Aquaphotomics and NIR water spectral patterns in dairy production: a review exploring potentials and challenges

Abstract

This review describes the potentials of a new omics science in the dairy sector, particularly regarding the improvement of animal health and welfare. The three-dimensional network of water hydrogen bonds is a dynamic entity, subject to the influence of its components and properties of the environment. For this reason, it is sensitive to any chemical, physical or biological perturbation of the system. Therefore, the aqueous matrix acts as a sensitive sensor, reflecting the state, behaviour, and functionality of the system and providing similar information on its components. Aquaphotomics builds upon these underlying assumptions. It is a scientific discipline that combines spectroscopy with multivariate spectral analysis to extract the absorbance spectral pattern of water, which describes how water molecular structure changes in response to perturbations. Many studies assessed the applicability of this approach, including veterinary diagnostics. The water spectral pattern can be used as a multidimensional biomarker for rapid and non-invasive discrimination between healthy and diseased systems, even in the subclinical phase. Adopting such an approach, focused on precision farming, can foster subsequent optimisation of animal production performance and improve the overall profitability of farming operations.

Introduction

The first part of this review will describe the background to the molecular systems of water, their interactions with dissolved substances and the exploitation of this information in NIR milk analysis.

The role of the water molecular system in the determination of the conformational structure of biomolecules in bovine milk

The beginning of the scientific research conducted on the molecular structure of water can be traced back to the second half of the 18th century to the English chemist Henry Cavendish, who discovered the chemical formula of water from the reaction between two parts of hydrogen and one part of oxygen (Cavendish, 1766). Since then, several studies have deepened this fundamental chemical compound's chemical, physical and biological properties. Water is a small, electrically neutral molecule with a polar identity resulting from the greater electronegativity of the oxygen atom capable of attracting the electronic cloud around the molecule with a greater force than the individual proton charges of the two hydrogen atoms. The polarity of this covalent bond creates a partial positive charge on the hydrogen atom and a partial negative charge on the oxygen atom, resulting in a dipole moment of the water molecule. Such a polar bond also enables water molecules to form hydrogen bonds with each other, causing them to attract one another, with the positive end of one molecule meeting the negative end of another (Latimer and Rodebush, 1920). Thanks to this interaction, neighbouring water molecules can assemble to create dimers, trimers, tetramers and other molecular conformations in a three-dimensional network of weak-intensity bonds that can break and reform quickly and continuously in a dynamic equilibrium, influenced by the chemical content and by physical conditions of the system (Chaplin, 2019).

Solvation is the interaction of a solvent with dissolved molecules that influences their properties and stability. As a solvent, the water molecular system behaves in a very flexible manner. Its elasticity reflects the structural complexity of the solutes, forming a solvation shell characterised by the assembly of three different states of water molecules: (i) the strongly bound ones, when the interaction occurs with polar groups or with ionic residues on the molecular surfaces; (ii) the weakly bound ones, resulting from the formation of electrostatic bonds with weaker polar groups; (iii) the free water molecules, not directly involved in the solvation of the solute (Laage et al., 2017). The hydrogen bonds have a fundamental role in the correct folding and stabilisation of the tertiary and quaternary structures of the macromolecules, determining their nature and, consequently, their biological function uniquely.

In the case of interaction with proteins, the structural nature of the three-dimensional network of water involved in their hydration results from many factors, including the chemical composition of the amino acid surface constituting any polypeptide (Haider et al., 2016). The stability of intermolecular interactions between water and proteins mainly involves acid, basic and hydrophilic side chain amino acids. By contrast, amino acids with hydrophobic side chains (alanine, proline, methionine, tyrosine and tryptophan) cause the destabilisation of the interaction between water and protein biomolecules, which creates favourable conditions for protein to consolidate their chemical bonding with other biomolecules or ligands (Schauperl et al., 2016; Chaplin, 2019; Rego et al., 2019).

Concerning interactions with lipids, the features of their surface result from the abundance of hydrophobic groups alternated with few hydrophilic groups accepting hydrogen bonds. Such a heterogeneous chemical character strongly influences the orientation of the water molecules in the solvation shell. Consequently, the interaction of water with phospholipids or triglycerides shows a substantial spatial heterogeneity in the first layer of the solvation shell (Laage et al., 2017). Specifically, water can form hydrogen bonds with the hydrophilic polar head of the phospholipid component (up to a maximum of 28 water molecules directly involved for the single biomolecule: Hishida and Tanaka, 2011), or it can establish an intermolecular bond with the oxygen ester in triacylglycerols (Groot et al., 2016).

The molecular properties of water affect the structure and dynamics of carbohydrates (Almond, 2005). In lactose, hydroxyl oxygens and hydroxyl hydrogens attract water more than other atoms constituting the ring (Imberti et al., 2019). Sugar hydration involves multiple layers of water molecules and, in particular, the hydration shell around α-glucose and β-glucose has fifteen water molecules distributed over three sites (Song et al., 2020).

Water is an essential component of bovine milk (87%). It is responsible for the formation of a molecular network of interactions with the remaining 13% of dry matter present in solution or suspension in the liquid, which is made up of fats (2.5–6%), proteins (2.9–5%), lactose (3.6–5.5%), minerals (0.6–0.9%) and traces of other substances (Bylund, 1995; Mansson, 2008). In milk, fat is present in suspended globules ranging in size from 10⁻² to 10⁻³ mm. A thin membrane with a variable composition of phospholipids, lipoproteins, glycolipids, trace elements and bound water molecules of the solvation shell of the lipid droplet covers them. Casein and whey proteins represent the milk protein component. Casein consists of four different sub-forms, namely αs1, αs2, β and k-casein, arranged in colloidal suspension micelles ranging from 50 to 500 nm and made up of smaller sub-micelles, connected by calcium phosphate bridges. This conformational model results from an orderly arrangement of the α and β-caseins at the centre of the micelle. At the same time, the k-caseins are oriented with the hydrophilic C-terminal end on the most superficial portion to promote the establishment of hydrogen bonds with the water molecules of the solution (Bylund, 1995). Whey proteins represent the water-soluble protein fraction of milk. They include α-lactalbumin, β-lactoglobulin and, to a lesser extent, immunoglobulin, serum albumin and bioactive peptides. β-lactoglobulin has a homodimer conformation consisting of two globular structures linked by non-covalent bonds, whilst α-lactalbumin is a metalloprotein with a binding site for the Ca²⁺ ion located near the junction ring of the two subdomains composing the protein (Pizzichini et al., 2006).

Using near-infrared spectroscopy in dairy production

Vibrational spectroscopy in the near-infrared region (NIR) is a highly sensitive, low-cost, non-destructive analytical technique. It allows rapid and reliable generation of objective data, an advantage compared to the more traditional laboratory methods that necessitate an initial sample preparation with chemical reagents, an operation requiring experienced and qualified personnel (Loudiyi et al., 2020). The basis of the technique rests on the principle that every compound possesses a unique absorption spectrum that distinguishes it from others. Light interacts with matter, which absorbs part of the incident radiations at specific wavelengths. This energy transfer promotes the transition of molecules to higher energetic vibrational levels. When the incident light has the same fundamental frequency as a bond, electrons catch part of it, causing the bond atoms to vibrate faster. As the electrons move closer or farther from the atom's nucleus, energy in the form of light is emitted or absorbed. The magnitude of energy that matter emits or absorbs across a range of wavelengths is its spectral emission or absorption pattern. Consequently, studying the spectral pattern enables the acquisition of qualitative information regarding specific functional groups within a sample (Molteni, 2013). Each type of molecule absorbs in a specific wavelength range, and the same is true for the different water species in solution. Water molecules absorb radiation over the entire range of the electromagnetic spectrum, with intramolecular vibrational transitions occurring in the infrared region. Due to the interaction between electromagnetic radiation and aqueous matter, the absorption of discrete values of NIR energy results in molecular vibrations in the water molecule (Fig. 1) occurring between the hydrogen and oxygen atoms involved in the covalent bond. These oscillations of interatomic distances can manifest as symmetrical (v1), asymmetrical (v3), or bending (v2) stretching modes, with the latter requiring less energy to occur compared to the former two (De Haseth, 2022).

Figure 1. Vibrational modalities of the water molecule hit by near-infrared radiation (from Holtrop et al., 2021).

The electromagnetic spectrum of pure water in the near-infrared region, between 750 and 2500 nm, shows four main absorption peaks (Fig. 2). The first peak (970 nm) results from the second overtone (a harmonic with a frequency higher than the fundamental frequency) of the –OH bond's symmetrical and asymmetrical stretching bands (av₁ + bv₃; a + b = 3; 3ν_1,3). The second peak (at 1190 nm) combines the first overtone of the symmetrical and asymmetrical stretching band of the –OH bond and the bending band of the –OH bond (2ν_1,3 + ν₂). At 1450 nm, the spectrum of water shows a large peak composed of many overlapping bands, attributable to the first overtone of the symmetrical and asymmetrical stretching band of the –OH bond (2ν_1,3). Finally, the most intense peak appears at 1940 nm: it results from the combination of the stretching and bending bands of the –OH bond (2ν_1,3 + ν₂) (Luck, 1973). Such dominant absorption bands of water pose a significant problem when NIR analysis focuses on aqueous solutions like milk. They hide the weaker ones belonging to analytes dissolved in solution at low concentrations, complicating their detection. For this reason, in former NIR studies, measurements considered absorption bands far from such wavelength regions or were performed on dried samples (Núnez-Sanchez et al., 2016). However, such procedures restricted the potential of NIR measurement technology for the daily management of dairy farming (Muncan et al., 2021b). Nowadays, there is wide use of NIR spectroscopy in dairy. Specifically, it offers the possibility of real-time monitoring of milk's qualitative/quantitative characteristics throughout the entire production chain to obtain a product with quality, safety and sustainability standards matching the regulatory requirements and consumer needs (Evangelista et al., 2021). In more detail, NIR spectroscopy can be utilised as a monitoring tool throughout the production line to characterise milk's chemical and physical parameters (Laporte and Paquin, 1999; Holroyd, 2013), as a guarantee of food safety and for the identification of adulterations (Liu et al., 2011; Cattaneo and Holroyd, 2013; Kamboj et al., 2020; Deshpande et al., 2021) or, directly in the milking parlour, to obtain real-time and non-invasive information on the health or reproductive status of each cow (Kawasaki et al., 2008; Maltz et al., 2009; Diaz-Olivares et al., 2020; Iweka et al., 2020a, 2020b). Rapidly detecting changes in milk composition allows the early identification of diseases in the animal, with timely health treatment and subsequent improvement of animal welfare and a reduction of productive and economic losses for the farmer (eg the early detection of the increase of the somatic cell counts that is the primary marker of mastitis disease: Tsenkova et al., 2001a, 2001b, 2001c).

Figure 2. Schematic representation of the position of the overtones and the absorption bands in the near-infrared region (from Eldin, 2011), with a focus on the electromagnetic spectrum of pure water (Muncan and Tsenkova, 2019). The main absorption peaks at 970, 1190, 1450, and 1940 nm have been highlighted.

Aquaphotomics: water–light interaction for characterisation of a system

As the name suggests, aquaphotomics is an ‘omics’ science, a molecular discipline aimed at representing and systematically interpreting matter complexity. It exploits the interaction between water and light radiation. However, unlike the other ‘omics’, aquaphotomics proposes an integrative approach instead of emphasising the single constituent element: it considers water a multidimensional biomarker capable of reflecting the overall effect of all the system's components. The system can be studied rapidly, non-invasively and non-destructively, with the possibility of real-time monitoring of the rearrangement of the chemical and structural composition of water resulting from any perturbation affecting it. The dynamism of such bonds makes it possible to explore any system using water as a possible descriptor. This approach can, therefore, be exploited to obtain indirect qualitative-quantitative information about the solutes present in the solution, in contrast with the conventional techniques, which focus on the direct measurement of the single analytes (Muncan and Tsenkova, 2019).

When near-infrared radiation hits water molecules, the energy released is converted into vibrational energy through oscillations of interatomic distances and bonding angles. Any variation in the concentration of analytes results in a change in the vibration mode of the water molecules that can be monitored along the spectrum (Muncan et al., 2016). The spectral pattern of water changes reflects accurately and sensitively the transformations at the molecular level that characterise the different species of water constituting the three-dimensional network of hydrogen bonds. Instead of tracing the individual solutes, it is possible to observe the cumulative effect on the spectral model of absorption of NIR radiation, which becomes a distinctive and identifying marker for each sample (Kovacs et al., 2022). Therefore, the aqueous matrix cannot be seen as an inert solvent as it constitutes the scaffolding of the entire system, with an interdependent connection with the rest of the constituent components of the system. For this reason, the objective of this new ‘omics’ science is to describe (in the future) the state of any system exclusively in terms of the structure of the water molecular system (Van De Kraats et al., 2019).

Water absorbance bands in the near-infrared region of the electromagnetic spectrum

Most aquaphotomics studies on system characterisation have focused on the electromagnetic spectrum wavelength range between 1300 and 1600 nm, corresponding to the first overtone of water OH stretching vibrations (Tsenkova et al., 2018). In this region of the NIR spectrum, from the experimental point of view, the adopted methodological approach involves executing a multivariate analysis of the spectral variations acquired in an aqueous system to find and identify the water absorption bands activated following a specific equilibrium perturbation.

An activated water absorbance band (WAB) is a spectral band in which the variation in light absorption is higher in response to a given perturbation. Combining multiple WABs produces a characteristic spectral pattern of water absorbances (WASP), reflecting the conditions of the entire aqueous molecular system (Tsenkova et al., 2015). In the spectroscopic models experimentally analysed, aquaphotomics research verified the existence of typical water absorption bands repeatedly activated with different intensity combinations depending on the type of perturbation and the system studied: the WAMACS (water matrix coordinates). These absorption coordinates are wavelength intervals in which the activated water bands are most likely to be found (Tsenkova, 2009). Each WAMACS corresponds to a specific molecular conformation of water, which can help to describe each frequency absorbed in terms of water structure (Weber et al., 2000; Robertson et al., 2003). All of these molecular conformations have distinctive spectral properties that aid in comprehending phenomena across various fields of application. In the first overtone region of the water spectrum, 12 WAMACS have been identified, providing insights into the structural organisation of water (Tsenkova et al., 2018), although recent studies point out the existence of several more (Muncan et al., 2022a, 2022b; Vitalis et al., 2023). These WAMACS are summarised in Table 1.

Table 1. Coordinates of the molecular matrix of water with their spectroscopic and structural characterisation

Multivariate spectral analysis and chemometric models are the primary tools for uncovering hidden information within the raw spectral data. The chemometric analysis combines mathematical and statistical tools to help identify the variables with a high contribution within the spectral pattern of water (ie those absorption bands that are constantly repeated during aquaphotomics analysis and can be considered active). Aquagrams are the graphical representation of this water spectral pattern. They are radar graphs reporting the normalised absorbance within the WAMACS, providing a quick and complete comparison of different systems or conditions of the same system regarding changes in the aqueous matrix (Muncan et al., 2014). The right part of the graph represents free water molecules, weakly bound to hydrogen or even water, involved in the hydration of solutes. In contrast, the area on the left of the graph corresponds to the water species most strongly bound to hydrogen and able to establish a more significant number of electrostatic intermolecular interactions with neighbouring molecules (Tsenkova et al., 2015; Muncan et al., 2020).

Aquaphotomics aims to construct a database containing all the water absorption bands that are linked to the respective molecular conformations. This database, called the Aquaphotome, represents an alphabet of interactions between water and light that can be used to describe the functionality of a system and provide a more thorough understanding of the phenomena (Tsenkova, 2008a). Aquaphotomics improves NIR spectroscopy by addressing the limitations of dominant water absorption bands. It uses the spectral characteristics of water as a biomarker to reflect the state, behaviour and functionality of the entire analysed matrix (Robertson et al., 2003; Van De Kraats et al., 2019). Essentially, aquaphotomics harnesses the spectral features of water as a comprehensive biomarker, capable of offering valuable insights into the properties of the investigated system (Tsenkova, 2005). The remainder of this review describes the potential of this technique, the achieved results, and their possible applications in animal health and dairy production.

Review procedure

The literature search of the most relevant articles for this scientific review was conducted until June 2023 using the Scopus database (Elsevier) and Google Scholar. The keyword used to search for relevant documents was ‘aquaphotomics’, which the document's title, abstract, or keywords had to include. No restrictions were placed on the type of documents, topic or year of publication to identify relevant articles. This approach revealed a continuously increasing number of publications from 2006 to the present (Muncan and Tsenkova, 2023). In the Scopus database from 2009 to 2018, relatively few articles were published on aquaphotomics, suggesting limited research in this field. However, since 2019, the number of published articles has increased significantly, and in the first four months of 2023 alone, 6 new articles on this topic were indexed in Scopus, indicating a growing interest among researchers (Supplementary Fig. S1). Online Supplementary Table S2 reports a topic-organised bibliography, these topics being elaborated in the following sections of the review and evaluating, for each, a potential application in the dairy sector as described by Fig. 3. After a brief overview of the technology and the main results that emerged from its application to experimental research activity, the remainder of the review focuses on the possibility of exploiting aquaphotomics as a non-invasive and real-time monitoring tool to point out the changes in the composition of milk in ruminants associated with the development of a subclinical form of some diseases (eg metabolic diseases). Aquaphotomics represents an opportunity for ethical and sustainable animal production that falls within the objectives of precision animal farming with high production standards.

Figure 3. Structural organisation of the review according to paragraph 3.1.

Application field and results obtained

Aquaphotomics has impacted a wide range of sectors, from fundamental molecular research to practical utilisation in the agri-food, medical, microbiological and industrial sectors. This continuous evolution promises to open new horizons of scientific knowledge (Kovacs et al., 2022). Water reflects any occurring molecular change and provides valuable information about the dynamics of the interactions occurring under the effect of any system perturbation through the three-dimensional network of water involved in the hydration of the molecules in the solution (Han et al., 2022). Aquaphotomics has revolutionised water's role, transforming it from a passive component to a fundamental pillar of any system that reflects molecular changes. Water's ability to form hydrogen bonds and influence the structure of macromolecules is essential to any solution (Tsenkova, 2009, 2010; Tsenkova et al., 2015; Cattaneo et al., 2015a; Van De Kraats et al., 2019) opening new perspectives for dairy research.

Applications in veterinary medicine

The studies conducted using aquaphotomics as an examination method have revealed that water can act as a biomarker for the diagnosis of mastitis in dairy cattle. Mastitis significantly alters the ionic concentration and chemical composition of many biomolecules. This bio-perturbation affects the molecular structure of the water in milk samples, resulting in differences in the spectral patterns of absorption between healthy individuals and individuals with mammary gland inflammation (Tsenkova, 2007a, 2008a; Tsenkova and Muncan, 2021). The technology allows for the continuous and in vivo observation of an animal's health status, and serves as an early warning system for abnormal physiological states, identifying deviations from individual markers in the animal's biological fluid spectra and prompting timely treatment (Muncan et al., 2023). Santos-Rivera et al. (2021), Santos-Rivera et al. (2022a), and Santos-Rivera et al. (2022b) have used aquaphotomics to rapidly identify the biochemical changes occurring in the early stages of bovine respiratory diseases from different aetiological agents to ensure timely intervention before the symptoms worsen. These studies considered exhaled breath, nasal secretions and blood (online Supplementary Fig. S2). Santos-Rivera (2022) pointed out the existence of a biological spectral signature associated with a specific pathogen profile.

Aquaphotomics technology is a highly versatile tool effectively utilised in animal reproduction research. Counsell et al. (2017) demonstrated the efficacy of this technology by using it to assess the health of swine offspring from in vitro fertilisation of young sows with nanopurified semen. Furthermore, aquaphotomics offers a quick and cost-effective alternative to traditional methods of analysing animal hormone levels during the oestrus cycle, which typically require chemical reagents and the expertise of trained personnel (Kinoshita et al., 2012, 2015; Takemura et al., 2015; Agcanas et al., 2017; Vance et al., 2017). Aquaphotomics technology has other potential benefits in animal breeding practices. Firstly, it can detect both organic and functional changes related to developing pathological conditions or resuming the oestrous cycle. Secondly, it can be applied to genetic breeding programs aimed at reducing the environmental impact of cattle farming. Studies have explored the possibility of phenotyping milk composition traits for more profitable and sustainable animal production with less impact on animal health and the environment by combining infrared spectroscopy with genomic and molecular data sources (Bresolin and Dórea, 2020; Tiplady et al., 2020). In a similar vein, Bittante and Cipolat-Gotet (2018) and Vanlierde et al. (2018) used mid- and near-infrared milk spectral data to predict the daily methane production in cows, pointing out the potential of breeding based on such an approach.

Applications in plant biology and agronomy

Ensuring the nutritional quality of the ration distributed to animals is crucial to promote their health. One way to achieve this is by monitoring the quality of plants used for feed production, especially when they are components of a total mixed ration. As feed quality directly impacts the quality of the milk produced, it is essential to monitor it promptly to support precision farming management of the herd. Properly managing dairy cow rationing influences the animal's individual production regarding product quality and quantity (Caccamo et al., 2012). Knowing the compositional characteristics of milk enables helpful information on the quality of the ration administered to the animal which nowadays undergoes increased levels of automation (Bahri et al., 2019; Romano et al., 2023). Aquaphotomics can assist farmers in real-time optimisation of the chemical and physical composition of the total mixed ration administered, thus improving the farm's productivity. Aquaphotomics also has great potential in regard to crop production. It can detect the early onset of plant diseases, even while they are still in the latent stage (Jinendra et al., 2010; Zahir et al., 2022). Additionally, it can assess the effectiveness of genetic improvement actions by detecting specific active substances in plant tissues (Cattaneo et al., 2015b).

Human medicine

Significant advances have been made in the diagnostic sector of human medicine, similar to those conducted in animal medicine. NIR spectroscopy, coupled with aquaphotomics analysis, can be complementary for rapid, in vivo and continuous monitoring of critical physiological parameters to reduce disease incidence or mortality rate (online Supplementary Fig. S3: Chatani et al., 2014; Cui et al., 2019b; Li et al., 2020). This method can also help in identifying zoonotic agents that husbandry staff in direct contact with animals may be exposed to, including tuberculosis, brucellosis, listeriosis and leptospirosis. According to Veleva-Doneva et al. (2010), when combined with multivariate classification methods, NIR spectroscopy is an excellent tool for quickly detecting bacterial contamination in bovine milk. Proactively detecting these pathogens in milk is a preventive and protective measure, mitigating the potential biological risks associated with farming and animal management activities. The resulting early diagnosis enables more targeted use of antibiotics on farms, resulting in the containment of antibiotic resistance in animal production and humans.

Studies on water solutions in dairy

Aquaphotomics is used in the dairy industry to evaluate the compositional, nutritional and economic value of milk and its by-products. This method involves monitoring milk as an aqueous solution, which helps analyse milk's components (fats, proteins, lactose). These components can affect water's behaviour, which has been demonstrated through studies conducted on solutions analysed with various analytes at different concentrations (Tsenkova, 2007b; Kovacs et al., 2015; Cui et al., 2016; Dong et al., 2019a, 2019b; Zhang et al., 2023a, 2023b). Analysing the spectral pattern of water in the visible (Bozhynov et al., 2019, 2020, 2021) or near-infrared region can help track the origin of a perturbation and provide valuable information about any element present in a solution, such as salvianolic acid, methanol, heavy metals, sugars and proteins (Tsenkova, 2008b; Vero et al., 2010; Meilina et al., 2011; Bazar et al., 2014, 2015; Putra et al., 2016, 2017; Cui et al., 2017; Li et al., 2019; Muncan and Tsenkova, 2019; Dong et al., 2022). Combining multivariate spectral analysis with chemometric models also enables the understanding of the dynamics of the system (Kojić et al., 2017; Tsenkova et al., 2018; Cui, 2022; Roger et al., 2022; Ye et al., 2022; Ma et al., 2023).

Fat is the most variable component of milk as numerous factors (season, feeding, breed, stage of lactation and individual animal traits) affect its composition, which results in variations in cheese and butter consistencies and, therefore, in the price of the commodity (Coppa et al., 2011; Borreani et al., 2013). Similarly, milk proteins, particularly casein, play a crucial role in determining its suitability for cheesemaking (Sturaro et al., 2015). The real-time analysis of lactose is also crucial for the dairy industry, as it helps ensure that specific dairy products are suitable for lactose-intolerant people. Online Supplementary Fig. S4 provides a practical example of aquagrams resulting from different concentrations of lactose solutions in water.

Besides serving as a potential monitoring tool for milk composition, aquaphotomics has significant potential in assessing milk quality and its derivatives following heating or other raw material treatments, which may impact the organoleptic and sensory characteristics of the products. This approach can detect any potential quality impairment resulting from oxidative phenomena, making it a valuable tool for verifying the effectiveness of conservation treatments, as monitoring the changes occurring at the water absorbance bands increases understanding of the system's dynamics (Kaur et al., 2017; Cui et al., 2018, 2019a; Stoilov et al., 2022).

Microbiology and process quality

Aquaphotomics helps optimise dairy processing by identifying critical points that need modulation (Nath et al., 2021). Specifically, each phase of industrial manufacturing processes significantly impacts the quality of the finished product. Therefore, production efficiency monitoring methods are necessary along the cheese production line or during the fermentation processes of the lactic acid contained in milk to obtain yoghurt and other derivatives (Muncan et al., 2021a) with subsequent prompt implementation of targeted corrective actions to address any critical issues arising. In microbiology, the NIR absorbance pattern of water offers a distinctive spectral fingerprint of cultured cells, primarily due to the synthesis of various extracellular metabolites (Nakakimura et al., 2012). Aquaphotomics also classifies bacterial strains with precision (Remagni et al., 2013) and allows for selective strain choice based on stability, resistance, or probiotic properties (Slavchev et al., 2015, 2017; Kovacs et al., 2019).

Food quality

Aquaphotomics integrates conventional technologies for food quality and hygiene control (Veleva-Doneva, 2017; Veleva-Doneva et al., 2017). It predicts chemical composition (Williams, 2009) and quantifies fundamental components, like dietary fatty acids in milk (Muncan et al., 2021b), for nutritional purposes. NIR spectroscopy has many uses, including: (i) studying the organoleptic properties of food by detecting the changes in the structure of the water molecular system that affect the sensory characteristics (Vanoli et al., 2018; Rajkumar et al., 2022); (ii) detecting biological alterations resulting from mechanical damage to a product (Gowen et al., 2009; Esquerre et al., 2009); (iii) determining the food safety (Su et al., 2022) and (iv) performing automated and real-time monitoring of industrial transformation processes (Muncan et al., 2014; Kato et al., 2021; Muncan et al., 2021a).

The various treatments adopted to preserve the safety or shelf life of food can affect the water absorbance spectral pattern (Barzaghi et al., 2014; Atanassova, 2015; Sannia et al., 2019; Brambilla et al., 2020; Cattaneo et al., 2021, 2022; Nagy et al., 2021; Bodor et al., 2022; Malegori et al., 2022; Kaur et al., 2022; Marinoni et al., 2022a, 2022b; Muncan et al., 2022b; Vitalis et al., 2023). Additionally, there have been specific applications of aquaphotomics that focus on the quick and non-destructive assessment of food adulteration (Bazar et al., 2016; Vitalis et al., 2019; Zaukuu et al., 2019; Kamboj et al., 2020; Yang et al., 2020; Aouadi et al., 2022; Raypah et al., 2022; Atanassova et al., 2023). Aquaphotomics can be an effective strategy for detecting food adulteration as it targets the milk molecular changes resulting from adding foreign substances to dairy products, protecting consumers against the sometimes highly harmful consequences of illegal practices (Liu et al., 2012; Obladen, 2014). Milk adulteration is a serious food safety risk due to the high variability of adulterating substances and dilution with water (Nagraik et al., 2021; Mafra et al., 2022).

Materials and nanomaterials

The materials and nanomaterials sectors are equally essential for the dairy industry. The performance of packaging and food stability properties during shelf-life depends on the choice of food coating material (Cattaneo et al., 2016; Barzaghi et al., 2017; Vanoli et al., 2019). The spectral pattern of water absorption (Supplementary Fig. S5) can help evaluate the effect of food coating material on these properties. For example, milk packaging allows the product to be isolated from the external environment, creating a barrier against microorganisms and against air, humidity, and light to extend the product's shelf life. However, there may be issues with the migration of contaminants in the coating material towards the food, which can pose risks to human health (Wu et al., 2010). Aquaphotomics can be applied in the dairy sector to design and validate food coating materials and study their impact on the organoleptic characteristics of milk.

General discussion

Rapid population growth linked to qualitative and quantitative changes in food demand is putting a severe strain on the entire primary sector, now facing the considerable challenge of producing more using fewer resources (CEMA, 2018). In this context, the dairy sector has undergone a profound intensification of the production model in recent years, with a worldwide production of cow's milk reaching more than 740 million tons in 2021, with a constant but progressive increase over time, as can be observed in the online Supplementary Table S1 (CLAL, 2023). Intensification of milk production has been made possible thanks to the enormous scientific advances obtained in the field of genetics, as well as to the progressive digitalisation of the cattle sector through the adoption of cutting-edge technological solutions in the field of nutrition, milking, or reproductive management of animals, with an almost complete transition from grazing systems to housing. This constant search for increasingly efficient production conditions has led to selecting high-yielding livestock with higher individual production, allowing farms to increase their economic stability and competitiveness in the market (Medeiros et al., 2022). However, intensive farming has also led to negative consequences, with problems related to the production model's economic, social, and environmental sustainability and an increased occurrence of metabolic disorders (Zachut et al., 2020). These pathological conditions (ketosis, ruminal acidosis, hypocalcaemia, hepatic steatosis, abomasal displacement) result from difficulty in maintaining homeostasis as a consequence of imposing metabolic demands on the animal, especially during the transition period and the negative energy balance that occurs post partum. If left untreated, these disorders may lead to other clinical diseases in dairy cattle (Fantini, 2016). Unsustainable management also has a significant impact on the perception of the consumer, who is increasingly attentive to the ethics of animal production (Raboisson et al., 2016). For this reason, managing livestock health is a critical issue for both the farmer and the consumer.

The early diagnosis of disease improves the sustainability of dairy production (Lovarelli et al., 2020). The opportunity to achieve this strategic objective results from using technologies related to precision livestock farming, particularly by implementing modern bio-sensing approaches in the bovine production chain. These innovative sensor technologies in veterinary diagnostics collect, combine, process, and interpret farmed animals' physiological and health parameters to generate helpful information for the breeder, thus ensuring sustainable and ethically acceptable livestock management (Rutten et al., 2013; Berckmans, 2017). Therefore, the main objective of these technologies is the automatic and continuous monitoring of individual animals, generating reliable data that can prevent the onset of metabolic disorders in the farm and optimising the production process (Caja et al., 2016). Neethirajan et al. (2017) provide an overview of emerging technological advances in monitoring the health status of farm animals, categorising the sensors currently available according to the type of biomarker analysed. An example is given by the sensors installed in line for real-time analysis of the composition of the milk produced, such as AfiLab^TM (S.A.E. Afikim, Afikim, Israel) (Leitner et al., 2011), or Herd Navigator^TM (DeLaval, Tumba, Sweden) (Mazeris, 2010), which find application directly in the milking parlour to provide an individual indication on the productive and metabolic performance of the cow. The benefits of such technological solutions are innumerable: milk sampling has the great advantage of not being invasive for the animal; furthermore, these diagnostic tools are precise and sensitive to the parameters analysed, promising, compared to laboratory analyses, a reduction in the costs necessary for the purchase of reagents and for the transport of samples, as well as the time necessary to carry out the analyses (Giannuzzi et al., 2022). However, such approaches focusing on individual milk components interpret the system according to a fragmentary logic that does not consider the molecular relationships underlying any biological system's structural and functional organisation.

In this regard, aquaphotomics represents a sustainable approach for monitoring production variables related to the early onset of metabolic disorders in intensive farms. Onset of disease significantly alters the molecular structure of the water contained in milk or any other biological fluid matrix in response to subtle biochemical changes that accompany the initial stages of the onset of an infection. An animal's physiological and health status can, therefore, be determined using the spectral pattern of water as a biomarker, a discovery of enormous scientific importance compared to the measurement of the individual analytes used for diagnosing pathology. If taken individually, these components often don't discriminate between healthy and diseased systems, especially in the subclinical phase (Muncan and Tsenkova, 2019). The knowledge gained from this discipline offers the potential to develop portable or milking parlour-installed diagnostic tools to characterise the milk properties of each individual cow in real-time. The resulting daily monitoring enables farmers to noninvasively and immediately control the health status of their animals. This makes it possible to automatically identify the animals that require more assistance and specialised care (Caja et al., 2016; Muncan et al., 2021b). Furthermore, this alarm system could help the farmer undertake the targeted interventions to correct and optimise the chemical and physical composition of the ration. Such a ration improvement reduces the stress factors, prevents the disease onset in the clinical form, reduces the economic expenses associated with the treatment, and avoids yield drops in milk production (Caja et al., 2016; Wisnieski et al., 2019). Implementing such systems would enhance the efficiency of animal breeding, ensuring that it meets the current and future market demands for food supply while safeguarding the welfare of farmed animals in compliance with regulatory standards and consumer interests.

In conclusion, aquaphotomics is a promising method that can be implemented throughout the milk supply chain to guarantee adherence to the highest quality standards for the production animal and the final product (Lovarelli et al., 2020). By applying this approach on the farm, during production and on the finished product, we can ensure that the end product complies with strict quality, safety, and sustainability guidelines. This approach represents an essential instrument of protection and guarantee in the dairy industry, ensuring that the final product is of the highest quality and meets the needs of both producers and consumers.