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Principles and Application of High Pressure–Based Technologies in the Food Industry V.M. (Bala) Balasubramaniam,1,2,∗ Sergio I. Mart´ınez-Monteagudo,1 and Rockendra Gupta1 1 Department of Food Science and Technology, 2 Department of Food Agricultural and Biological Engineering, The Ohio State University, Columbus Ohio 43210; email: [email protected]

Annu. Rev. Food Sci. Technol. 2015. 6:19.1–19.28

Keywords

The Annual Review of Food Science and Technology is online at food.annualreviews.org

high pressure, pasteurization, sterilization, homogenization, microbial safety, quality, process design

This article’s doi: 10.1146/annurev-food-022814-015539 c 2015 by Annual Reviews. Copyright  All rights reserved

Abstract High pressure processing (HPP) has emerged as a commercially viable food manufacturing tool that satisfies consumers’ demand for mildly processed, convenient, fresh-tasting foods with minimal to no preservatives. Pressure treatment, with or without heat, inactivates pathogenic and spoilage bacteria, yeast, mold, viruses, and also spores and extends shelf life. Pressure treatment at ambient or chilled temperatures has minimal impact on product chemistry. The product quality and shelf life are often influenced more by storage conditions and packaging material barrier properties than the treatment itself. Application of pressure reduces the thermal exposure of the food during processing, thereby protecting a variety of bioactive compounds. This review discusses recent scientific advances of high pressure technology for food processing and preservation applications such as pasteurization, sterilization, blanching, freezing, and thawing. We highlight the importance of in situ engineering and thermodynamic properties of food and packaging materials in process design. Current and potential future promising applications of pressure technology are summarized.

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INTRODUCTION

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The food and beverage manufacturing industry represents one of the largest manufacturing sectors in the United States and generated an economic output of more than $1.08 trillion in 2010 (Myles 2013). In 2011, food processing plants accounted for 14.7% of the value of shipments from all US manufacturing plants (http://www.ers.usda.gov). In a consumer-driven market, food processors are constantly challenged to develop food products with consumer-desired characteristics at affordable costs without compromising food safety. Consumers’ health and wellness-oriented lifestyles lead to a preference for mildly processed, fresher-tasting foods with minimal or no preservatives. In a way, health-conscious consumers are increasingly considering foods as delivery systems for nutrients, embracing a long-known connection between healthy eating and human well-being. Interestingly, developments in mass communications and social media have significantly increased the consumer awareness of microbiological outbreaks in processed foods as well as many unintended adverse effects of conventional food preservation methods such as nutrient destruction, preservative use, and formation of toxins and off-flavor compounds. However, the beneficial effects of industrial food manufacturing are often underappreciated (van Boekel et al. 2010). Researchers from the food industry and academia are responding to the consumer’s desire for microbiologically safe, yet minimally processed, foods by developing different advanced thermalbased technologies, such as aseptic processing and ohmic, microwave, and radio-frequency heating, and nonthermal processing methods, such as irradiation and high pressure, pulsed electric field, and UV processing. Although advanced thermal processing technologies employ rapid volumetric heat to overcome limitations of the conventional conduction and convection heat transfer to make food safe, nonthermal technologies employ lethal agents (such as pressure, electric field, and irradiation doses) with or without the combination of heat to kill pathogens and spoilage organisms (Floros et al. 2010, Zhang et al. 2011). This review summarizes various basic principles and the current and future promising applications of high pressure processing (HPP) in the food industry. Topics discussed include typical processing steps, key pressure equipment components, packaging material selection, and the role of engineering and thermodynamic properties in process uniformity and design. Attention must be paid to understanding the synergistic, additive, or antagonistic effects of combining pressure and heat treatment on product microbiological safety, instrumental quality, and nutritional attributes. Table 1 summarizes some key advantages and limitations of high pressure applications in the food industry. Due to space constraints, the works cited should be considered as representative and not comprehensive.

HISTORICAL DEVELOPMENT The history of high pressure food processing applications dates back to the late nineteenth century (1880–1899) (Table 2). In 1882, the conversion of starch into glucose was studied under pressure (Soxhlet 1881). Years later, the acid inversion of sucrose was inhibited by the application of 50 MPa, 500 times the atmospheric pressure (Stern 1897). The ability to operate vessels at pressure levels of more than 100 MPa opened a window to an unexplored region. The effectiveness of pressure in inactivating spoilage bacteria was demonstrated by pressure treating milk at 680 MPa (Hite 1899). In the early twentieth century (1909–1959), Percy W. Bridgman investigated extensively the engineering aspects of HPP. Specifically, he investigated compressibility (Bridgman 1909), thermal conductivity (Bridgman 1923), phase change (Bridgman 1914a), and polymorphic transformations (Bridgman 1912). The water phase diagram is one of the most significant impacts of Bridgman’s research (Figure 1). This is particularly important in the field of food science, given water is a major component of most food systems. An investigation on the coagulation of albumen showed that 19.2

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Table 1 Unique advantages and limitations of high pressure food processing

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Description

Advantage

Limitations

Hydrostatic pressure

Rapid, quasi-instantaneous uniform distribution throughout the sample

Batch or semicontinuous operation

Thermal distribution

Minimal or reduced thermal exposure Instant temperature increase and subsequent cooling upon depressurization

Preheating step for pressure-assisted thermal processing (PATP) Thermal nonuniformity during PATP

Physical compression

Suitable for high moisture–content foods

Not suitable for products containing dissimilar compressibility materials such as marshmallows

Product handling

Suitable for both liquid and pumpable foods

Throughput limited due to batch operation

Process time

Independent of product shape and size

Functionality

Opportunity for novel product formulation Distinct products through pressure effects such as protein denaturation, carbohydrate gelatinization, and fat crystallization

Reaction rate

Within some pressure-thermal boundary conditions, pressure accelerates microbial inactivation

Variable efficacy in enzyme inactivation; pressure alone cannot inactivate bacterial spores

Consumer acceptance

Consumer acceptance as a physical process

Higher processing costs and batch operations are barriers for commodity product processing

the appearance of a pressure-treated egg resembles that of a hard-boiled egg (Bridgman 1914b). Another important discovery was by Larson et al. (1918), who showed that bacterial spores (Bacillus subtilis used in this experiment) could survive applications of 1,200-MPa treatments at ambient temperatures. The effects of pressure on chemical reactions were acknowledged in the 1920s with a remarkable example reported by Brown (1920), who manufactured phenol from chlorobenzene under pressure using a continuous reactor. Throughout the twentieth century, there were successful developments in areas such as chemical, material, and process engineering. Sheet metal forming, polymerization of ethylene, synthesis of diamonds, and isostatic pressing of advanced materials are notable examples of industrial applications of high pressure. In the 1970s, the food industry investigated use of supercritical pressures (30–50 MPa) for decaffeinating coffee (King 2014). In contrast, elevated high pressure (400–900 MPa) applications in the food industry were explored as recently as 1990 in Japan with the commercial introduction of pressure-treated jams and jellies. By 1997, pressure-treated guacamole was commercialized in the United States. Spanish sliced cooked ham was among the first of pressure-treated products introduced in Europe (Tonello 2011). To date, HPP represents a $2.5 billion market with various products. HPP has been considered as one the most important innovations in food processing during the past 50 years (Dunne 2005). In 2009, the Food and Drug Administration (FDA) approved the application of high pressure to a preheated sample for commercial sterilization of low-acid foods ( Juliano et al. 2012, Stewart et al. 2015). This is a significant milestone for the commercialization of sterile foods preserved by pressure-assisted thermal processing (PATP). More recently, Park et al. (2013b, 2014) developed a new method that utilizes simultaneous application of pressure, heat, and electric field for inactivation of bacterial spores. More importantly, they found that the application of electric field under pressure yielded enhanced volumetric heating, which significantly reduced the thermal load. In short, high-quality, shelf-stable products can be obtained by the simultaneous application of pressure, heat, and electric field, also known as pressure-ohmic-thermal sterilization (POTS) (V.M. Balasubramaniam, S. Park, S.K. Sastry, patent application No. 61/733,608 submitted). www.annualreviews.org • Principles and Application of High Pressure

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Selected scientific and commercial milestones in high pressure food processing applications

Year

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Application

Reference (or URL)

1881

Conversion of starch into sugar

Soxhlet 1881

1897

Inversion of cane sugar

Stern 1897

1897

Starch saccharification

Lippmann 1897

1899

Experiments on milk preservation

Hite 1899

1900

Homogenization for stabilization of dairy emulsion

Dons`ı et al. 2009

1909

Experiments on compressibility

Bridgman 1909

1912

Water phase diagram

Bridgman 1912

1914

Coagulation of albumen

Bridgman 1914b

1918

Pressure effects on bacteria

Larson et al. 1918

1920

Continuous manufacture of phenols

Brown 1920

1923

Experiments on thermal conductivity

Bridgman 1923

1943

Mutarotation of glucose

Sander 1943

1969

Biological reaction rates

Yayanos 1969

1970

Decaffeinating coffee via supercritical fluid extraction

King 2014

1980

Beef protein quality

Elgasim et al. 1980

1990

Meidi-Ya Food Co. launched high pressure–treated products in Japan

Mozhaev et al. 1994

1995

Thermodynamic properties of water under pressure, NIST/ASME Steam database

http://www.nist.gov/srd/upload/STEAM302.pdf

1997

Pressure-treated Gucagmole by Avomex (now Fresherized Foods)

Sizer et al. 2002

1998

Spain introduced pressure-treated sliced cooked ham

Tonello 2011

2002

Hormel introduced pressure-treated deli meat products in the US market

Sizer et al. 2002

2005

Cited as one of the best innovations in food processing

Dunne 2005

2009

FDA issued no objection to an industry petition for PATP (research sponsored by Army-Industry consortium)

Juliano et al. 2012

2012

Introduction of pressure-treated juices

http://www.starbucks.com/promo/evolutionfresh-juice

2013

Development of pressure-ohmic-thermal sterilization

http://tco.osu.edu/technologies/#/tech/959

Abbreviations: ASME, American Society of Mechanical Engineers; FDA, US Food and Drug Administration; NIST, National Institute of Standards and Technology; PATP, pressure-assisted thermal processing.

HIGH PRESSURE PROCESSING: BASIC PRINCIPLES, PROCESS DESCRIPTION, AND CHARACTERIZATION HPP (also known as high hydrostatic pressure processing and ultrahigh pressure processing) involves the use of pressures in the range of 100–800 MPa, with or without the application of heat, for inactivating a variety of pathogenic and spoilage vegetative bacteria, yeasts, molds, viruses, and spores to ensure microbiologically safe foods. In practical food processing applications, the combined intensity of both thermal and pressure effects can cause various physical, chemical, or biological changes in foods.

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VII VI

1,000 V

II

Liquid

100

Solid

Melting

1

Freezing

ati

riz

0.1

on

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Tc = 370°C Pc = 22 MPa

po Va

Pressure (MPa)

10

Critical point

n tio sa en nd Co

I

Gas 0.01

Triple point 0.001

0.0001

P = 6 × 10–4 MPa T = 0.01°C –75

Sublimation 25

125

225

325

Temperature (°C) Figure 1 Water phase diagram as influenced by pressure-thermal effects. I–VIII represent different types of ice. Abbreviations: P, pressure; Pc , critical pressure; T, temperature; Tc , critical temperature. Source: Phase Equilibria Diagrams database, version 3.4.0 (http://www.nist.gov/srd/nist31.cfm).

Basic Governing Principles As with heat, pressure is a basic thermodynamic variable. Strictly speaking, during HPP the effects of temperature cannot be separated from the effects of pressure. This is because for every temperature there is a corresponding pressure. Thermal effects during pressure treatment can cause volume and energy changes. However, pressure primarily affects the volume of the product being processed. The combined net effect during HPP may be synergistic, antagonistic, or additive (Gupta et al. 2011). Mathematically, the impact of pressure ( p) and temperature (T ) can be quantitatively related using Gibbs’s definition of free energy G: G ≡ H − TS,

(1)

where H and S are the enthalpy and entropy, respectively. Further, H ≡ U + pV ,

(2)

where U = internal energy and V = volume. It can be deduced from Equations 1 and 2 that d (G) = V d p − Sd T .

(3)

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Therefore, reactions such as phase transitions or molecular reorientation depend on both temperature and pressure and cannot be treated separately. The following are some basic governing principles behind HPP.

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Isostatic principle. The first consideration involving the application of high pressure is the isostatic principle, which presumes that the uniform application of pressure acts equally in all directions. A true hydrostatic condition should be independent of time and space. It can be established when a fluid is used to transmit the pressure throughout the food. In high pressure applications, the pressure and more importantly, its effects, are instantaneously and homogeneously distributed within the food item, regardless of food geometry and size. This unique characteristic has enabled the development of processes that have been successfully commercialized. This principle helps explain why nonporous foods with high-moisture content are not damaged macroscopically by pressure treatment. Because air and water differ in compressibility under pressure, the structure and shape of the foods containing air pockets (as in the case of marshmallows) may be altered upon pressure treatment, unless the food is perfectly elastic and consists of closed-cell foam from which air cannot escape (Balasubramaniam et al. 2008). Le Chatelier’s principle. This principle addresses changes to equilibrium as a result of pressure application. It states that any phenomenon (phase transition, change in molecular configuration, chemical reaction) accompanied by a decrease in volume is enhanced by pressure. If pressure (extensive variable) changes, the equilibrium shifts in a direction that tends to reduce the change in the corresponding intensive variable (volume). Thus, pressure shifts the system to that of the lowest volume. Principle of microscopic ordering. At constant temperature, an increase in pressure increases the degree of ordering of molecules of a given substance. Therefore, pressure and temperature exert antagonistic forces on molecular structure and chemical reactions (Balny & Masson 1993). Arrhenius relationship. As with thermal processing, various reaction rates during HPP are also influenced by thermal effects during pressure treatment. The net pressure-thermal effects can be synergistic, additive, or antagonistic. Hydrostatic pressure alters interatomic distances, affecting those interactions for which bonding energy depends on distance (Mart´ınez-Monteagudo et al. 2012). For instance, the force of electrostatic interactions is inversely proportional to the distance between charged particles, whereby the application of pressure will affect its bonding strength. Hydrogen bonding and van der Waals forces are also distance-dependent and therefore are greatly affected by pressure. However, covalent bonds are unlikely to be affected by pressure because their bonding distance can be only minimally compressed further. Indeed, studies have revealed that covalent bonds that constitute the primary structure of proteins are unaffected by pressure (up to 1,500 MPa) (Mozhaev et al. 1994). The ability of hydrostatic pressure to keep covalent bonds unaffected has been the central hypothesis for the preservation of biological activity of functional compounds, such as ascorbic acid (Oley et al. 2006), folates (Butz et al. 2004), antioxidants (Matser et al. 2004), anthocyanins (Verbeyst et al. 2010), lycopene (Gupta et al. 2010), and conjugated linoleic acid (Mart´ınez-Monteagudo & Saldana ˜ 2014). The consequences of altering the interatomic distance by means of high pressure can be viewed as (a) changes in physical properties, such as melting point, solubility, density, and viscosity; (b) effects on equilibrium processes, such as dissociation of weak acids, acid-based equilibria, and ionization; and (c) effects on rates of processes, such as delaying or accelerating the rate at which a particular reaction occurs. In pressure-treated products, some intrinsic quality attributes are the 19.6

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result of the above phenomena or their combinations. For instance, inactivation of microorganisms is influenced by a combination of changes in physical properties of membrane lipids, changes in the chemical equilibrium that modify the internal pH, and changes in the rate of specific physiological functions that cause irreversible or lethal damage to bacteria cells (Molina-Guitierrez et al. 2002). Table 3 provides examples on how food quality attributes are dictated by the ways pressure affects

Table 3 Examples of how pressure on physical properties, equilibrium, and rate processes influence food quality attributes Pressure treatment effect

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Quality attribute

Physical process

Equilibrium process

Rate process

Volatile formation: aroma development in onions (Butz et al. 1994)

Disruption of cell integrity that brought phenols and enzymes together

Changes in the pH value that affect enzyme activity

Decrease in the concentration of dipropyl disulfide

Color and appearance: color degradation in broccoli juice (Van Loey et al. 1998)

Cell damage that releases different enzymes

Shifts the equilibrium that triggers different mechanisms

Decrease in the reaction rate

Changes in brightness in cow milk (Needs et al. 2000)

Disruption of casein micelles Changes in the solubility of ions and minerals

Shifts the equilibrium between colloidal and soluble calcium Shifts the pH value

Upon pressure release, some changes are partially reversible

Nutritional properties: stability of thiamin and riboflavin in pork and model systems (Butz et al. 2007)

Physically bound vitamins were released by pressure

Shifts the pH value that could trigger different mechanisms

Pressure increases the rate constant without affecting the activation energy

Ascorbic acid degradation (Oley et al. 2006)

Solubility of small compounds creates a protective effect

Degradation mechanisms of ascorbic acid are highly dependent on its pH

Temperature and pressure act synergistically on the degradation reaction rate

Functionality: milk fat crystallization in emulsion (Buchheim & Abou El-Nour 1992)

Changes in the melting curve measured by DSC Changes in the solubility that yields different solid fat content

Shifts the equilibrium between attractive and repulsive forces

Pressure affects fat crystallization

Starch dispersion (Buckow et al. 2007)

Changes in viscosity of starch suspension Changes in rheological properties

Shifts the chemical balance

Retrogradation rate is affected by pressure

Microorganism inactivation: pressure-induced inactivation in microorganisms (Smelt 1998)

Cell damage: membrane lipids change physical state

Intracellular pH shifts Changes in equilibrium in solute transport

Decreased DNA synthesis

Spore inactivation: bacterial spore inactivation (Black et al. 2007)

Cortex degradation and release of dipicolinic acid

Shift the osmotic equilibrium Shifts the equilibrium of electrolytes

Inactivation rate is affected by pressure

Sensory and texture: cheddar cheese treated by HPP to improve visual appearance (Serrano et al. 2004)

Surface adhesion and flexibility of protein Disruption of the paracasein network

Shift the equilibrium between solutes and water

Accelerates hydrolysis of amino acids

Abbreviations: DSC, differential scanning calorimetry; HPP, high pressure processing.

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the physical, equilibrium, and rate processes. A rate process is considered when pressure increases or decreases the concentration of a particular compound.

High Pressure Processing Equipment HPP primarily employs batch equipment, although semicontinuous equipment is also available. The equipment is typically made up of high strength steel alloys with high fracture toughness and corrosion resistance. The following are typical components of batch high pressure equipment (Ting 2011):

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1. 2. 3. 4. 5. 6.

pressure vessel (thick-wall cylinder), two end closures to cover the cylindrical pressure vessel, yoke (structure for restraining end closures while under pressure), high pressure pump and intensifier for generating target pressures, process control and instrumentation, and a handling system for loading and removing the product.

HPP could be performed as a batch process (both solid and liquid foods) or be developed into a semi-continuous process for pumpable foods. A batch process is analogous in operation to thermal processing in a retort system. The food product is typically vacuum packaged and placed inside a sample loading basket, which is then loaded into the pressure vessel containing pressure-transmitting fluid. The pressure vessel and its content are closed with the end closures. A yoke structure slides across the closed vessel to restrain top and bottom closures under pressure. The desired process pressure is achieved through compression of pressure-transmitting fluid using the combined action of a pump and intensifier. During HPP, the product is held for the desired time at the target pressure, the vessel is depressurized at the end of the treatment time, and the product is unloaded. Product holding times of less than 10 min may be required to develop a commercially viable process. In a modified high pressure treatment method (also known as pulsed high pressure processing), the product is subjected to compression-decompression cycles of fixed pressure holding time. Microbial efficacy of the process has been reported to be more effective than an equivalent single pulse of equal time. However, rapid compression and decompression increases the number of cycles on the vessel and subjects the vessel material to enormous stress and the risk of early failure. Semicontinuous systems for processing pumpable foods use two or more pressure vessels containing a free floating piston for compression (Ting 2011). The vessels are connected such that when one vessel discharges the product, the second system pressurizes, while the third vessel is loaded with the food sample. Thus, a continuous output is maintained.

Pressure-Transmitting Fluid Pressure-transmitting fluid is used to transmit pressure to prepackaged food samples. Water is the most commonly used pressure-transmitting fluid in industrial-scale equipment. Alternatively, laboratory- or pilot-scale equipment may use glycol, glycol and water, silicone oil, sodium benzoate solution, or castor oil as pressure-transmitting fluids, as the equipment may be prone to corrosion due to low-cost steel structures. The selection of pressure-transmitting fluid is in part dependent on its ability to seal under pressure, corrosion prevention properties, fluid viscosity changes under pressure, and heat of compression. The composition of the pressure-transmitting fluid, its thermal characteristics, and the fluid-to-sample ratio play an important role in governing the thermal behavior of foods under pressure. The importance of considering the compression heating behavior of pressure-transmitting fluid on microbial inactivation kinetics has been addressed in several articles (Balasubramanian & Balasubramaniam 2003, de Heij et al. 2003, Matser 19.8

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et al. 2004, Otero & Sanz 2003). Differences in compressibility and heat of compression values of different pressure-transmitting fluids can affect the inactivation kinetics of bacterial spores (Balasubramanian & Balasubramaniam 2003).

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Packaging A variety of flexible, high barrier packaging materials can be used to contain samples of HPP. The volume, geometry, and composition (polymer type, film thickness, and sealing and barrier properties) of the packaging material are important considerations for selecting packaging materials for HPP (Balasubramaniam et al. 2004). At least one interface of the package should be flexible enough to transmit pressure. Thus, rigid metal containers may not survive the pressure treatment (Rastogi et al. 2007). The presence of headspace air, oxygen in particular, can adversely affect product quality at elevated pressure-temperature conditions. Dissolved oxygen becomes more reactive (ground oxygen) when pressure is applied. Okamoto (1992) reported that the lifetime of singlet oxygen increases significantly with pressures of up to 400 MPa, which can initiate detrimental reactions. Furthermore, air has different compressibility properties than water and more effort is needed to compress the air. Thus, it would be desirable to minimize the presence of headspace air in the packages or preferably vacuum package the product. It is noteworthy that the intensity of the pressure-thermal treatment can alter the barrier properties of packaging material (Schauwecker et al. 2002; Caner et al. 2004; Galotto et al. 2008, 2009; Halim et al. 2009; Fairclough & Conti 2009; Yoo et al. 2014). Schauwecker et al. (2002) studied propylene glycol (PG) migration in different pouches under various pressure-temperature conditions. PG migration in EVOH/PE-EVOH pouches decreased significantly in pressure-treated samples (400, 600, and 827 MPa at 30, 50, and 75◦ C for 10 min) compared with thermally treated samples (30, 50, and 75◦ C for 10 min under an atmospheric pressure of 0.1 MPa). Pouch structures having a plastic-metallic interface as used in meal, ready-to-eat pouches processed at elevated pressures (≥200 MPa) and at 90◦ C for 10 min showed signs of delamination between polypropylene and aluminum layers. An investigation of pressure-assisted thermally processed carrots packaged in flexible pouches (Nylon/EVOH/EVA, Nylon/EVA, and MetPET/PE) revealed that the processing conditions (600 MPa and 110◦ C for 10 min) induced physical changes in the structure of the polymers. More importantly, it was determined that the oxygen transmission rate of Nylon/ EVOH/EVA was minimally impacted by the processing conditions (Ayvaz et al. 2012). More research is needed on the effect of elevated pressure-heat treatments on different packaging materials. Development of nanocomposite packaging material with superior barrier properties is desired.

Typical Processing Steps and Pressure-Temperature Response Figure 2 illustrates the basic steps involved in a combined pressure-thermal treatment. First, the food to be treated is preferably vacuum packaged in a flexible, high barrier package. At least one interface of the package should be flexible enough to transfer pressure to the packaged food (Balasubramaniam et al. 2004). Then, the prepackaged food material along with the pressuretransmitting fluid is preconditioned (preheated or cooled) to a certain initial temperature (e.g., heating from T1 to T2 for a given time t1 , Figure 2) inside the sample-loading basket. Subsequently, the sample basket containing prepackaged food material is loaded inside the pressure vessel. The pressure vessel is also preconditioned to a predetermined initial temperature using an external jacket. The remaining volume of the pressure vessel is filled with the preconditioned pressuretransmitting fluid. The loading time (t2 ) is the time needed to insert the sample, adjust the transmission fluid volume, and close the high pressure vessel. Upon loading, the sample is pressurized www.annualreviews.org • Principles and Application of High Pressure

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P2

P2

T3

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ΔT T2

T4

Pressure (MPa)

Temperature (°C)

T3

T1

P1

P1 t1

t3

t2

t4

t5

Time (min) Figure 2 Schematic representation of a typical pressure-temperature profile experienced by a food sample during preheating and compression, and holding time during pressure-assisted thermal processing. T1 , T2 , T3 , and T4 are the initial, preheating, target and final temperature, respectively; P1 , and P2 are the atmospheric and target pressure, respectively; t1 , t2 , t3 , t4 , and t5 are the preheating, loading, come-up, holding, and decompression time, respectively; T is the difference between the product temperature increase due to adiabatic compression during pressure treatment. Figure adapted from Balasubramaniam et al. (2004), with permission from Elsevier.

from atmospheric pressure (P1 ) to a target pressure (P2 ). The time interval between P1 and P2 represents the pressure come-up time (t3 ). Typical commercial equipment may have a pressure come-up time of approximately 2 min to reach 600 MPa (5 MPa per s). The pressure come-up time is characterized by the existence of nonisothermal and nonisobaric conditions. During compression, the temperature of both the food sample and medium rises (indicated by T in Figure 2) due to adiabatic heating. This rise in temperature enables reaching the target process (or working) temperature (T3 ). The time at which the target process temperature and pressure are reached is considered the beginning of the holding time (t4 ). Then, samples are processed for the desired holding time typically under isothermal and isobaric conditions. For economic reasons and to minimize adverse thermal effects on product quality, pressure holding times of less than 10 min may be desired. After processing, the samples are depressurized back to atmospheric pressure (P1 ). Most of the commercial-scale high pressure equipment have short (100 MPa). Abbreviations: HPH, high pressure homogenization; HPP, high pressure processing; PAF, pressure-assisted freezing; PAT, pressure-assisted thawing; PATP, pressure-assisted thermal processing; POTS, pressureohmic-thermal sterilization; UHT, ultrahigh-temperature pasteurization. Figure is not drawn to scale.

Solid

Log pressure (MPa)

0.1

Liquid Blanching

Freezing

Pasteurization

Vacuum impregnation 0.01

0.001

Gas Freezedrying 0

–20

20

40

60

80

100

Temperature (°C)

50

b

Pressure (MPa)

40

Liquid

30 l

ica rit rc ids e p u Su fl

20 Spraydrying

10

Extrusion Retort

0.1 30

90

Gas

UHT 150

210

270

330

390

Temperature (°C)

c

1,000

Solid

800

Pressure (MPa)

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Figure 3

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Standard homogenization

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Liquid 600 PATP 400

POTS

HPP PAF PAT

200

HPH –30

–20

–10

0

30

40

60

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100

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Balasubramaniam

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Mart´ınez-Monteagudo

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Gupta

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FO06CH19-Balasubramaniam

ARI

18 February 2015

15:1

Annu. Rev. Food Sci. Technol. 2015.6. Downloaded from www.annualreviews.org Access provided by Mahidol University on 02/26/15. For personal use only.

Practical applications of vacuum impregnation in diverse food products have been reviewed by Zhao & Xie 2004 and Fito et al. 2001 (Figure 3b). The dairy industry uses moderate pressures (up to 50 MPa) in the homogenization and stabilization of various dairy products. This has been widely used in the manufacture of ice cream mixes, yogurt, and milk-based beverages (Huppertz 2011). Combinations of pressure and temperature yielding water in the subcritical state, i.e., water above the boiling and below the critical point (100 MPa) is an increase in the fluid temperature, which is roughly 16◦ C per 100 MPa for water. Studies in which target microorganisms were inactivated by HPH are summarized in Table 6. In general, the thermal effect is the predominant factor causing microbial inactivation. For instance, a 6-log reduction of S. aureus was obtained at 300 MPa and an inlet temperature of 50◦ C. However, with an inlet temperature of 25◦ C and 300 MPa, only a 3-log reduction of S. aureus was observed (Diels et al. 2003, Wuytack et al. 2002). Unlike conventional HPP, the product temperature does not return to its initial value upon depressurization. Dense phase CO2 . Dense phase CO2 is a method that utilizes moderate pressures (

Principles and application of high pressure-based technologies in the food industry.

High pressure processing (HPP) has emerged as a commercially viable food manufacturing tool that satisfies consumers' demand for mildly processed, con...
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