Journal of Food Processing & Technology

In the present world, food scientists, manufacturers and consumers became more health conscious and demand for processed foods without loss of the nutritive value and organoleptic properties of foods along with improving the shelf life on inhibiting or killing microorganisms [1]. This demand paved way for non-thermal technologies without the use of preservatives or additives for processing food products maintaining color, flavor, texture, nutritive and functional qualities at acceptable levels that are minimally processed, easily handled, good quality and safe [2]. Emerging non thermal methods particularly used in milk processing are cold plasma technology [3-5], atmospheric pressure encapsulated Dielectric Barrier Discharge (DBD) plasma technology [6], ohmic heating [7-9], high pressure processing [10,11], pulsed electric field [12,13], supercritical fluid technology [14], ultra-violet irradiation [15,16], pulsed light treatment [17], microfiltration [18,19] and ultrasound treatment [20,21] resulted in efficient inactivation of pathogens and significant improvements in nutritional, sensory and quality attributes with prolonged shelf life compared to conventional methods. Among these technologies, SCFE has fruitful benefits like s eparation of desired or selective compounds with no traces of toxic solvent residues in the resulting food products, lesser chances for thermal degradation of the processed products, an appreciable level of nutrient retention and higher efficiency [22,23]. The selective separation of compounds through SCFE mainly depended upon two reasons i.e., supercritical fluids has higher diffusivity than gases and lower viscosity than liquid solvents. These factors resulted in a higher rate for mass transfer of solutes into the fluid [24] In this review, the various types of supercritical fluid techniques used in milk processing and dairy industry, how the treatment affected various physical, chemical, microbial and other related properties were discussed here.

The supercritical condition is the state at which component (substance) temperature and pressure are above its critical values where gas and liquid are indistinguishable from each other. Supercritical coordinates and supercritical region of Carbon Dioxide are illustrated in Figure 1.

Figure 1: Supercritical point of CO₂.

In case of a fluid, when it forced to a pressure and temperature higher than its critical limit, then it termed as Supercritical Fluid (SCF). Under these conditions, the SCF behaves like both gas and liquid i.e. properties of SCF lies between gases and liquids. SCF is extensively using in extraction processes due to their functional physiochemical properties such as density, diffusivity, dielectric constant, viscosity, solvating power, flow properties etc. SCF having some properties similar to gases and liquids hence, they are indiscernible. SCF is having a density close to that of liquids, viscosity near to that of gases and diffusivity is moderate between gases and liquids. They behave like compressible fluids (gases) and have similar density and solvating power of those liquids.

Usage of SCF in extraction processes is increasing tremendously day by day because of various physiochemical characteristics of SCF which helps in providing operational benefits over traditional extraction techniques. Since, conventional extraction methods are time-consuming, laborious, low selectivity and low extraction yields. Furthermore, traditional techniques use toxic solvents in excessive amounts. These drawbacks of traditional methods can be overcome by SCF extraction technology. SCF provides attractive features that deal with the limitations facing in conventional extraction methods. SCF are possessing significantly lower viscosity and relatively higher diffusivity which promotes the transport properties (flow properties) of fluid. In addition to higher diffusivity, SCF is also possessing the stupendous capacity to dissolve. Thus, enhanced diffusion and solubility take place in the solid matrix resulting in accelerated extraction rates with higher yields than those of traditional solvent extraction methods. The main aid of SCF is that its density since, solvent strength of SCF is a function of density which plays a vital role in controlling solvent power by altering temperature or pressure. Another unique characteristic of SCF is their selectivity. By tuning, temperature or pressure or both extraction selectivity of SCF towards target compound can be altered [25]. This allows SCF for extracting numerous ranges of target compounds [25,26] mentioned that extraction selectivity can be altered by adding co-solvent such as ethanol, methanol, hexane, acetone, chloroform, water etc. to promote or demote polarity depending on requirement.

Numerous fluids can be used as supercritical fluids listed in Table 1. Among the list of supercritical fluids, supercritical Carbon dioxide (SC-CO₂) is the mostly used fluid in SCF extraction process because of its low critical coordinates. The critical point of SC-CO₂ is 3°C of temperature and 7.38 MPa of pressure. Moreover, SC-CO₂ is having some specific advantages made it more suitable fluid in the extraction process are abundance, non-in flammability, cheap, its volatility, nonpolarity. Due to its volatile nature, target extract obtained is solvent free i.e. extract always in its pure form or without any SC-CO₂.

Table 1: Properties of supercritical fluids (da Silva et al. 2016) [27].

Steps involved in SCF extraction

The extraction process involves two steps namely, extraction and separation. In the first step, fluid is heated and pressurized above critical point depending on target compound and allow it to the extractor setup where extraction takes place. The second step is to separate the solvent and extracted material in the separator. SCF turns into gaseous form after depressurization. Here, separation is based on gravity. Subsequently, SCF can be recycled or released into the atmosphere.

Scientists and researchers have been acquainted with SCF from a long back but SCF solvents have been the focus on active research and development from past few decades. Even though the SCF extraction method has several advantages over conventional methods, high capital investment is required to set up the equipment. This is the main obstacle for industry scale commercialization of SCF technologies. However, SCF techniques reduce the processing steps resulting in a significant reduction in processing costs. Additionally, the supercritical extraction process is a green extraction technology.

Application of SCFE in the milk industry

Milk is the lactic secretion produced by the mammary gland of all female mammals which provides the nourishment to humans [27,28]. Milk is consumed by all ages of people, but it is highly perishable and being spoiled by microorganisms. So the safety, microbial quality and shelf life of milk are the most considerable factors during milk processing, preservation, storage and marketing [29]. During thermal processing of milk, changes occur in certain heat-labile nutrients, vitamins, minerals etc., which impairs the nutritive value of milk, precipitation of calcium phosphate, the occurrence of Maillard reactions and water-soluble vitamins are also mainly affected. Severe heating or Prolonged storage or severe heating causes whey protein denaturation which may affect the flavor of the milk [30]. But these effects can be controlled to appreciable levels with the application of SCFE. The milk fat, vitamins, and cholesterols extracted from milk using SCFE technique have resulted in inappropriate results.

SCFE in milk fat analysis

The food industry utilizes milk fat mostly in butter and anhydrous milk fat [31]. Generally, simple bench-top Supercritical Fluid Extraction (SFE) systems utilizing CO₂ proved to be effective in measuring total fat content in the food matrices such as dairy products, meats, and seeds [32].

Determination of fat content is very much essential in all milk processing industries. The organic solvents used in soxhlet apparatus causes the environmental release of hazardous materials. The SCFE with on-line piezoelectric detection used very small amounts of samples to analyze the fat content in milk products and significantly reduced the dangers with exposure to hazardous materials [33].

SCFE in milk fat vitamins and flavor determination

Turner et al. [34] used SFE followed by a saponification and HPLC for the fat-soluble vitamins A and E determination in milk powder with SFE conditions: 37 MPa, 80°C, 15 min static and 15 min dynamic extraction, 1.0 ml/min, collection into 16 ml of ethanol-diisopropyl ether (1:1) at 58°C. The milk samples were wetted with 2.0 ml of methanol or ethanol and a reference sample with no modifier added was extracted with CO₂ containing 5% of methanol. The SFE with good recoveries above 99% for vitamin A and 96% for vitamin E were occurring in milk powder [34].

Haan et al. [35] extracted flavor components from milk fat using supercritical fluid extraction. Since carbon dioxide has a critical temperature (31.1°C), nontoxic, non-flammable, inexpensive and has a high purity and these properties made to choose CO₂ as an ideal solvent for extracting flavor components from the milk fat which also helped in retaining its natural value. The process conditions for flavor extraction depended upon the carbon dioxide density, temperature, solvent-tofeed ratio on the extracted content, the concentration factor and the fraction of flavor components extracted from the milk fat. Mainly, two steps were used to analyze the flavor components in the feed, i.e., the extract and the raffinate. Firstly, the flavor components were separated from the triglycerides with HPLC followed by capillary gas chromatography for the flavor fraction analysis. Here the extraction has two stages. In the very first stage, the flavor components from the triglycerides were separated and the second stage was used to obtain a maximum concentration factor. The decrease in CO₂ density increased the concentration factor in the first extraction stage and decrease in the extraction temperature resulted in higher concentration factors in the second stage. The fraction of flavor components was increased as much as possible particularly at low extraction temperature, increasing the number of equilibrium stages in the column and a small solvent-tofeed ratio [35]. Low-fat cheese products were developed in the dairy industry which utilizes SFE technology and retains flavor compounds [36].

Milk fat is composed primarily of triglycerides (or triacylglycerides) is 98% of the total milk fat (by weight). Other milk lipids (as a percentage of the total milk fat) include: diacylglycerides (0.25- 0.48%); monoacylglycerides (0.02-0.04%); phospholipids (0.6-1.0%); cholesterol (0.2-0.4%); glycolipids (0.006%); and free fatty acids in milk (0.1-0.4%). Crystallization at a different temperature is a common method for milk fat fractionation with or without the use of solvents [37,38]. Though solvents or surfactants are able to separate triglyceride effectively, solvent removal creates a problem. Chemical methods such as inter-esterification [39] and hydrogenation [40] for milk fat modification results in loss of many desired characteristics and extinguish natural flavor. Such restrictions are not found with SCFE.

Shishikura et al. [41] conducted an experiment using a single pass unit at a pressure of 150 Kg/cm² and temperature of 40ºC. They reported that triglyceride with less than 36 carbons i.e. short chain triglyceride was especially concentrated in the initial two fractions, while those with 38 and 40 carbon units were concentrated constantly all through the extraction process. Triglycerides with more than 40 carbon units show an increment through the process of extraction and especially, glycerides with more than 46 carbons were concentrated in final extraction and in residual oils. Arul et al., [42] fractionated milk fat into 8 fractions using supercritical carbon dioxide for the temperature of 50ºC and 70ºC and, for a pressure range of 10-35 MPa. The 8 fractions are shown in Table 2.

Table 2: Yield of different milk fat fractions by supercritical carbon dioxide (Arul et al. [42]).

The authors found that the fraction of the short chain triglycerides (C24-C34) gradually decreased from Ll to S3, while L2 and I-3 were rich in medium chain triglycerides. On the other hand, compared to milk fat, fractions S2 and S3, were low in both short and medium chain triglycerides. There is an increase in long-chain triglyceride from L1 to S3 and are mostly concentrated in fractions S2 and S3. Another study was conducted by Bhaskar et al., [37], they divided the milk fat into 5 fractions namely S1, S2, S3, S4, and S5 respectively in the pressure range of 241-34 bar and temperatures range of 40ºC-75ºC. They observed that the fractions S1 to S5 show a gradual increase in short-chain (C4-C8) and medium-chain (ClO-C12) fatty acids, while a reverse case was observed with the long-chain and unsaturated fatty acids (C14-C18).

Milk fat solubility in supercritical carbon dioxide

The solubility of a substance tends to increase with the increase in pressure with temperature being constant, because solubility of any substance is mostly dependent on the van der walls, but influence of temperature on solution equilibrium is slightly complicated, beyond a certain pressure (which is specific for solvent-solute) the solubility increases with increasing temperature [43]. Arul et al. [43] found that smaller triglycerides exhibited a greater supercritical carbon dioxide solvent capacity than the larger ones. This is because with the increase of pressure at constant temperature volatility or fugacity of the triglyceride phase increases. The vapor pressure of triglycerides decreases with their carbon number [44] and therefore, the short-chain triglycerides are, in general, more volatile than long chain molecules.

Short chain triglycerides, on the other hand, have higher absolute vapor pressure due to their higher volatility, up to a certain pressure than that of long-chain molecules. Taking into consideration all these factors short chain triglycerides versus have more solubility than the long chain molecules at low pressure. However, long-chain triglyceride has higher solubility at higher pressure short chain triglyceride.

Arul et al. [43] found that solubility of milk fat triglyceride was higher for 50ºC than for 80ºC at a constant pressure (say 200 bar) because at that pressure the heat of mixing is exothermic, and hence, solutesolvent attractive orientations are favored at a lower temperature. But, that was not the case always, because above a pressure of 250 bar, the heat of mixing becomes endothermic and solubility of triglyceride increases with increasing temperature [45].

Extraction of milk fat globule membrane (MFGM) phospholipids and proteins using a combination of techniques

Costa et al., [46] developed a dairy ingredient with the use of whey buttermilk as raw material and produced dairy powder enriched in Milk Fat Globule Membrane (MFGM) phospholipids and proteins using the combination of ultrafiltration/diafiltration membrane filtration and supercritical fluid extraction technologies. Concentrate of whey buttermilk was spray-dried and subjected to three extraction cycles using Supercritical Fluid Extraction (SFE) with conditions: 1.5 kg of CO₂ at a flow rate of 0.02 kg/min, extraction pressure of 35 MPa at a temperature of 50⁰C, after the membrane filtration. Phospholipids enriched dairy powders have found their application in food formulations like ice cream, powdered soup mix or bakery delicacies [46].

The Milk Fat Globule Membrane (MFGM) contains phospholipids in buttermilk. Polar MFGM lipids and proteins can be concentrated from buttermilk using microfiltration and supercritical fluid extraction technologies and were used at various levels in different dairy components [47]. Also, microfiltration, when coupled with supercritical fluid extraction, can be incorporated as nutritional valued lipids into a novel ingredient. SFE provide phospholipid enrichment and used for optimizing the lipid removal effectiveness. With the addition of physical aids like removable Teflon beads, fluidized bed mixing systems makes adequate removal of nonpolar lipids for enriching polar lipid [48].

Phospholipids (PLs) were associated with health and nutritional benefits related to cognitive development and repair. A Pilot scale production of a dairy ingredient enriched in phospholipids (PLs) was generated from a Buttermilk Powder (BMP) substrate. In this respect, a combined process of targeted enzymatic hydrolysis of the innate milk proteins and ultrafiltration of the smaller molecular weight peptide material was followed. Purified PLs fraction was achieved through SFE. From an industrial point, the dairy products which were enriched in PLs fortified nutrition in infants and aged people in promoting brain health [49].

Microbial inactivation in milk using SCFE

Milk is a low acid food, thus allowing development of a range of microorganism, including important pathogens, such as Salmonella, Escherichia coli, Listeria monocytogenes, and Coxiella burnetti, among others [50]. The microbial reduction of aerobic mesophilic bacteria and Escherichia coli in human milk depends upon temperature, pressure and exposure time of SCFE processing using carbon dioxide as a solvent. This reduction will be greater at a higher pressure of CO₂. The exposure time for 120 min and the pressure of 20 MPa had a positive effect on microbial inactivation [51].

The use of supercritical CO₂ technology to inactivate microorganisms is a novel method and well reported in the literature and the factors involved were shown in Table 3. Many terms have been used by several authors, such as Dense Phase Carbon Dioxide (DPCD), Liquid Carbon Dioxide (LCD) and High-Pressure Carbon Dioxide (HPCD). The use of supercritical carbon dioxide technology has also aroused interest in the dairy area. In 1987, in Germany, a patent (DE3734025 A1) proposed the use of supercritical CO₂ to increase the shelf life of dairy products. Table 3 explains the different reasons behind microbial inactivation using supercritical carbon dioxide.

Table 3: Microbial inactivation mechanism of supercritical CO₂ (SOURCE: Adapted from Perrut [52]).

Milk pasteurization using SCFE

Supercritical fluid extraction using CO₂ as the solvent can be considered as a mild effective pasteurization process which increases the shelf life for skim milk. This process when used with HTST or ultra-pasteurization (cream separated by centrifugation) to produce mild pasteurized whole milk. This whole process is similar to Bactocatch process with 3 sections: homogenization section and thermal pasteurization section (for centrifugal separation of cream) and the SCFE pasteurization treatment section. The optimal operating parameters were the pressure of 15 MPa, temperature 35-38°C, CO₂/skim milk-feed ratio of 0.30:0.33 and residence time of 0.25 hr with a shelf life of 35 days. Also, SCFE pasteurized skim milk has significantly better taste than thermally pasteurized milk. Continuous SFE process with CO₂ solvent was developed for industrial production of non-thermal pasteurization of whole liquid milk [52,53]. Table 4 summarizes the studies on the use of microbial inactivation in milk using supercritical carbon dioxide.

Table 4: Main current studies on the use of supercritical CO₂ for microbial inactivation in milk and dairy products (Source: Amaral et al. [50]).

Enzyme activity in milk involved with SCFE

Alkaline phosphatase was an endogenous enzyme present in milk and slightly more resistant to heat compared to the most pathogenic bacteria. It was used as good indicators for the effectiveness of pasteurization of milk and milk products [54]. In recent times, SCFE processing using CO₂ can inactivate enzymes and pathogenic microbes and also the degradation of thermo-labile nutrients of foods at a minimal level, thereby extending their shelf lives and preserves sensory and nutritive values. This process using CO₂ is cost effective and also improves mass transfer due to higher CO₂ diffusivity. The enzymes increase their activities with SCFE treatment using CO₂ [55].

The effects of temperature, pressure, CO₂ to milk mass ratio, alkaline phosphatase activity and reduction of activity after the continuous treatment with SCFE with CO₂ and with initial alkaline phosphatase activity: 5.5 U/mL were 30°C, 80 bar, 0.45 (wt.%), 4.2 U/mL and -23.6% with apparent residence time of 30 min. Initial alkaline phosphatase activity was 5.5 U/mL. The optimized condition for continuous inactivation of alkaline phosphatase activity in milk was CO₂ to milk mass ratio of 0.05 wt%, 70°C, 80 bar and residence time of 30 min was 1.1 U/mL with initial alkaline phosphatase activity of 6.4 U/mL [56].

SCFE for cholesterol determination and removal

Cholesterol, a sterol is an essential structural component of the animal cell. 100-gram milk contains 13.6 mg of cholesterol. Saturated fat and cholesterol are the primary risk factors for coronary heart diseases if taken above a certain limit. So, there is a growing concern among the consumers about low cholesterol foods. It is being recommended by the American Heart Association to reduce the daily intake of cholesterol and saturated fat [57]. An average American consumes about 500 to 600 mg/d cholesterol. A maximum intake limit of 300 mg/d is being suggested by many national institutes of health [58,59]. For all of these reasons, there is a serious concern among the consumers for low cholesterol food, which has forced the researches or scientist to focus on producing low cholesterol products. Steam distillation, crystallization, supercritical fluid extraction, complexing cholesterol with β-cyclodextrin are some methods to remove cholesterol from food.

Cholesterol extraction from anhydrous milk (AMF) fat using SCCO₂ has been widely studied. Anhydrous milk fat is a complicated mixture of triglycerides, which contain several fatty acids of varying degree of saturation and carbon chain length [60]. The solubility of cholesterol and AMF in SC-CO₂ has also been intensively studied [61-63]. Bradly [64], used digitonin procedure for β-3-OH sterols for cholesterol assay. He designed a two-stage processing equipment, the first stage, which operate at low pressure is used for the separation of flavors and triglyceride with low melting points and remainder with cholesterol is conveyed to high-pressure unit i.e., a second stage where cholesterol and high melting triglyceride gets separated. They reported an extraction efficiency of 90% of cholesterol from milk.

Huber et al. [60] also studied the selective separation of cholesterol from anhydrous milk (AMF). They prepared a mixture of AMF and supercritical fluid (SCF) by dissolving AMF in SCF and passed this mixture through a suitable adsorbent [65]. The extraction process parameters were varied from 80-400 bar and temperature from 40 ºC-70ºC for determination of solubility of AMF in SCF and selectivity of cholesterol over AMF. It was found that alone SCF is not feasible for extraction of cholesterol from AMF because the solubility of AMF was poor even at best selectivity conditions (80-100 bar and 40ºC). Whereas with the incorporation of a silica gel based absorbent of particle size 5-40 μm a selective removal of 97% of the cholesterol from the extracted AMF is possible.

Huber et al. [66] determined the cholesterol in milk fat using supercritical fluid chromatography (SFC). They found that although SCF was more accurate, it was costlier and more time consuming than Gas Chromatography (GS) for analysis of cholesterol in milk fat. Kankare and Alkio [67] found that 99% of milk cholesterol can be removed by an SCF extraction system equipped with a silica gel column. Their studies also indicated that the fractionation of fat on the basis of the different fatty acid composition can be achieved simultaneously by performing the milk fat extraction using supercritical carbon dioxide and connecting an adsorbent column filled with silica gel to the extraction system.

Isolation of polychlorinated biphenyls from milk using SCFE

Polychlorinated Biphenyls (PCB) which is widely spread in the environment can make a very highly toxic pollutant and some adverse biological effects have been reported in the literature. The primary causes of PCB are many human activities. PCBs may cause cancer development, immune deficiency disorders and reproductive, nervous or other biological systems malfunction [68]. Cow milk has been reported to contain PCB residues which sometimes exceeds the level 0.2 mg/l (equivalent to 5.0 mg/l in milk fat), which is being set by the FDA. Therefore, it is required to remove the excess amount of PCB from milk. A typical process using Soxhlet extraction with an organic solvent would require 3-8 hours or overnight after that is requires evaporation to a low volume and then to isolate PCB from fat a chromatographic clean-up is needed. A study by Mills and Jefferies [69] shows SFE and SFC as a replacement to the traditional practices which reduces cost and time of operation. They used a mixture of freeze-dried skim milk, equivalent to 10 ml of original milk (fat content 0.1% w/w). They found that the extraction conditions were the same for extraction of both fat and PCB from milk. The extraction condition which they used was: 50ºC and 160 bar, 45º C and 160 bar and 50ºC and 200 bar.

PCB distribution in milk fat globule for four different lipidic fractions (short-chain triglyceride, medium-chain triglyceride, long chain triglyceride, and cholesterol) by SC-CO₂ was studied by Ramos et al. [70]. The experiment was conducted with 6 g powdered full-fat milk with a moisture content of 4.2% and at a constant temperature of 50ºC for varying pressures of 133, 167, 200, 233 bar respectively. Levels of PCB congeners extracted by SFE increased with the pressure from 133 to 167 bar and then decreased for subsequent SFEs. Short-chain and medium-chain triglyceride constituted 59% and 81% respectively and long-chain triglyceride and cholesterol constituted in the range of 11- 19% and 0.0-30% respectively of the total endogenous congener levels extracted by the four consecutive SFEs. Figure 2 below summarises the percentage of PCBs detected in four consecutive lipid fractions extracted by SFE.

Figure 2: Percentage of PCB found in the four consecutive lipidic fractions extracted by SFE (Source: Ramos et al. [70]).

SCFE has grabbed considerable attention in a variety of industries due to its higher solubility, improved mass transfer rates and better optimization of temperature and pressure in the process gave good results. SCFE with CO₂ as solvent proved as the green alternative technology providing safety to health and environment in the commercial applications of the dairy industry in various functional dairy food products having good economic and nutrient value. The cholesterol extracted from cream powder using SCFE provided functional foods which satisfied the health-conscious consumers. When buttermilk is subjected to SCFE, concentrates of polar lipids like phospholipids from Milk Fat Globule Membrane (MFGM) were obtained. SCFE can be used as a replacement technology for milk fat analysis instead of traditional methods which requires intensive time and labor and also huge amounts of hazardous organic solvents were used. This process controlled the microbial inactivation at appreciable levels compared to cold pasteurization with minimal loss of vitamins, flavors, nutrients, etc. This extraction process is environmentally friendly since it reduces the amount of the number of solvents, efficient utilization of industrial by-products and also filters the toxic compounds which possess hazards to the environment.