**Alena Smirnova 1,2,\*, Georgii Konoplev 3, Nikolay Mukhin 3,4,\*, Oksana Stepanova <sup>3</sup> and Ulrike Steinmann <sup>5</sup>**


Received: 10 September 2020; Accepted: 12 October 2020; Published: 15 October 2020

**Abstract:** Milk is a product that requires quality control at all stages of production: from the dairy farm, processing at the dairy plant to finished products. Milk is a complex multiphase polydisperse system, whose components not only determine the quality and price of raw milk, but also reflect the physiological state of the herd. Today's production volumes and rates require simple, fast, cost-effective, and accurate analytical methods, and most manufacturers want to move away from methods that use reagents that increase analysis time and move to rapid analysis methods. The review presents methods for the rapid determination of the main components of milk, examines their advantages and disadvantages. Optical spectroscopy is a fast, non-destructive, precise, and reliable tool for determination of the main constituents and common adulterants in milk. While mid-infrared spectroscopy is a well-established off-line laboratory technique for the routine quality control of milk, near-infrared technologies provide relatively low-cost and robust solutions suitable for on-site and in-line applications on milking farms and dairy production facilities. Other techniques, discussed in this review, including Raman spectroscopy, atomic spectroscopy, molecular fluorescence spectroscopy, are also used for milk analysis but much less extensively. Acoustic methods are also suitable for non-destructive on-line analysis of milk. Acoustic characterization can provide information on fat content, particle size distribution of fat and proteins, changes in the biophysical properties of milk over time, the content of specific proteins and pollutants. The basic principles of ultrasonic techniques, including transmission, pulse-echo, interferometer, and microbalance approaches, are briefly described and milk parameters measured with their help, including frequency ranges and measurement accuracy, are given.

**Keywords:** milk composition; multiphase polydisperse system; near-infrared spectroscopy; mid-infrared spectroscopy; Raman spectroscopy; milk optical and acoustical properties; milk spectral analysis; speed of sound; attenuation; ultrasonic techniques

#### **1. Introduction**

Milk is one of the most common foods. Dairy products are important dietary sources of calcium because of its high content, high absorption rate, availability, and relatively low cost. They contain more protein, calcium, potassium, magnesium, zinc, and phosphorus per calorie than any other popularly food found in our diet [1]. Regular consumption of milk improves enamel in children, promotes better absorption of vitamins, and prevents dehydration of the body. According to research by the Annals of Internal Medicine, dairy consumption reduces acidity throughout the body. But when using milk, it should be borne in mind that some people may have lactose intolerance and may have problems digesting milk because of the lactose contained in it [2–4]. The dynamics of growth in the production and consumption of dairy products is observed annually [4].

The safety and quality of raw milk is now a key competitive factor in the industry. The main reason for the low quality of milk is the lack of effective organization of technological processes. Optimization of quality control of dairy products remains a pressing issue. Gradually, along with traditional methods of analysis, which take a long time, require chemical reagents and preliminary sample preparation, express analyzers are being introduced more and more. Now the market for equipment for the analysis of milk and dairy products is represented by very diverse new models. Such equipment can perform analysis on several indicators in a short time.

All express analyzers are united by the fact that working with them is not difficult for a non-professional, they do not require additional reagents for operation, they are capable of operating at least 8 h, they can be transported and calibrated according to their samples. But despite all the advantages of modern analyzers, there are issues that have not yet been resolved. The most common and interesting devices for further modernization work on the basis of optical, spectroscopic, and ultrasonic methods of analysis.

Optical spectroscopy is among the most widely used methods for the routine quality control of milk and dairy products since it provides fast, simple, and reliable analytical procedures with minimal sample preparation.

Mid-infrared (MIR) spectroscopy has been successfully used for the quantitative determination of the principal milk constituents (protein, fat, lactose) for decades and became some kind of a gold standard; apart from improving accuracy and reliability, the recent developments in this area were aimed at the detection of milk adulteration, the problem which is extremely acute worldwide. Fourier transform infrared (FTIR) spectrometers with attenuated total reflection (ATR) accessories are largely used for the MIR spectral analysis of milk. While there is an obvious market demand for on-site and in-line applications, the high cost of FTIR spectral equipment and the intrinsic limitations of ATR measurements hamper the implementation of MIR spectroscopy outside off-line, laboratory conditions.

Near-infrared (NIR) spectroscopy provides relatively low-cost and robust solutions suitable for in-line applications on milking farms and dairy production facilities, including devices incorporated in milking robots and industrial processing equipment. Because of the significant advances in chemometrics NIR technologies of milk analysis have now achieved the level of accuracy and reliability only slightly inferior to MIR spectroscopy.

Different aspects of milk spectral analysis are discussed in this section of the review: optical properties of milk in a wide spectral range (0.2–25 μm), various analytical techniques, the spectral equipment. The emphasis was made on the methods, which are widely employed for routine milk quality control, namely MIR and NIR molecular absorption spectroscopy (spectrophotometry). Spectroscopic techniques, which are not so extensively used, including ultraviolet (UV) absorption spectroscopy, Raman spectroscopy, atomic spectroscopy, and molecular fluorescent spectroscopy are also briefly described.

The following are acoustic methods that are suitable for non-destructive analysis of milk. This article discusses the acoustic properties of milk and acoustic methods for measuring the speed of sound, attenuation, density. Questions about how the acoustic properties of milk are related to its ingredients are discussed. It is discussed how acoustic characterization can provide information on fat

content, particle size distribution of fat and proteins, and the kinetics of changes in the biophysical properties of milk over time, the content of specific proteins and pollutants.

The basic principles of ultrasonic measuring techniques, including transmission, pulse-echo, interferometer, and microbalance approaches, are briefly described and examples of milk parameters measured with their help, including frequency ranges and measurement accuracy, are given.
