Wijayasinghe, Rangani (2024) Properties of Simple Sugars and their Interactions as Affected by Presence of Acids and Salts. PhD thesis, Victoria University.

Abstract

Lactose stands as the primary carbohydrate and predominant solute in both milk and whey. While sucrose is identified as the most abundant among low molecular weight carbohydrates. The role of these simple disaccharides is crucial for human life, the environment, and the dairy industry. The dairy industry encounters challenges in extracting lactose from acid whey, a byproduct of Greek yoghurt and soft cheese manufacturing, primarily attributed to the inadequacy of lactose to crystallize appropriately. The presence of lactic acid and Ca and their interaction have been found to hinder the crystallization of lactose and negatively impact the environmental issues associated with the acid whey waste stream. Sucrose on the other hand is present in combination with acids and salts in most food products including sugar-sweetened carbonated drinks. Sucrose-acid interactions on the properties of sucrose in soft drinks during the production process remain unclear, which is critical to the health and nutritional value of the product. Therefore, it is crucial to examine the interaction among prevalent disaccharide sugars, acids, and salts to comprehend their behaviour adequately. Therefore, the present study aimed to investigate the physico-chemical and thermal behaviour of simple disaccharide sugars under different compositional and processing conditions to gain a molecular-level understanding of the interactions between sugars and acids and salts. Understanding the properties of the concentrated lactose solution is paramount in determining the crystallization behaviour of lactose in the presence of different components. Initially, the physicochemical and thermal characteristics of lactose in concentrated solutions were established while considering various acids (lactic, citric, and phosphoric acid) and their concentrations (0.05, 1, or 4% (w/w)), commonly present in acid whey and other food systems. Thermographic analysis found that water evaporation from lactose solutions was hindered by water-acid interaction. This was due to the formation of a strong hydration layer around lactose molecules through hydrogen bonding. The extent of lactose hydrolysis into glucose and galactose varied depending on factors like acid concentration and molecular interactions. The influence of all three acids on the crystallization patterns of lactose exhibited variations, predominantly contingent on their concentrations. The addition of 1% or 4% citric or phosphoric acid led to a notable reduction in crystal yield, diminishing by at least 18% compared to the crystal yield of pure lactose, which stood at approximately 82%. The observed acid-induced hydrolysis during the concentrated solution phase has the potential to influence the crystallization behaviour of lactose. This impact is attributed to the presence of glucose and galactose in the solution, influencing supersaturation. The thermographic analysis also indicated that the inclusion of 1% lactic acid, 0.05% and 1% citric acid, and 4% phosphoric acid in lactose solutions resulted in the formation of amorphous lactose. The hindrance of lactose crystallization in the presence of elevated salt concentrations poses significant challenges for downstream processing in acid whey. Hence, a meticulous study was conducted using a model-based approach, utilizing various cations (Mg, Ca, K, and Na) at concentrations (8, 30, 38, and 22 mM, respectively) that are typically encountered in acid whey. The thermal analysis of concentrated solutions discovered, the introduction of individual cations and their combinations led to an increase in the enthalpy of water evaporation, in contrast to pure lactose. Consequently, there was a significant decrease in crystal yield, which exhibited an exact reversal of the enthalpy order. The salt mixture resulted in the most substantial reduction (63%), followed by Ca (67%), compared to pure lactose (79%). This reduction in yield was inversely related to lactose solubility. It was observed that the involvement of divalent cations played a crucial role in the isomerization of lactose molecules. The structural properties of sucrose molecules are subject to modifications under the influence of acids and salts. In the relevant study, we established the behaviour of sucrose in the presence of citric acid, phosphoric acid, and sodium. The primary objective of this research was to elucidate the underlying molecular mechanisms that govern the structural modifications of sucrose in the presence of such additives. The behaviour of sucrose exhibited distinct responses to the presence of acids and salts. The addition of phosphoric acid and citric acid-induced structural modifications in sucrose molecules via a concentration-dependent hydrolysis process. The hydrolysis was notably facilitated by the presence of citric acid and phosphoric acid. Specifically, in the presence of citric acid, lactose hydrolysis resulted in a higher release of fructose, whereas in the presence of phosphoric acid, sucrose underwent separation into its constituent monosaccharides, yielding more glucose than fructose. The alterations in the hydration water layer, stemming from the molecular rotation of sucrose, fructose, and glucose molecules, coupled with the formation of robust hydrogen bonds, played a pivotal role in modifying the enthalpies of water evaporation (ΔH) in the presence of sodium, phosphoric acid, and citric acid. Notably, the influence of phosphoric acid in the sucrose solution demonstrated a more pronounced impact on sucrose behaviour compared to citric acid and sodium. The behaviour of lactose and sucrose is influenced by a complex interaction of acids and salts, which involves interactions between water, acids, cations, and sugar molecules. This interaction leads to changes in sugar solubility, ion-dipole interactions between water and cations, and alterations in the structure of water molecules. The creation of a strong hydration layer hinders the removal of water. Therefore, the present study established that for lactose to be crystallized in acid whey the composition should be altered. By manipulating the Ca concentration and the lactic acid concentration, lactose crystallization can be improved and sequentially, achieve the better processability of acid whey. Furthermore, the reduction of acid concentration and the use of an alternative for phosphoric acid may be a feasible strategy in beverage production to mitigate the health implications.

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