**4.3 Gum rheology**

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groups in polysaccharides in the fingerprint region [44, 46].

cooling [62] are definitions attributed to thermogravimetry.

registered, simultaneously with the sample mass [63].

between functional groups (NH3<sup>+</sup>

**4.2 Thermal analysis of gums**

with the peak areas [60].

through a temperature gradient [64].

versatility for use as food additives [57].

spectroscopy and their respective functional groups present in polysaccharides. It is also possible to see that FTIR can provide information on important functional

In polysaccharides, the infrared spectroscopy can be used to qualitatively observe possible structural changes. Quelemes et al., [56] demonstrated the structural change in cashew gum when submitted to quaternary ammonium reagent, which also improved some properties such as biocompatibility and antimicrobial action. FTIR was also efficient to demonstrate that the interaction of gum arabic and chitosan was formed by electrostatic complexes, a result of the interaction

improved viscoelastic characteristics at different pH's, demonstrating its complex

Most polymers, synthetic or natural, suffer degradation when subjected to thermal stress [58]. This is attributed to chain depolymerization, point splits, or even the elimination of low molecular weight fragments, which cause mass loss due to the increase in temperature [59]. They cause thermal effects related to physical or chemical changes, and are associated with thermodynamic events [58]. These changes in energy and mass can be measured by thermogravimetry (TG), derivative thermogravimetry (DTG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC), which make it possible to obtain information such as changes in the crystalline structure, reaction kinetics, melting and boiling point, glass transition, and others [60]. Changes in mass as a function of temperature and/or time [61] and continuous registration of mass subjected to heating or

Being the combination of an electronic microbalance and an oven, associated with a linear temperature programmer, thermogravimetric analysis consists of submitting a known mass of sample inside a crucible, suspended by a platinum wire, to a programmed temperature gradient, for a predefined time, which is automatically

In DTG, the mass variation derivative (dm/dt) is registered as a function of temperature or time. In this method, the levels observed in TG are replaced by peaks that delimit areas which are proportional to the changes in mass suffered by the sample and can indicate the exact initial temperatures and maximum speed of reactions. DTG allows a clear distinction of successive reactions (not detected by TG), by quantitative determinations of loss or gain of mass which are associated

DSC and DTA are analyses that measure energy gradients between the sample and a reference material subjected to controlled temperature. DSC is a calorimetric method in which energy differences are measured, whereas in DTA, temperature differences between the sample and the reference material are registered [59]. DTA provides a qualitative analysis of the thermal events experienced by the sample, whereas DSC is able to quantify such events because it measures the heat flow

Changes in composition, food processing temperatures or ingredients result in changes in phase transitions of the product [65]. Quantifying the variables involved in these phenomena, such as temperature or thermodynamic quantities, is important for understanding processes such as evaporation, dehydration, and freezing [66]. Being the responsible for plasticizing effects and important component of food, water and its state transitions (gaseous or crystalline) guide such processes, and can also be used to describe the effects of temperature on physical properties [59].

and ▬COO-) of both macromolecules. Also, it

**240**

Natural polymers are of particular interest in rheological studies [67]. Their thickening, emulsifying, gelling, and stabilizing properties, which enable them to be used in food, pharmaceutical, and cosmetic industries are supported by a series of inter and intramolecular association mechanisms inherent to each polymer. Such mechanisms lead them to particular applications in different processes and products [68].

Gum arabic *(Acacia senegal)* 3% (m/v), originating from African regions such as Sudan, Senegal, and Mali, has typical behavior of a liquid. Sanchez, Renard, Robert, Schmitt, & Lefebvre, [69] investigated G' and G" in gum arabic, where G' is the storage modulus and indicates the portion of energy (from the applied voltage) that is temporarily stored during the test, and it provides information on the elastic characteristic of the fluid. On the other hand, G" is the loss modulus, which indicates the portion of energy used to initiate flow. It is irreversibly transferred in the form of heat and provides information on the viscous characteristics of the fluid [70]. The authors state that gum arabic presented a viscous modulus (G') greater than its elastic modulus (G'), but after 5 hours of rest, gel characteristics were identified, consequently showing a more elastic structure [69].

*Acacia tortuosa* gum, originating from species located in South America (Venezuela) (15% m/v), presented elastic modulus (G') greater than its viscous modulus (G"), indicating the occurrence of a gel material that became progressively weaker with increasing temperature [71]. In both studies, gums showed transition from Newtonian to non-Newtonian behavior with increasing concentration. Also, the influence of inter and intramolecular structural interactions as agents responsible for rheological changes was observed [69, 71].

The emulsifying and rheological characters of chemically modified gum arabic (*Acacia senegal*) (esterified with octenyl succinic anhydride (OSA) at different concentrations) was measured by [72]. The study revealed that the gum presented an increase in its emulsifying capacity and a gradual increase in apparent viscosity with increasing OSA content, indicating satisfying emulsion stability and potential use as microencapsulant. The electrostatic interaction between gum arabic and soy protein β-conglycinin was the mechanism that improved the flocculating action of *Acacia senegal*, in addition to providing greater elasticity at the oil/water interface of the gum, consequently improving its emulsifying capacity [73]. The interaction of gum arabic with native tapioca starch also provided improved product elasticity and adhesiveness [74]. Chenlo, Moreira, & Silva, [75], studied the rheology of aqueous dispersions of tragacanth gum and guar gum (10 g/L) during storage for 47 days. In general, the apparent viscosity decreased significantly (α = 0.05) for both systems at low values of γ ̇ (< 10s−1) and remained constant above this value. The decrease in viscosity was lower for tragacanth gum and lasted until the 15th day, whereas for guar gum, the decrease occurred until the 20th day.

Mixtures of corn starch (5% m/m) and locust bean gum (0; 0.125; 0.25; 0.50; and 1% m/v) were rheologically evaluated by Hussain, Singh, Vatankhah, & Ramaswamy, [76], who found that the addition of locust bean gum at low concentrations (0.125%) made the mixture behave as a liquid at low oscillatory frequencies (0.1 to 10 rad/s). It also presented increased elasticity, with typically solid behavior at concentrations of 0.5 to 1%, at higher frequencies (15 to 100 rad/s). Thus, locust bean gum has potential to specifically modify the structure and texture of corn starch products.

The research results showed that there are many variables that influence the rheological characteristics of gums. Among them, the fine chemical structure of the polysaccharide, their interactions, and molecular conformations can be highlighted, which confirms the importance of characterizing the structure of new gums.
