Effect of different heat treatments on the Maillard reaction products, volatile compounds and glycation level of milk
Introduction
Milk and dairy products are usually submitted to thermal procedures prior to being marketed and consumed. Heat treatment can kill spoilage bacteria, pathogenic bacteria, and some spores and inactivate enzymes (Elliott, Dhakal, Datta, & Deeth, 2003). Thermal treatment ensures the microbiological safety of milk, but induces Maillard reactions that modify proteins and generate Maillard products, such as furosine and 5-hydroxymethylfurfural (5-HMF) (Sunds, Rauh, Sørensen, & Larsen, 2018). Additionally, many other physicochemical changes occur in milk after heating. These changes include damage to the creaming properties, nonenzymatic browning, degradation of lactose to lactulose and acids, denaturation of whey proteins, dephosphorylation and hydrolysis of caseins and protein glycation (Aktağ, Hamzalıoğlu, & Gökmen, 2019; Elliott, Datta, & Amenu, 2005).
Due to the various physical and chemical reactions occurring during heat treatment, the flavour of the milk changed and was different from that of the raw milk (Al-Attabi, Arcy, & Deeth, 2014). The sensory characteristics of UHT and HTST milk are quite different due to differences in heating time, temperature and total heat load (Lee, Barbano, & Drake, 2017). Key changes in flavour during thermal milk processing are related to the Maillard reaction, lipid degradation, and thermal denaturation of whey proteins and milk fat globule membranes (Troise et al., 2014).
Lactose and the amino groups in the milk protein undergo Maillard reactions during heat treatment, and the process produces a variety of volatile compounds, such as sulphuric acid, nitrogen-containing compounds, maltol, and diacetyl. Lipid degradation during thermal milk processing produces methyl ketones among other compounds (Jo, Benoist, Barbano, & Drake, 2018).
Glycation of milk proteins has been associated with the harshness of thermal processing (Dalsgaard, Nielsen, & Larsen, 2010; Sharma, Kumar, Betzel, & Singh, 2001). The most thoroughly studied modification of milk proteins is the Maillard reaction, a nonenzymatic glycation reaction in which the carbonyl group of reducing sugars, such as lactose, primarily reacts with the ε-amino group of Lys residues, leading to the initial formation of lactosyl-lysine, which structurally rearranges into the more stable Amadori product lactulosyl-lysine (Majsvan, 1998; Siciliano, Mazzeo, & Arena, 2013). Maillard reaction results in the loss of lactose and essential amino acids, which reduces the nutritional value of dairy products (Singh, Wakeling, & Gamlath, 2007). In addition, some by-products of Maillard reaction are harmful to human health, such as 5-HMF that has cytotoxicity, indirect mutagenicity, carcinogenic, hepatotoxicity and nephrotoxicity. It could also reduce the level of cellular glutathione and cause allergic reactions (Lee, Chen, & Lin, 2019). Maillard reaction is closely related to heat processing conditions. It is necessary to explore the effect of different heating temperatures and heating times on the degree of glycation of milk.
With different degrees of heating, the glycation and binding site of glycoproteins (such as lactoferrin, glycomacropeptide and the glycoproteins of the milk fat globule membrane) in bovine milk changed (O'Riordan, Kane, Joshi, & Hickey, 2014). The first glycation site was identified in β-lactoglobulin (β-LG) and mapped to Lys100 in 1998 (Fogliano & Ritieni, 1998). The main glycated products are milk proteins such as α-lactalbumin (α-LA), β-LG, and κ-casein (Milkovska-Stamenova & Hoffmann, 2016). However, there have been few studies on whether glycation occurs during the heat treatment of milk to produce glycated casein and whey protein, as well as glycation binding sites and the extent of glycation. The objectives of this study were to evaluate the impact of different heating treatments on the characteristics of milk, including fluorescence intensity, Maillard reaction products, volatile compounds and glycation level of milk. In particular, odour active volatile compound characterisation and the degree of glycation by the number of glycated proteins and binding sites for a milk sample treated at 135 °C for 5 s of milk were also evaluated.
Section snippets
Sample collection and heat treatment
Raw milk sample (160 L) was collected from Beijing Sanyuan Lvhe Farm. The raw milk sample was a mixture of milk from all the cows at the farm. The total bacteria count of the mixed sample was 2.0 × 105 cfu mL−1, the number of somatic cells was 1.68 × 104 mL−1, the fat content was 3.74% (w/w), the protein content was 3.34% (w/w), and the lactose content was 4.45% (w/w).
Raw milk was divided into sixteen 10-L samples. The first sample was unheated and served as the control sample, and the
Changes in fluorescence value
Maillard reaction can produce fluorescent substances, which can reflect the degree of heat processing of milk. Fig. 1 shows the change in fluorescence values under different heat treatment conditions. The milk samples after heat treatment at different temperatures and times were scanned at 500–600 nm, and the maximum value of fluorescence intensity appeared near 520 nm. As shown in Fig. 1, the three different heat treatment times of 5 s, 15 s and 30 s showed different amplitude enhancements as
Conclusion
The contents of FRS, LCT and 5-HMF significantly increased with increasing heat treatment temperature and time. It was found that the fluorescence value increased greatly when the heat treatment temperature was higher than 105 °C. The content changes of LCT and FRS were similar when the heat treatment temperature was lower than 105 °C and increased significantly in samples treated at 105 °C. After the heating temperature reached 135 °C, the FRS content reached a maximum. The content of 5-HMF
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
We would like to thank the National Key R&D Program of China (2018YFC1604301, 2017YFE0131800), National Natural Science Foundation of China (31871834) for financial support.
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Yumeng Zhang and Shengnan Yi contributed equally to this work.