain) and 20 % (bold) sucrose (gsucrose/g starch)

(also called defects or free volume) which are sometimes referred to as "mobility islands" for the possible greater mobility they offer throughout the greater space. When considering hydration, the water molecules "first" added to the systems (below 9 % for extruded bread) would localize in the low density regions, filling the voids and therefore causing an increased density (at a molecular level) and thus an increased hardness of the glasses. Further hydration will see water interacting with polymers through hydrogen bonding in competition with the interpolymer Η-bonding. When this level of hydration has been reached, the dominant effect of the water uptake will be a "softening" of the matrix by replacement of the interpolymer network by water-polymer interactions. When the matrix contains a mixture of starch and sucrose, the hydrogen-bonding could first establish between the water molecules and the solutes before affecting the biopolymer. Therefore crispness would be maintained up to a greater hydration level in presence of sucrose. 32

Moreover, the development of interactions reinforcing the polymer network, possibly throughout water bridges could also be suggested to interpret the observed phenomena. Indeed, several works based on relaxation studies have shown that localised motions such as secondary relaxations observed in dry systems were hindered when the water content was increased . The susceptibility to this antiplasticization could be confered by the polarity of the polymer and solutes constituting the systems. The settlement of this network would inhibit the motions, and thus increase the rigidity of the systems and therefore contribute to the increase of both perceived and instrumental hardness accompanied with a change in the fracture properties and thus a crispness loss. 27,

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Throughout this work, the texture changes of crispy cereal products observed upon rehydration during storage have been considered with a particular focus on their physical origin. The loss of crispness occuring above 9 % of water was attributed to localised processes occuring in the glassy state, involving hydrogen bonding and building up of interactions between water and polymers. The role of polar solutes (such as sugars) is under investigation based on mixtures with solutes of different polarities. Preliminary studies showed that low resolution proton N M R was able to detect differences in the dipolar interactions between the samples at different sugar and water contents. Moreover, techniques such as R A M A N , or positron annihilation spectroscopy could be suitable to further investigate and test the hypothesis.

Acknowledgements The work was carried out with financial support by the Commission of the European Communities, Agriculture and Fisheries (FAIR) specific R T D program, CT96 1085, "Enhancement of quality of food and related systems by control of molecular mobility". It does not necessarily reflect its view and in no way anticipates the Commission's future policy in this area.

Littérature cited 1. 2. 3.

Szczesniak, A. S.; Kahn, E . L . J. Texture Studies. 1971, 2, 280-295. Vickers, Z . M. J. Food Science. 1983, 48, 783-786. Lee, W . Ε. III; Deibel, A . E.; Glemblin, C.T; Munday, E . G . J. Texture Studies 1988, 21, 165-178. 4. Katz, E.E.; Labuza, T. J. Food Science. 1981. 46, 403-409. 5. Attenburrow, G . E.; Davies; A . P.; Goodband; R. M.; Ingman; S. J. J. Cereal Science, 1992, 16, 1-12. 6. Nicholls, R.J. ; Appelqvist, I.A.M. ; Davies, A.P. ; Ingman, S.J. ; Lillford, P.J. J. Cereal Science. 1995, 21, 25-36. 7. Roudaut, G.; Dacremont, C.; Le Meste, M. J.Texture Studies. 1998, 29,199213. 8. Valles Pamies, B . ; Roudaut, G.; Dacremont, C.; Le Meste, M.; Mitchell, J.R. 2000, J. Sci Food Agric. 80, 1679-1685. 9. Vincent, J.F.V. J. Sci Food Agric. 1998, 78, 162-168. 10. Kinloch, A . J.;.Young, R. J. Fracture behaviour of Polymers; Applied Sciences Pub. : London, U K , 1983.



234 11. Le Meste, M. ; Roudaut, G. ; Aynié-Davidou, S. J. Thermal Analysis. 1996. 47, 1361-1375. 12. White, G . W . ; Cakebread, S. H. J. Food Technol. 1963. 1, 73-82. 13. Levine, H . ; Slade, L . Carbohydrate Polymers 1986. 6, 213-244. 14. Ablett, S.; Attenburrow, G . E . ; Lillford, P. J. In Chemistry and Physics of Baking : Material, Process and Products,. J. M . V . Blanshard; P. J. Frazier and T. Galliard, Ed.; Royal Society o f Chemistry : London, U K , 1986; pp 30-41. 15. Hutchinson, R. J.; Mantle, S. Α.; Smith, A . C.. J. Material Sci. 1989, 24, 3249-3253. 16. Guraya, H . S.; Toledo, R. T. J. Texture Studies. 1996, 27, 687-701. 17. Desrumaux, Α.; Bouvier, J.M.; Burri, J. Cereal Chemistry. 1999. 76, 5, 699704. 18. Slade, L . ; Levine, H . In Frontiers in Carbohydrate Research 1. Food applications. Millone, R.P. BeMiller, J . N . Chaudrasekaran, R. (Eds) Elsevier Applied Science, 1989, pp 215-270. 19. Wang, Y . J . ; Jane, J. Cereal Chemistry. 1994. 71, 6, 527-531. 20. Wetton, R. E . In Development in PolymerCharacterization, Dawkins, J.V. Ed.; Applied Science Publisher, London, U K , 1986, pp 179-221. 21. Le Meste, M.; Huang, V.T.; Panama, J.; Anderson, G . ; Lentz, R. Cereal Food World. 1992, 3,264-267. 22. Hallberg, L .M.; Chinachoti, P. J. Food Science.1992, 57, 1201-1204. 23. Vodovotz, Y .; Chinachoti, P. J. Food Science.1996, 61, 932-937. 24. Roudaut, G.; Maglione, M .; van Dusschoten, D . ; and Le Meste, M. Cereal Chem.1999.76, 70-77. 25. Benczedi D. Trends in Food Science & Technology. 1999, 10, 21-24. 26. Chang Y.P.; Cheah P.B.; Seow C.C. J. of Food Science. 2000. 65, 3, 445451. 27. Starkweather, H . W . In Water in Polymers. S. P. Rowland E d ; A m . Chem. Soc., Washington, D.C. 1980, pp 433. 28. Heux, L.; Lauprêtre, F.; Halary, J.L. Monnerie, L. Polymer. 1998. 39, no 67, p 1269-1278. 29. Fontanet, I .; Davidou, S.; Dacremont, C.; Le Meste, M . J. of Cereal Science. 1997, 25, 303-311. 30. Kalichevsky, M.T.; Jaroszkiewicz, E . M .; Blanshard, J.M.V. Polymer.1993. 34, 2, 346-358. 31. ten Brinke, G.T.; Karasz, F.E.; Ellis, T..S. Macromolecules. 1983. 16, 2, 244-249. 32. Johari, G.P.; Goldstein, M. J. Chemical Physics. 1970, 53, 6, 2372-2388. 33. Lourdin, D.; Bizot, H.; Colonna, P. J. Applied Polymer Science. 1997, 63,1047-1053. 34. Gaudin, S.; Lourdin, D . Forssel, P . M . ; Colonna, P. Carbohydrate Polymers. 2000. 43, 33-37.


Rehydration of Crispy Cereal Products: A Tentative Explanation of

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