Bottiroli, Riccardo (2021) Towards the development of high quality UHT hydrolyzed-lactose milk. [Tesi di dottorato]

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Lactose is present at high concentration in mammals' milk and it is metabolized by β-1,4-galactosidase (lactase) that splits the molecule into galactose and glucose, which are then absorbed in the small intestine. Lactose intolerance is a worldwide issue with both nutritional and economic implications. In traditional dairy consuming countries, the consumer perception about lactose intolerance is mounting; it is estimated that 70% of world population has non-persistence lactase. Lactase is always present in the newborn, but its activity naturally decreases after weaning. Undigested lactose is fermented in the gut increasing the abdominal pressure and intestinal transit causing unpleasant symptoms such as abdominal pain and diarrhea. Low Lactose (LL) or lactose free (LF) products represent the simplest solution to deal with the issue keeping the nutrients intake related to milk and dairy products consumption but eliminating intestinal discomfort. This product category will reach a turnover of €9 billion by 2022 and today represent the fastest growing market in the dairy segment. The most popular methodology applied to produce LL and LF products is through lactose hydrolysis by addition of free soluble lactase, which lowers the content of lactose in the milk from 4.5-5.0% down to <0.01%. The availability of several commercial lactase preparations (LPs) favored the development of different technological approaches to manufacture ultra-high temperature hydrolyzed-lactose milk (UHLM). On the other hand, depending on the production system employed, manufacturing poses several technological problems in relation to the stability upon production and shelf-life of the milk, with potential economic implications as well. Furthermore, the commercial LPs intended for UHLM production contain enzymatic side activities that, if not properly controlled and mitigated, can change the physicochemical and sensory properties of the final product during shelf-life, which may end up in consumers rejection. In particular, off-flavor formation is the major factor limiting the shelf-life of UHLM. UHLM with unaltered sensory characteristics during storage can be produced as long as the chemical nature of off-flavor formation is well understood. Therefore, there is a clear need to identify those molecules responsible for the off-flavor in UHLM in relation both to the processing technology applied and the sensory characteristics of the final product. In this context, the aim of this PhD project was to unravel the chemical nature of the changes in the flavor profile of UHLM occurring during shelf-life. In particular, the project attempted to explore the effect of different production and storage conditions on the formation and release of volatile compounds in different production and storage stages, elucidating the relationship between process technology and the variables implicated in the definition of the final product quality. Utmost attention was given to Maillard reaction and its secondary pathways because, upon lactose hydrolysis and secondary proteolysis from the lactase, the reaction is facilitated causing unwanted modifications of the UHLM sensory properties. ¬Priority was given to the UHLM produced by the "in batch" system, one of the two production technologies involving free soluble lactase currently available for dairy producers. The production system involves the thermal inactivation of the lactase after lactose hydrolysis which, according to the literature, may alleviate the proteolytic and arylsulfatase side effects of the commercial LPs. This aspect would be particularly beneficial in comparison to the other production technology available (the so-called "in pack" system): in this case, the lactase is added to the milk after thermal sterilization. This strategy allows the mitigation of Maillard reaction (MR) upon processing as the reducing sugars generated by lactose hydrolysis are not exposed to high temperature. On the other hand, the LP remains active throughout the product shelf-life and its side activity may alter the milk components until consumption. Starting from this hypothesis, these specific goals have been addressed along the PhD project: 1. Mapping the volatile organic compounds (VOCs) profile of UHLMs throughout the product lifecycle, both during production and shelf-life. 2. Development and fine-tuning of analytical methodologies for studying the chemical parameters associated with quality changes in UHLM (e.g. GC-MS, PTR-MS, LC-MS).¬¬ 3. Identifying specific VOCs formed via Maillard reaction, elucidating the pathways of formation in relation to different lactases employed for production. 4. Defining how the flavor profile is correlated to the sensory characteristics of UHLM during shelf-life, explaining the chemical nature of the changes limiting the shelf-life of UHLM. 5. Proposing effective strategies to mitigate those reactions limiting the shelf-life of UHLM by looking at the response of the product to different storage temperatures.

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