What is food engineering within the dairy field?

There are many early definitions of food engineering, all of which have the primary goals of design, construction, operation and formulation of raw materials aiming at economic benefits and preservation of foods (Parker et al. Reference Parker, Harvey and Stateler1952; Charm, Reference Charm1963). In addition, some definitions include engineering research and their outcomes applied to real applications in the industry environment (Heldman, Reference Heldman1975). According to Roos et al. (Reference Roos, Fryer, Knorr, Schuchmann, Schroen, Schutyser, Transtram and Windhab2016), and as defined by the European Academy of Food Engineering; ‘Food engineering covers the study, modelling and design of ingredients and foods at all scales using technological innovations and engineering principles in the development, manufacturing, use and understanding of existing and emerging food processes, food packaging and food materials from food production to digestion and satiation enabling development and design, production, and availability of sustainable, safe, nutritious, healthy, appealing and affordable supply of high-quality ingredients and foods.’ Thus, food engineering is a multidisciplinary field with the applications of agricultural, mechanical and chemical engineering principles to food materials and matrixes.

Food engineering across the dairy field mainly employs modern tools and processes, technology, and knowledge in developing new products primarily to increase consumer perception and convenience and simultaneously increase the cost effectiveness of the production. However, improving quality, nutrition, safety, and security remain critical issues (Fig. 1). New packaging materials and techniques are being developed to provide more protection to foods, and novel preservation technologies for enhanced shelf life are emerging (Rysstad & Kolstad, Reference Rysstad and Kolstad2006). Additionally, process control and automation appear among the top priorities identified (Goff & Griffiths, Reference Goff and Griffiths2006). Furthermore, energy saving and minimisation of environmental problems continue to be important food engineering issues, and significant progress is being made in waste management, efficient utilisation of energy, and reduction of effluents and emissions within the dairy sector for a sustainable dairy industry. Significant improvements in non-thermal processing technologies are in place, due to increased demand for food with minimal changes in food quality.

Fig. 1. Key challenges in the dairy sector.

History

The use of milk as a beverage began with the domestication of animals. Cattle were being herded in about 7000 B.C., while goats and sheep were domesticated in around 9000 B.C. Raw milk then became an important food source for many cultures. By exploiting the preservative benefits of fermentation due to lack of refrigeration, the method of cheese and yoghurt making was known to the ancient Greeks and Romans.

Gradually, with the much more recent development of the dairy industry, a variety of machines for processing milk were developed. In 1856, Gail Borden patented a method for making condensed milk by heating under a partial vacuum. Not only did this method remove much of the water so that the milk could be stored in a smaller volume, but it also protected the milk from microbes. In 1863, Louis Pasteur of France developed a method of heating to kill microorganisms which later became known as pasteurisation. In 1895, commercial pasteurising machines for milk were introduced in the United States. Although many dairy operators opposed pasteurisation as an unnecessary expense at the beginning; gradually it became a must. In 1899, Auguste Gaulin obtained a patent for the homogeniser which was used to break down the large fat globules in milk which thus prevented the separation of cream from milk.

Coming up to date, in 2015 milk production in Australia has been increasing by ~6% due to the favourable rainfall and pasture conditions, and adoption of new technologies. As a measure of recovery from the low farm gate prices for fresh milk, manufacturers have channelled into production of cheese, Skim Milk Powder (SMP) and butter (Dairy Australia, 2014). Traditionally, liquid milk is in demand in urban communities and fermented milk in rural areas, but processed products are becoming increasingly important in many countries within the European Union (EU) in general. China remains the main importer of fluid milk especially UHT, while India has seen reinforced growth in demand for dairy products due to the rise in incomes, population growth, changes in habits and lifestyles and urbanisation. New Zealand faces a similar situation to the European Union. Thus, the growing demand for milk and milk products offers a great opportunity for the increased consumption of milk (Rysstad & Kolstad, Reference Rysstad and Kolstad2006). The dairy industry's perpetual drive to optimise production has led to a willingness to adopt new technologies that enable more to be done with less (Roos et al. Reference Roos, Fryer, Knorr, Schuchmann, Schroen, Schutyser, Transtram and Windhab2016). Farmers are producing more milk per cow and dairy processors are increasing output and reducing operating costs by maximising efficiency through modern engineering concepts and processes. Due to a focus on efficiency, the dairy industry has shown steady growth in the past 5 years despite an economy that has slowly recovered from a hard-hit decline. Thus, food engineering within the dairy sector as a whole has many benefits but processing may also have some drawbacks as stated in Fig. 2.

Fig. 2. Benefits and drawbacks of food engineering within the dairy sector.

Thermally engineered milk products

In the early decades when domestic refrigeration was not widespread, pasteurised milk was kept for only a short time. However, in the 21^st century, improved access to refrigeration has resulted in the engineering of this simple thermally processed milk into many products available for the consumer. Although the main purpose of heating in early days was to minimise the pathogenic load to reduce spoilages and thereby extend the shelf life, nowadays the increased scale of operation with automated large scale instruments that utilise less energy has led to production of different products with various properties (Rysstad & Kolstad, Reference Rysstad and Kolstad2006).

Increasing distribution times for the delivery of milk and milk products have resulted in the development of processes and packaging concepts to increase the shelf life of these products in cold chain distribution. For example, modern pasteurised milk (72 °C/15 s) undergoes less thermal damage to the quality of raw milk and under refrigeration conditions can be stored for up to 3–7 d. The shelf life can be extended to 21–28 d under refrigerated conditions, if thermal treatment is performed at slightly elevated heat treatments and accompanied by aseptic packaging. Even higher temperatures, ultra-high temperature (UHT – 140 °C/2 s) treatment extends the shelf-life of milk for months without refrigeration. In contrast to pasteurised milk, UHT milk results in substantial changes in colour, flavour and appearance due to the high temperature treatment (Chandrapala, Reference Chandrapala2008). The increased shelf life of UHT without refrigeration has gained widespread acceptance and popularity by many consumers, although UHT milk has not been accepted by all because of the perceived ‘cooked’ taste of the product. Consequently, the need for extending the shelf life of pasteurised milk, without detectable changes to the flavour profile is sought and has proven to be an engineering challenge. The challenge lies in the need for heat treatment severe enough to obtain sufficient microbial kill while maintaining the sensory and nutritious quality of the product.

Currently, commercial UHT processing of milk relies on indirect plate heat exchangers (PHE) or direct steam injection (DSI). Electrical resistive (ER) heating is a new method of heating of milk and involves the release of heat as an electrical current is passed through milk. This induces very rapid heating without exposure of the milk to hot steam or metal heat transfer surfaces (McMahon et al. Reference McMahon, Ganesan, Qian and Brothersen2012). Such heating provides opportunities for processing of heat induced milk with fewer flavour changes. McMahon et al. (Reference McMahon, Ganesan, Qian and Brothersen2012) studied the flavour attributes and consumer preferences of UHT processed milk through ER, DSI and PHE. The ER milk was preferred over PHE and DSI treated milk. After 8 months of storage, the PHE and DSI treated UHT milks were considered bitterer than pasteurised or ER milk. So according to their study, ER produced better-flavoured UHT milk than current UHT processes, although ER is yet to be implemented commercially. However, the safety of these processes is still under investigation.

Increasing the shelf life of pasteurised milk, while keeping the fresh taste and quality profile is considered a priority in extended shelf life (ESL) technology (Rysstad & Kolstad, Reference Rysstad and Kolstad2006). Some methods including bactofugation and microfiltration (MF) are used widely in the dairy industries. However, these methods can only be used under good cold chain distribution conditions. If the quality of the cold chain is poor, these methods will not work. During MF, removal of bacterial cells and spores from milk occurs. However, the size of fat globules is similar to the size of the cells and spores, so fat can be unintentionally removed. Therefore MF can only be used for skim milk processing, and is not to be used on whole milk (Daufin et al. Reference Daufin, Esudier, Carrere, Berot, Fillaudeau and Dedoux2001). In addition, due to the overlap of the particle size distribution of cells and spores with that of casein micelles, careful consideration of the selection of the membrane pore size is required in order to minimise changes in milk composition. These days, ceramic membranes with pore diameters ranging from 0·8–1·4 mm are commonly used commercially (Kessler, Reference Kessler1997). The MF permeate is then standardised for fat and pasteurised at a low temperature for a short time to produce an ESL milk with high flavour quality. Elwell & Barbano (Reference Elwell and Barbano2006) studied the shelf life of pasteurised (72 °C, 15 s) skim milk with and without MF (1·4 and 0·8 µm) at different storage temperatures (0·1–6·1 °C). A maximum bacterial log reduction of 5·63 and 92 d of milk shelf life was obtained. MF milk is marketed in several countries as more ‘pure’ and ‘natural’ than standard heat-treated milk, and has achieved a higher retail price as a branded product. However, the shelf life of MF milk is strongly influenced by raw milk quality as well as processing and storage conditions (Schmidt et al. Reference Schmidt, Kaufmann, Kulozile, Scherer and Wenning2012), although the microbiological quality of MF milk at the end of shelf life can be excellent. Thus, further advances of this technique require high standards of production hygiene as well as an exceptionally good raw milk quality to ensure a safe and palatable end product.

Other non-thermal techniques for pasteurisation of milk such as pulsed electric field (PEF), high pressure processing (HPP) and ultrasound (US) have been investigated. A recent trend towards the development of ultrasound as an alternative non-thermal technique is further driven by other advantages including reduced energy consumption, ability to target specific organisms and no requirement for the introduction of preservatives (Chandrapala et al. Reference Chandrapala, Oliver, Kentish and Ashokkumar2012). Gera & Doores (Reference Gera and Doores2011) found reductions of E. coli and L monocytogenes significantly under US conditions of 85 W/cm², 24 kHz, 100 µm amplitude with 80% of pulsing. The main mechanism responsible for the ultrasonic deactivation effect is the physical forces generated by acoustic cavitation. The asymmetric collapse of a cavitation bubble leads to a liquid jet rushing through the centre of the collapsing bubble. Microorganisms that have hydrophobic surfaces will promote the collapse of cavitation bubbles on the surface and lead to severe damage of the cell wall (Chandrapala et al. Reference Chandrapala, Oliver, Kentish and Ashokkumar2012). In addition, micro-streaming effects can lead to the erosion of cell walls, which then again result in inactivation of the microorganisms. Gera & Doores (Reference Gera and Doores2011) showed that the presence of lactose diminished the microbial reductions in milk which may be due to the stabilisation of the bacterial membrane by lactose. Thus, the resistance of different microorganisms to ultrasound varies widely and many process treatment parameters and ultrasound variables have to be carefully selected to compensate if any further advances are to be made within this technology.

PEF processing is a non-thermal method of food preservation that uses short bursts of high voltage pulses in the order of 20–80 kV for microbial inactivation. It offers minimal detrimental effect on quality such as aroma, flavour, appearance and nutritional value (Ruan et al. Reference Ruan, Deng, Cheng, Lin, Chen and Metzger2012). Guerrero-Beltran et al. (Reference Guerrero-Beltran, Sepulveda, Gongora-Nieto and Barbosa-Canovas2010) studied the L. innocua inactivation of milk after application of pulsed electric fields at 30 and 40 kV/cm respectively as a function of the number of pulses. When applying 30 kV/cm of electric field intensity, a maximum microbial reduction of 4·3 log cycles was achieved with 20 pulses. At 40 kV/cm of intensity, a maximum of 5·5 log bacterial inactivation was achieved when applying ~12 pulses. However, commercial application of this technology has not been implemented yet, mainly due to lack of regulatory approval, high initial investment, and elevated processing costs.

High pressure processing (HPP) is also used to inactivate microbial growth to prolong the shelf-life of milk. This recent commercial technology has enormous potential in the food industry, controlling food spoilage, improving food safety and extending product shelf life while retaining the characteristics of fresh, preservative-free, minimally processed foods. Over the last 20 years, significant advances in HPP technology have been made, in the form of semi-continuous systems to the scaling up of pilot units to successful commercially viable processes (Moreau, Reference Moreau, Ledward, Johnston and Earnshow1995). Current industrial HPP treatment of food is carried out using a batch or semi-continuous process (Hogan et al. Reference Hogan, Kelly, Sun and Sun DW2005). The type of substrate and composition of the food can have a dramatic effect on the response of microorganisms during pressure treatment. Carbohydrates, proteins, lipids and other food constituents can confer a protective effect (Simpson & Gilmour, Reference Simpson and Gilmour1997). Thermodynamic properties and phase equilibrium of any system as well as transport properties such as viscosity, thermal conductivity, or diffusivity have to be considered in their functional relationship with pressure. The reduction of microbial pathogens can be achieved by inducing structural changes to the cell membrane. It is known that Gram negative bacteria are inhibited at lower pressure than Gram positive bacteria (Considine et al. Reference Considine, Kelly, Fitzgerald, Hill and Sleator2008). The inhibition of microbial spores can be managed by combining the high pressure treatment with low temperatures. Rising concerns over food safety and the growing demand for processed food all over the world drives the growth of the HPP technologies market (Considine et al. Reference Considine, Kelly, Fitzgerald, Hill and Sleator2008). Furthermore, consumers increasingly appreciate high quality products with natural and fresh flavour. The reduction of casein micelles of milk at pressures >230 MPa can lead to losses in colour and consistency (Chawla et al. Reference Chawla, Patil and Singh2011). In addition, HPP can induce crystallisation of milk fat. These draw backs that have implications on the quality of the milk have resulted in the technology been picked up at a slower pace by the dairy industry. However, with continued research and reducing equipment costs, these problems will soon be overcome.

The combined use of several methods, possibly physical and chemical, is used more nowadays to meet the requirements of safety. Hurdle technologies have been commonly applied by the dairy industry to ensure food safety and stability (Roos et al. Reference Roos, Fryer, Knorr, Schuchmann, Schroen, Schutyser, Transtram and Windhab2016). In recent years combining several techniques has been developed not just to understand why a certain food is safe and stable, but to improve the microbial quality of the food by optimisation and intelligent modification of the combined technologies. Mandatory pasteurisation regulations require that microfiltration be accompanied by thermal pasteurisation. A maximum extended shelf life of 74 d was found for milk after the combination of microfiltration and direct heat treatment at 125–130 °C and storage at room temperature (Garcia & Rodriguez, Reference Garcia and Rodriguez2014). Comparing PEF, MF, and thermal pasteurisation (TP) for the reduction of the native microbial load in milk led to a 4·6 log10 CFU/ml reduction in count for TP, which was similar to 3·7 log10 CFU/ml obtained by MF, and more effective than the 2·5 log10 CFU/ml inactivation achieved by PEF inactivation. However, combined processing with MF followed by PEF (MF/PEF) produced a 4·8 log10 CFU/ml reduction in count of the milk microorganisms, which was comparable to that of TP. Reversed processing (PEF/MF) achieved comparable reductions of 5·7 log10 CFU/ml and a higher inactivation of 7·1 log₁₀ in milk than for TP. Thus, PEF/MF represents a potential alternative for ‘cold’ pasteurisation of milk with improved quality (Walkling-Ribeiro et al. Reference Walkling-Ribeiro, Rodrigeuz-Gonzalez, Jayaram and Grifiths2011). Due to high potential for commercialisation, further research on the optimisation of this non thermal hurdle technology with a focus on the specific requirements for different milk products is recommended. In addition, the interactions or mechanisms at the molecular level following PEF milk treatment, which were responsible for an increased antimicrobial efficacy of the PEF/MF over the MF/PEF processing sequence should be analysed carefully.

Intense ultrasound power and long contact times are required for microbial inactivation when ultrasound is applied alone. The most effective sonication approaches to inactivate microbes for industrial purposes is the combination of ultrasound with heat (thermosonication, TS), pressure (manosonication, MS), or heat and pressure (manothermosonication, MTS; Chandrapala et al. Reference Chandrapala, Oliver, Kentish and Ashokkumar2012). For example, ultrasound treatment combined with mild heat (57 °C) for 18 min resulted in a 5-log reduction of L. monocytogenes in milk and a 5-log reduction in the total aerobic bacteria in raw milk (D'Amico et al. Reference D'amico, Silk, Wu and Guo2006). In addition, inactivation of microorganisms using ultrasound depended on the amplitude of the ultrasonic waves, the exposure/contact time and the temperature of treatment (Juraga et al. Reference Juraga, Salamon and Herceg2011). Bermúdez-Aguirre et al. (Reference Bermúdez-Aguirre, Corradini, Mawson and Barbosa-Cánovas2009) compared batch pasteurisation (63 °C, 0–30 min) with TS (400 W, 24 kHz, 63 °C, 0–30 min) inactivation of L. innocua and mesophilic bacteria in raw full fat milk. Batch pasteurisation resulted in 0·69 log and 5·5 log reductions for 10 and 30 min treatments, respectively. With a combination of ultrasound (60, 90 or 100% of 400 W) and heat (63 °C) a 5 log reduction was achieved after just 10 min. Furthermore, the thermosonicated milk had similar physicochemical properties compared to pasteurised milk. Similarly as for US, synergistic interaction between PEF and mild thermal treatment was found. A maximum microbial reduction of 4·3 log cycles was achieved using 10 and 25 pulses when processing milk at 30 kV/cm and initial temperatures of 43 and 13 °C, respectively. Milk treated with 40 kV/cm of electric field intensity, few pulses, and initial temperature close to 55 °C showed the best balance between L. innocua inactivation of milk (Guerrero-Beltran et al. Reference Guerrero-Beltran, Sepulveda, Gongora-Nieto and Barbosa-Canovas2010).

The introduction of pasteurisation in 1886 improved both the safety and keeping quality of milk and milk products. Newer methods like UHT, HPP, US, PEF and MF treatments have further increased the safety and the shelf-life of modern dairy foods as well as allowing non-refrigerated distributions. However, there are still challenges such as reducing energy requirements, and improving the flavour of longer life products. As new technologies are explored, it is important to recognise not only the science but also the regulatory requirements for successful implementation. Well-designed experimental approaches should reflect both scientific discovery and regulatory overview.

Fermented dairy products

Yoghurt

Fermentation has long been a processing technique used to improve the shelf life of milk. Nowadays, the purpose has changed to consumer demands for healthier foods that retain their original nutritional properties and consumer preferences for more natural food with no processing and fewer chemical preservatives. Preservation can disturb one or more of the homeostasis mechanisms of the micro-organisms, thereby preventing them from multiplying and causing them to remain inactive or even die. The best and most efficient way is to disturb several homeostasis mechanisms simultaneously, thus a combination of multiple technologies could increase the effectiveness of food preservation. It has been suspected for some time that combining different hurdles for good preservation might not have just an additive effect on microbial stability, but could produce synergistic benefits.

The word ‘yoğurt’ is Turkish in origin. In history, the natural enzymes in the carrying containers curdled the milk, making yoghurt. The longer shelf life and the preferred taste continued the practise. The first industrialised production of yogurt is attributed to Isaac Carasso in 1919 in Barcelona in his company ‘Danone’. The perceived health benefits of yoghurt led its consumption to spread throughout the world. Yoghurts are now available in many varieties to suit every taste and lifestyle.

Modern dairy yoghurt making involves steps such as standardisation, pasteurisation, homogenisation and fermentation (Fig. 3). When milk arrives at the factory, the standardisation process is performed to reduce the fat and increase the TS content. Yoghurt should contain at least 3·25% of milk fat and at least 11–12% of Solids Non Fat (SNF) with a titratable acidity of not less than 0·9%, expressed as lactic acid according to the USDA specifications for yoghurt. For yoghurt manufacture, the solids content of milk is increased to 16%. This increasing total solid content increases the nutritional value of the product and also helps to produce firmer and more stable yoghurt. This can be achieved either by evaporation or simply by fortifications. However, the modern dairy industry uses membrane processing a great deal due to major advancements in this field. Thus, ultrafiltration is commonly used in concentrating the yoghurt milk base. UF not only increases the protein concentration, but also result in a lower lactose level. Lankes et al. (Reference Lankes, Ozer and Robinson1998) reported that set and stirred yoghurts made from UF of skim milk had better gel strengths compared to fortification and concentrating through the use of evaporators. Pasteurisation follows, having the primary purposes of destroying the microbes that may interfere with the controlled fermentation process and to denature whey proteins, giving rise to a firmer gel network. Homogenisation follows immediately after heating where fat globules are broken into smaller particles which help to produce a smoother, creamier and more uniform end product without a phase separation.

Fig. 3. Basic layout of a modern day yoghurt factory (Adapted from R Sharma's yoghurt presentation).

However, with advancements in technology, US and HPP have been trialled for improving the quality of the yoghurts. Vercet et al. (Reference Vercet, Oria, Marquina, Crelier and Lopez-buesa2002) studied the use of MTS to obtain tailored functional properties of yoghurts. The application of ultrasound allowed yoghurts with superior rheological properties such as flow curves, apparent viscosity, yield stress and viscoelastic properties to those of control prepared with untreated milk. Furthermore, yoghurts made from TS milk had higher gelation pH values, greater viscosities and higher water holding capacities (Reiner et al. Reference Riener, Noci, Cronin, Morgan and Lyng2009). The authors further stated that the gel network showed a honeycomb like network and exhibited a more porous nature. Similarly, Reiner et al. (Reference Reiner, Noci, Cronin, Morgan and Lyng2010) found superior rheological properties of yoghurts prepared from ultrasonicated milk than the yoghurts prepared from thermally treated milk. Further, Bermudez-Aguirre & Barbosa-Canovas (Reference Bermudez-Aguirre and Barbosa-Canovas2010) found only minor changes to the nutritional quality of milk after ultrasound, with the advantage of extending the shelf-life of the product for more than 16 d at 4 °C without the use of intensive heat treatments. With the added advantage of minimal nutritional loss compared to thermal treatments but still retaining long stable shelf-lives, the use of ultrasound technology in dairy streams shows promise (Chandrapala et al. Reference Chandrapala, Oliver, Kentish and Ashokkumar2012).

Similarly, Penna et al. (Reference Penna, Subbarao and Barbosa-Canovas2007) showed that the microstructure of HPP yoghurt (676 MPa for 5 min) has more interconnected clusters of densely aggregated protein and reduced particles with smoother surfaces as compared to heat-treated milk yogurt (85 °C for 30 min). The latter had fewer interconnected chains of irregularly shaped casein micelles, forming a network that enclosed the void spaces. Furthermore, HPP prevents any rise in acidity during storage to maintain the consistent texture of full fat yoghurts and to maintain the creamy, thick consistency with no additions of polysaccharides for low fat yoghurts. In addition, use of the combined effect of HPP and thermal treatments in preparing low fat yoghurt exhibited improved elastic modulus, yield stress and reduced syneresis (Harte et al. Reference Harte, Luedecke, Swanson and Barbano-Canovas2003). Thus, the use of HPP offers microbiologically safe and additive-free low-fat yogurt with improved performances, such as reduced syneresis, high nutritional and sensory quality, novel texture, and increased shelf-life (Harte et al. Reference Harte, Luedecke, Swanson and Barbano-Canovas2003).

Although new advanced technologies are sought out by industries, consumer perception has led more towards health benefits. Yoghurt in general is considered as a nutrition-dense food due to its nutrient profile in terms of Ca and proteins which provide almost all the essential amino acids necessary to maintain good health. Yoghurt is a probiotic carrier food that can deliver significant amounts of probiotic bacteria into the body which can claim specific health benefits once ingested. Due to these beneficial effects, yoghurt has become the fastest growing dairy category and is now available in many varieties with different fat contents, flavours and textures.

Yoghurt can be categorised mainly as standard or probiotic yoghurt. Standard yoghurt refers to those made with L. bulgaricus and S. thermophiles, while probiotic yoghurts are manufactured by culturing beneficial microorganisms that claim to have numerous health benefits once ingested, typically the probiotic strains of Bifidobacteria and L. acidophilus. Probiotic yoghurts aid in digestion and promote good health, although these probiotic strains should remain alive at adequate numbers if health effects are to be claimed (Chandan & O'Rell, Reference Chandan, O'rell and Chandan2006). Thus, according to the National Yogurt Association's guidelines, refrigerated and frozen products should contain at least 100 million or 10 million live cultures per gram, respectively, at the time of manufacture in order to obtain the live and active culture seal with the consumption (De Brabandere & de Brabandere, Reference De Brabandere and de Brabandere1999). Probiotic yogurt occupies a very strong position in the dairy market, and there is a clear trend to increase its consumption in the next few years.

Apart from this main classification, yoghurts are available in a wide range of flavours, textures and forms. For example, based on the fat content, yoghurts are classified as regular, low-fat yogurt and non-fat yogurt where the fat contents varied from 3·25%, 0·5–2·0% and <0·5%. Yoghurts are also classified according to their physical nature as set yoghurt, stirred yoghurt and drinking yoghurt. Stirred yogurts are produced by incubating the mix in a tank followed by stirring prior to cooling and packaging. Thus, the possibility of adding both fruits and stabilisers give the manufacturer several options for controlling the texture and physical properties of the yoghurts (Lucey, Reference Lucey2002). Additional health aspects of the additive-free product favours increased consumption of traditional yoghurts. However, it is a challenge to produce low-fat and non-fat yoghurt products that do not whey-off during storage without using stabilisers (Lucey & Singh, Reference Lucey, Singh, Fox and McSweeney2003). Drinking yoghurts usually go through a homogenisation process in order to reduce the particle size that assured hydro colloidal distribution and stabilisation of the protein suspension.

Due to the high consumer appeal for yoghurts with some flavour, yoghurts are now categorised as plain/natural, fruit and flavoured yoghurts. Natural yogurt is the simplest (Chandan & O'Rell, Reference Chandan, O'rell and Chandan2006). Natural yoghurt is closer to the nutritional value of milk which it is made of, and provides all of the benefits associated with fermentation while supplying fewer amounts of calories. Moreover, plain yogurt gives the pure yogurt taste and contains the richest calcium content among the yogurt products (Chandan & O'Rell, Reference Chandan, O'rell and Chandan2006). Flavoured yoghurts are available in a vast array of flavours including fruit (apple, apricot, black cherry, black currant, blue berry, lemon, mandarin, raspberry, strawberry, peach), vegetables, chocolate, vanilla, caramel, ginger, etc. In general, flavours are added to yoghurt during the production stage and the addition of flavours not only results a wide array of tastes, but also increases sweetness of the product. In recent years, there is an increasing trend towards the production of herbal yogurts by incorporating natural food additives and health promoting substances. Addition of neem (Azadirachta indica) into yogurt has shown increased acidification, antioxidant activity and inhibition of enzymes related to diabetes and hypertension (Shori & Baba, Reference Shori and Baba2013). Bio yogurts prepared from cow milk and camel milk with Cinnamomum verum are reported to inhibit enzymes such as α-amylase and α- glucosidase related to diabetes, whereas higher counts of Lactobacilli were observed in herbal yogurts prepared with camel milk than that of plain yogurt (Shori & Baba, Reference Shori and Baba2013). Herbal yogurts prepared with peppermint, dill and basil showed higher ACE inhibitory activities, α- amylase and α-glycosidase activities which suggested that these herbal yogurts may possibly be beneficial to treat hypertension and diabetes mellitus (Amirdirvani & Baba, Reference Amirdirvani and Baba2011). In general, health benefits of this sort are claims rather than proven efficacy.

Greek yoghurt is technically defined as a semi-solid product derived from regular yoghurt by straining away part of its water and water soluble components, mainly lactose and salt. In accordance with this definition, the Greek yoghurt production process is an extension of a regular yoghurt production process. Concentration of insoluble parts in the yoghurt mixture and strengthening of the gel structure takes place when converting regular yoghurt into Greek yoghurt. Generally, Greek yoghurt is characterised by its white colour, soft and smooth body, good spreadability and a slightly acidic flavour. These favourable characteristics place Greek yogurt at a highly competitive market position compared to regular yogurts. Consumer demand for Greek yogurt has rocketed in recent years. Greek-style yogurts accounted for nearly 26% of all new yogurt introductions in the U.S. in 2011 vs. around 9% in 2008, according to Datamonitor's Product Launch Analytics database of new products. Further, Greek yoghurt has been used as a ‘workout’ and ‘between meals’ snack due to claimed health benefits. In the USA, there as an increased consumption of Greek yoghurt for breakfast by the 18–34 age group.

In ancient times, Greek yoghurt was derived from regular yoghurt using an animal skin. Fermented milk was stored within an animal skin until it was consumed. During this process, a portion of the water soluble parts was absorbed by the animal skin, while another portion penetrated through the skin and evaporated. However, this processing method was considered unhygienic. Thus, continued evolution of Greek yoghurt processing has resulted in advancing from this animal skin to modern technological methods such as ultrafiltration and reverse osmosis. Traditionally, a bag made from a double layer of cheese cloth or similar material is used for the filtration process. The bag is left hanging in a cold room (4 °C) overnight to allow complete draining. This traditional Greek yoghurt production method results in excellent sensory qualities including good viscosity, pleasant mouthfeel, increased acidity up to 18-20 g/kg and fat content raised to 100 g/kg. Although this technique can achieve a high quality final product, it has two major drawbacks limiting the widespread industrial use; reduced production capacity due to high labour involvement and production time. In addition, this method can lead to unhygienic conditions, due to growth of undesirable microorganisms during processing (Tamime, Reference Tamime2006). With technological advancement, ultrafiltration has become the most suitable technique developed in dairy industry for selective concentration of dairy ingredients. While technology has provided new equipment, all these methods share the same basic principle of separating water soluble particles from water insoluble particles. The main intentions of using more modern technologies have been to gain advantages such as reducing processing time, less labour requirement and increased product shelf life. Milk is not subjected to any phase separation during UF processing. In Greek yoghurt manufacturing, ultrafiltration can be applied either prior to or after milk fermentation. The chemical and physical properties of final Greek yoghurt are highly dependent on the process stage at which filtration is applied (Tong, Reference Tong2013).

Since the introduction of commercial yoghurt production in 1919, the yoghurt industry has bloomed significantly with increased consumer consumption and also has used different technological advancements to produce a huge variety of yoghurts. The use of new technologies not merely targets the production yields. Consumer satisfaction, safety and health consciousness were also taken into account, while increasing the shelf life for convenience. Whether these products can carve out a significant part of the market is unclear. However, if yogurt products of all types can persuade consumers to buy in order to improve digestive health, the future will be good for this dairy product. Continuous innovations, introduction of new flavours, novel packaging and new technologies will prove to be key in increasing demand.

Novel technologies for energy savings & cost reductions

Consumers increasingly demand dairy products which retain their natural flavour, colour and texture with fewer additives such as preservatives. Food safety is vital during this process. However, profit incentives drive the sustainability of the dairy industry. Thus, rising energy costs have driven the industry towards new and potentially energy-saving technologies such as US, HPP and PEF. In addition, the huge demand of dairy plants for water usage means that the industry faces challenges to cope with environmental change. Thus, there is a huge demand for reducing water usage and increasing the efficient utilisation of dairy waste water streams. Better understanding of how processes work and use of new knowledge for improved process control will minimise waste and use the dairy by-products more efficiently.

Extensive fundamental research has been done on a range of novel food processes, in which preservation occurs through non thermal means, such as US, HPP and PEF, or through heat generation, such as ohmic and microwave processes which reduces the energy usage substantially. For example, Koh et al. (Reference Koh, Chandrapala, Zisu, Martin, Kentish and Ashokkumar2013) studied whether high shear mixing or homogenisation was able to replicate similar effects to that of sonication using 5 wt% whey protein concentrate. Smaller reductions in particle size and viscosity were observed in the high shear mixed samples, suggesting lower energy efficiency. Both sonication and homogenisation provided similar particle sizes and viscosity reductions. This study suggested that these novel technologies provide advantages over the traditional methods. Comparing the total energy consumption required for PEF in comparison to HTST pasteurisation of milk, Fernandez-Molina et al. (Reference Fernandez-Molina, Bermudez-Aquirree, Altunakar, Swanson and Barbosa-Canovas2006) concluded that the averaged 100–200 kJ/l was substantially lower than the 300 kJ/l for heating. US was found to have a very high energy efficiency where overall efficiency of the industrial ultrasonic devices is ~80–90% from the power source into the liquid (Fig. 4). Although energetic efficiency was improved, consumer perception of the products was a key issue. An ‘ill-defined’ aroma was identified for the pasteurised homogenised milk through a 24 kHz US (Riener et al. Reference Riener, Noci, Cronin, Morgan and Lyng2009). In contrast, there was no unusual odour detected using continuous sonication to treat concentrated milk delivering a significantly lower energy density of 4–7 J/ml with 10 s contact time (Zisu et al. Reference Zisu, Bhaskaracharya, Kentish and Ashokkumar2010).

Fig. 4. Efficiency of using Ultrasound (Adapted from Hilcher website).

There are optimisation needs for better and more efficient process monitoring and control methods for quality assurance of the final product. These methods are often lacking in the dairy industry and the need is to have systems that can be validated for online monitoring and process control (Chandrapala et al. Reference Chandrapala, Oliver, Kentish and Ashokkumar2012). Low intensity, high frequency ultrasound used for detection purposes is typically <1 W/cm² and >100 kHz and is gaining more popularity as it can provide a rapid, accurate, inexpensive, simple and non-destructive method to monitor the properties of foods on-line during process operations (Bermudez-Aguirre et al. Reference Bermudez-Aguirre, Mobbs, Barbosa-Canovas, Feng, Barbosa-C_anovas and Weiss2011). Low intensity ultrasound (at 1 MHz) has been successfully used to monitor different stages of the cheese manufacturing process (Benedito et al. Reference Benedito, Carcel, Gisbert and Mulet2002). Ultrasonic parameters such as attenuation and velocity were used to determine the renneting times and maturation degree of cheeses by relating the ultrasonic variables to enzyme activity in milk during coagulation and storage. Recently, Telis-Romero et al. (Reference Telis-Romero, Vaquiro, Bon and Benedito2011) proposed a method to estimate the fat and moisture content of fresh and blended cheeses from measurements of acoustic velocity at 1 MHz and various temperatures ranging from 3−29 °C. In addition, low intensity ultrasound-based techniques are able to detect and identify foreign bodies in cheese (Benedito et al. Reference Benedito, Carcel, Gisbert and Mulet2001) and yoghurt (Knorr et al. Reference Knorr, Zenker, Heinz and Lee2004). These efficient online processing methods will lead to huge cost reductions in batch replication, as one can monitor a change in advance and then can take precautions at an early stage.

Membrane processing is currently utilised to a very great extent in the dairy industry. In recent years, much work has been done on fouling of membranes, although little focus has been given to the cleaning process. With the increasing restrictions on the use of water, novel technologies for cost efficient cleaning are sought. There are a number of different chemical (use of acid, alkali, enzymes and hypochlorite), physical (forward flushing and back flushing) methods currently used for cleaning fouled membranes. However, these methods are time consuming, unsafe and/or expensive. UF membranes within the dairy plants are cleaned on a regular basis to ensure hygienic operations and to maintain membrane performance. Muthukumaran et al. (Reference Muthukumaran, Lalchandani, Kentish, Stevens, Ashokkumar, Mawson, Stevens and Grieser2005a, Reference Muthukumaran, Kentish, Stevens and Ashokkumarb) studied the ultrasonic cleaning of the fouled UF membranes. Their results revealed that ultrasound can significantly enhance the permeate flux, with an enhancement factor of between 1·2 and 1·7 across the full range. An increase of the mass transfer coefficient within the concentration polarisation layer was also observed. In another study (Muthukumaran et al. Reference Muthukumaran, Kentish, Stevens, Ashokkumar and Mawson2007), they extended this aspect to consider the effect of ultrasonic frequency and the use of intermittent ultrasound. Their results showed that the use of continuous low frequency (50 kHz) ultrasound is most effective in both the fouling and cleaning cycles. Use of these novel technologies could potentially save the industry a significant amount of money.

Limited studies of the environmental impact of novel processes have been made, and it is possible that there are environmental advantages over conventional products. A highly hygienic manufacturing plant is critical in the dairy industry, but cleaning is the single largest water-consuming process. Frequent cleaning is needed to ensure process sterility and to remove product residues, and both of these processes require huge amounts of water. Nowadays, dairy industries are re-utilising the water discarded from production after purifying through membrane processing. In addition, to minimise washings, new coatings for equipment surfaces have been developed that can significantly reduce fouling and speed cleaning (Afroz et al. Reference Afroz, Swaminathan, Karthikeyan, Pervez, Sudhir and Kumar2012). The new coating materials potentially offer a step change reduction in waste and energy usage.

Growing food production and consumption can result in major environmental problems as the world population exceeds 7 billion and is expected to reach 9·2 billion in 2050. Thus, utilising waste from manufacturing will play a key role in the sustainability of the industry as well as providing a solution for future food production. For example, the dairy industry produces large volumes of whey, accounting for more than half of the total solids present in the original whole milk and including whey proteins (20% of the total protein) and most of the lactose, minerals, and water-soluble vitamins (de Wit, Reference de Wit2001). In the past, sweet whey was traditionally disposed of by simply dumping into the ocean, which in turn created environmental pollution (Prazeres et al. Reference Prazeres, Carrvalho and Rivas2012). Nowadays, concentrated demineralised sweet whey is successfully utilised in a variety of food products such as beverage powders, nutrition bars, soups, bakery, confectionery coatings and ice cream and frozen dairy dessert products (Young et al. 1980). Furthermore, whey protein concentrates and isolates are also used as food ingredients in a number of different food products including hams, custards, confectionery, crab-sticks, cakes, infant formulae, sports drinks and formulated stock foods (Morr & Ha, Reference Morr and Ha1993). Companies typically produce 1 kg of creamy yogurt from 7–9 l of milk, therefore creating an average of 7 kg of acid whey that requires additional processing. Acid whey has been identified as a global problem in recent years; for example in USA, the Greek yogurt industry tripled in size over the last 5 years with companies producing a total of 800 million litres of acid whey in 2015. Chobani (USA), which is the largest Greek yoghurt company in the world, increased its revenue from zero to $1 billion in 5 years, a growth rate on par with Facebook and Google. The inclusion of acid whey into manure – for use as an organic fertiliser was trialled, supplementation of cattle feed was also tested, but caution was required due to energy overload and the suppression of metabolic activity of cows. Additionally, converting whey into biogas using anaerobic digesters is commonly practised by large dairy manufacturers, where they use the energy to maintain daily operations.

Future trends

A key trend in the global market for dairy products during recent years has been the widening range of products available to consumers. Cheaper production and storage technology combined with higher level of disposable income, has resulted in the development and distribution of premium products such as probiotic yoghurts and more exotic milk drinks. Public concern about food safety and quality has increased dramatically in recent years. The fluid milk and frozen dairy dessert processing industries have undergone tremendous improvements in technology in the last 25 years (Goff & Griffiths, Reference Goff and Griffiths2006). Emphasis has been on automation, increasing capacity, and improving hygiene while producing a wider range of safe, high-quality, convenient-to-use dairy products for consumers than has ever been seen before. The competition for market share of the consumer's food spending dollar is ever-increasing, which forces the dairy industry to be diligent in its search for new products or for new images for old products (as an example, better packaging for added convenience). Despite an increasing body of knowledge about foodborne pathogens and disease transmission, keeping dairy products safe for human consumption remains a challenge, due in part to the changing nature of pathogens and their control within today's production and processing environment.

Consumers have shifted away from full-fat milk towards low fat products. Consumers are now offered fat-free, 0·5% fat, 1% fat, and 2% fat products. Today, 35% of the fluid milk consumption is whole milk containing 3·25% milk fat, whereas 65% is consumed as reduced fat or non-fat milk as compared to 25 years ago when only 14% of total milk consumption was of low-fat products. This trend is expected to continue in the future, and dairy products with reduced fat and sugar contents are likely to receive greater attention. Such products may be developed by replacing some of the fats and sugars with other ingredients, or by using nano particles to prevent the body from digesting or absorbing these components from the food.

Nanotechnology is a powerful new technology for taking apart and reconstructing nature at the atomic and molecular level. It involves atomic level manipulation to transform and construct a wide range of new materials, devices, and technological systems. For example, milk bottles will have smart sensors or antimicrobial activators that can detect spoilage or harmful contaminants. Future products will enhance and adjust their colour, flavour, or nutrient content to accommodate each consumer's taste or health needs. Furthermore, smart packaging could release a dose of additional nutrients to those which it identifies as having special dietary needs (for instance, bioavailable calcium to people suffering from osteoporosis). Nano-sensors, embedded into dairy products as tiny chips that are invisible to the human eye, would also act as electronic barcodes, emitting a signal that would allow food, including fresh food, to be tracked from paddock to factory to supermarket and beyond. New advances in packaging will dramatically extend food shelf life. There will be invisible edible nano wrappers which will envelope foods, preventing gas and moisture exchange.

Food fortifications in the future will likely involve nano-encapsulated nutrients which will increase the mouthfeel. Food fortification will also be used to increase nutritional claims. For example, the inclusion of medically beneficial nano-capsules may soon enable yoghurt to be marketed as health promoting. Nanotechnology will also enable ‘junk’ foods like ice cream and chocolate to be modified to reduce the amount of fats and sugars that the body can absorb. Shanmugam & Ashokkumar (Reference Shanmugam and Ashokkumar2014) demonstrated the possibility of incorporating novel food oils into milk systems by ultrasonic emulsification. A 20 kHz ultrasound horn was used to emulsify flaxseed oil into skim milk. No addition of surfactants was required to stabilise the emulsion, as it was found that a small amount of partially denatured whey proteins surrounded the emulsified oil droplets, providing sufficient stability for a minimum of 9 d. The advantage of US is further highlighted since no stable emulsions could be produced when emulsification was performed with high energy mechanical mixing.

Food companies such as Mondelez and Nestlé are designing smart foods that will interact with consumers to personalise the food, changing colour, flavour or nutrients on demand. Kraft is developing a clear tasteless drink that contains hundreds of flavours in latent nano capsules. A domestic microwave could be used to trigger the release of the colour, flavour, concentration and texture of the individual's choice. Smart foods could also sense when an individual was allergic to a food's ingredients, and block the offending ingredient.

Functional foods are any foods or food ingredients that may provide a health benefit beyond the traditional nutrients it contains. The increased consumer demand for functional foods and nutrition is a key opportunity for the dairy sector. Dairy products are functional foods. They are one of the best sources of calcium, an essential nutrient which can prevent osteoporosis and possibly colon cancer. In addition to calcium, recent research has focused on probiotics. Probiotics are defined as ‘live microbial feed supplements which beneficially affect the host animal by improving its intestinal microbial balance’. Nutraceuticals inclusion to dairy products will aid health and medical benefits, including the prevention and treatment of disease.

There has traditionally been a very low level of R&D investment in the dairy sector. One of the major constraints on innovation in dairy is the operating costs. However, more resources should be invested in research and development where there is increasing demand for innovative and fresh products. The competitiveness challenge at the primary agriculture level combined with the necessary focus on environmental sustainability, means that a key strategy of the agri-food sector including the research community must be to focus on improving communication and adoption of best practise, new technology and research and advice. In this regard more energy saving and waste management opportunities will be demanded. However, the major concerns are the further automation and alienation of food production, serious new toxicity risks for humans and the environment, and the further loss of privacy as nano surveillance tracks in each step of the food chain. Concurrently, increasing allergic reactions with dairy related products also need attention.