Emerging thermal modifying methods in milk protein: A review

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Trends in Food Science & Technology 146 (2024) 104407

Available online 24 February 2024

Emerging thermal modifying methods in milk protein: A review

Jialun Hu

, Heyang Xu

, Ruijie Shi

, Munkh-Amgalan Gantumur

, Zhanmei Jiang

Juncai Hou

College of Food Science and Engineering, Guiyang University, Guiyang, 550005, China

Key Laboratory of Dairy Science (Northeast Agricultural University), Ministry of Education, College of Food Science, Harbin, 150030, China

ARTICLE INFO

Handling editor: AR Jambrak

Keywords:

Milk protein

Ohmic heating

Microwave heating

Radio frequency heating

Infrared heating

Extrusion heating

ABSTRACT

Background: Milk protein plays a crucial role in human diets. Thermal modication is one of the most widely used

food processing techniques. Traditional heating methods used in food processing, especially for heat-sensitive

products, may lead to unintended loss of quality. Therefore, it is urgent to preserve the nutritional value,

taste, and appearance of milk as much as possible and to improve the utilization efciency of thermal modifying

heat energy during the thermal processing of dairy products.

Scope and approach: This review provides an up-to-date overview of studies focusing on the fundamental prin-

ciples, features, and impacts on the structure and functional characteristics of dairy proteins. Furthermore, it

provides insights into the current application status of ve emerging thermal technologies: ohmic heating, mi-

crowave heating, radio frequency heating, infrared heating, and extrusion heating. Additionally, this review

highlights the existing problems of these novel thermal technologies and presents potential solutions.

Key ndings and conclusions: These technologies modify protein structures, enhancing their functional and bio-

logical characteristics. At the same time, microwave heating is studied by most countries because of its simple

operation and remarkable modication effect. Although microwave heating has the potential to be applied to

large-scale industrial dairy processing, it is crucial to explore the characteristics of other technologies to broaden

the thermal modifying methods.

1. Introduction

Protein is required for a variety of human metabolic activities, and it

also plays an important component in food processing due to its sig-

nicant impact on the nutritional value and technical functional prop-

erties of food, thereby affecting food quality and durability (Li, Li et al.,

2023). Casein, which accounts for about 80% of the total protein in milk,

is known for its insolubility around pH 4.6. The remaining 20% of the

protein composition in cow’s milk is comprised of globular proteins

known as whey proteins. These proteins have an isoelectric point (pI, the

isoelectric point is dened as the pH at which the protein/peptide has a

net of charge zero) around 5.0 (Beliciu & Moraru, 2013). In milk, caseins

are naturally present as supramolecular micelles, which are complex

structures with hydrodynamic diameters ranging between 150 and 200

nm. These casein micelles have the following ratios of the four poly-

peptide chains

-, β- and қ-casein: 4.0％, 1.0％, 3.5％, and 1.5％

(Beliciu & Moraru, 2013). The integrity of casein molecules is

maintained by calcium phosphate (CaP) nanoclusters, along with hy-

drophobic, hydrogen, and electrostatic interactions (Broyard & Gau-

cheron, 2015). The complex mixture known as “whey proteins” includes

various proteins such as bovine serum albumin (BSA),

-lactalbumin

(

-LA), β-lactoglobulin (β-LG), and numerous other minor proteins

(Jiang et al., 2023). Whey proteins are substantially smaller than casein

micelles, with hydrodynamic diameters measuring just over 10 nm

(O’Mahony & Fox, 2013).

Currently, the heat treatment of dairy products predominantly em-

ploys conventional heating technology, which generates heat by burning

fuel or using electricity, transferring heat to milk and dairy products via

convection and conduction (Fig. 1) (Qi, Ren, Xiao, & Tomasula, 2015).

However, during the processing of foods containing milk proteins, The

application of heat can lead to structural modications, resulting in

protein denaturation, the exposure of hydrophobic groups, and subse-

quent aggregation or occulation (Raikos, 2010). The thermal conduc-

tivity of foods varies with composition and changes in internal

* Corresponding author.

** Corresponding author. College of Food Science and Engineering, Guiyang University, Guiyang, 550005, China.

E-mail addresses: zhanmeijiang@neau.edu.cn (Z. Jiang), houjuncai88@126.com (J. Hou).

Contents lists available at ScienceDirect

Trends in Food Science & Technology

journal homepage: www.elsevier.com/locate/tifs

https://doi.org/10.1016/j.tifs.2024.104407

Received 11 October 2023; Received in revised form 15 February 2024; Accepted 23 February 2024

Trends in Food Science & Technology 146 (2024) 104407

composition due to increased temperature and pressure (Sweat, 1986,

pp. 87–99). The surface effects of typical heating methods can greatly

reduce the quality of heated dairy products, resulting in a low utilization

rate of heat energy (Bassey, Cheng, & Sun, 2022). High temperatures in

these processes can detrimentally affect food quality for heat-sensitive

structures (Tian, Shao, Yu, & Dai, 2020). Therefore, preserving the

nutritional components, color, aroma, and avor of milk during thermal

processing is crucial, as well as enhancing the energy efciency of

thermal modication. To meet the demands of modern dairy processing,

new heating technologies have been developed. The new heating tech-

nology has the advantages of easy operation, no pollution, high utili-

zation rate of heat energy, and good processing quality. Due to their

unique heat generation principles, these technologies variably affect

dairy products and their components (Guzik, Kulawik, Zajac, & Migdal,

2021; Li, Li et al., 2023; Nunes & Tavares, 2019; Li et al., 2022).

The food industry and researchers are currently focusing on

emerging food thermal processing methods such as ohmic heating (OH),

microwave heating (MW), radio frequency heating (RF), infrared heat-

ing (IR), and extrusion heating (ET) (Fig. 2). The emerging technology

means the new technology to replace (or combine with) conventional

heat treatment (Nunes & Tavares, 2019; Raikos, 2010). Nunes and

Tavares (2019) emphasized how the MW and RF heating altered the

structure, solubilities, and gelling properties of caseins and whey pro-

teins. More recently, Avelar, Vicente, Saraiva, and Rodrigues (2021)

studied the effects of OH on plant protein structure proposing that OH

could enhance food quality and preservation and efciently modify the

functional properties of proteins, including solubility, water/oil holding

capacity, foaming, and emulsication characteristics. Hence, the exist-

ing studies unequivocally establish that emerging thermal processing

techniques efciently alter proteins to enhance the quality of the

product (Bassey, Cheng, & Sun, 2021).

Although several new technologies have been proposed as viable

alternatives to conventional thermal modifying methods, there is

limited information available on how these technologies might affect the

structure and functionality of dietary milk proteins. The effects of

various emerging thermal technologies on milk protein have not been

thoroughly compiled. Currently, the available references tend to focus

on specic technologies. This review provides a brief description of the

principles underlying these techniques and details how processing im-

pacts the structures and functionalities of milk proteins. It also presents

the challenge of applying and developing these unique techniques. The

objective of this review is to stimulate further research and adoption of

emerging thermal modifying methods to enhance the quality and utility

of dairy products.

2. Innovative emerging thermal modifying technology

2.1. Ohmic heating (OH)

Ohmic heating is an advanced thermal processing technique which

the food material (which acts as an electrical resistor) is heated by

putting an electric current through it (Joule effect). (Lian et al., 2022).

2.1.1. Basic composition of equipment

The OH system primarily comprises a pump, a column ohmic heater,

an insulation tube, and a control instrument. The most crucial compo-

nent is the column ohmic heater, which includes over four electrode

chambers. In the process of OH, the application of alternating current

between two electrodes within food products generates internal heat.

Different from other electrical heating methods, OH involves the pres-

ence of electrodes that make direct contact with the food matrix. It also

allows for the usage of unlimited frequency and unrestricted waveform

types, with sinusoidal waveforms being a typical example (Torgbo,

Sukatta, Kamonpatana, & Sukyai, 2022).

2.1.2. Inuence factors

As shown in Fig. 3A, the factors affecting OH heating efciency are as

follows.

2.1.2.1. Physical and chemical properties of food materials. Conventional

heating rate is directly proportional to the temperature difference be-

tween food and the heat source. In OH, if the absorbed power is con-

stant, the temperature rises almost linearly. If the conductivity of the

material is too high or too low, or the impedance of the foods is too large

or too small, it is difcult to heat the food. Moreover, conductivity

changes nonlinearly with material temperature. Thus, considering the

impact of nonlinear temperature variation on raw material conductivity

is crucial in ohmic heating (Bhale, 2004).

2.1.2.2. Power supply type of heating system. Ohmic heating primarily

uses direct current (DC) and alternating current (AC). Initially, low-

frequency AC was common in ohmic heating. Subsequent research

revealed that low-frequency power sources could cause electrolysis in

water and other components. High-frequency power sources, however,

can reduce electrochemical corrosion of electrodes and prevent

contamination (Ayadi et al., 2005, 2008).

2.1.2.3. Electrode material. Electrodes are an indispensable part of

ohmic heating, and each ohmic heating device contains two or more

electrodes (Icier & Ilicali, 2004). Electrodes are typically selected based

Fig. 1. Effect of innovative emerging thermal modifying methods in milk protein.

J. Hu et al.

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