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4.1 Introduction

The consumer demand for minimally processed foods has always been a challenge for food processing sector. Therefore, novel food processing technologies are implemented to meet the challenges with the aim to ensure safe, fresh -like, nutritive foods without the use of heat or addition of food additives including preservatives. The recent scientific research for the food industry has now focused on non-thermal processing techniques, with high pressure processing (HPP) being amongst the few experiencing great potential in commercial settings (Norton & Sun, 2008; Rastogi & Raghavarao, 2007). High pressure processing is a cold pasteurization technique which consists of subjecting food, sealed in flexible and water -resistant packaging, to a high hydrostatic pressure up to 600 MPa (87,000 psi) for few seconds to few minutes. It is the same effect as subjecting the food to an ocean depth of 60 km deep. The technique was named after Blaise Pascal, a French scientist of the seventeenth century whose work included the effects of high pressure processing on fluids. The history of high pressure processing (HPP) dates back to nineteenth century. In 1899 Bert H. Hite at the West Virginia Agricultural Experimental Station examined the effect of pressure on dairy product like milk , meat , fruits and vegetables. Compared to today’s high pressure processing equipment , the model system (HPP) utilized in the 1890s by Hite was very primitive. Today, with the advancement in techniques like computational stress analysis and new materials, high capacity pressure systems can be manufactured to allow reliable high-pressure treatment of food products at even higher pressures (Hoover, 1993). Experiments into the effects of pressure on microorganisms have been recorded as early as 1884 (Jay, Loessner, & Golden, 2005). In general, high-pressure technology can supplement conventional thermal processing for reducing microbial load, or substitute the use of chemical preservatives (Rastogi, Subramanian, & Raghavarao, 1994). High pressure processing is a natural, environment friendly process with a real alternative to traditional thermal and chemical treatments.

There are two principles applicable to the use of high pressure processing in the food industry. The first is Le Chatelier’s Principle, which applies to all physical processes and states that, when a system at equilibrium is disturbed, the system responds in a way that tends to minimize the disturbance (Pauling, 1964). It means with an increase in pressure the volume decreases and vice versa. Second principle is the Isostatic Rule which states that pressure is transmitted instantaneously and uniformly throughout the sample either in direct contact with the pressure medium or hermetically sealed in a flexible packaging material (Olsson, 1995). The pressure applied to food system is transmitted isostatically and instantaneously, irrespective of the shape and size of the food commodity (Smelt, 1998) which offers distinct advantage over conventional thermal processing of large or irregular shaped food products (Kiera et al., 2008).

The fermented food industry is mainly focused on standardizing the properties and extending the shelf life of the product, in order to control the growth of microbes, and to shorten the time-consuming ageing processes required for flavour development. The synergistic effect of high pressure processing and additional hurdles like low pH in case of fermented foods ensures food commodities with better quality attributes and extended shelf life. For example, Bacteriocins work synergistically with HPP inactivating pathogens and increasing their death rate (Galvez, Abriouel, Lucas López, & Omar, 2007). Surviving pressure, cells of the pathogenic bacteria become injured and can be easily inhibited by bacteriocins (Liu et al., 2012). This synergistic action is the basis of the hurdle concept, which implies simultaneous or sequential use of several treatments to achieve product preservation and prolonged shelf life. These treatments include induced changes in aw, pH , temperature and the addition of bacteriocins (Jay et al., 2005). Several reports are available on the use of combination of hurdles with novel techniques like pulse electric field, high pressure processing, irradiation, etc. Hence, an intelligent combination of “hurdles” including non-thermal method of food preservation technique like high pressure processing and competitive microflora may lead to the development of minimally processed foods with added advantage in terms of quality attributes. This approach is described by the phrase “Hurdle technology” (Leistner, 1999; Leistner & Gorris, 1995). The chapter outlines various applications of HPP to extend shelf life of fermented food products .

4.2 Application of HPP for Fermented Food Products

4.2.1 Fermented Meat Products

The meat industry has developed guidelines to control safety of raw meat and other ingredients used in the production of fermented meat products. Some of the measures used to assure the safety of dry fermented sausages involve heating , salting or preservation using vinegar (acetic acid). The use of high temperatures alters the natural characteristics of the product and sensory qualities . Thus, there is a growing need for non-thermal technologies that meet relevant product specifications including improved product safety while maintaining or enhancing sensory characteristics. In HPP, pressure up to 900 MPa is used and its effectiveness on the inactivation of vegetative bacteria in foods has been reported (Aymerich, Picouet, & Monfort, 2008; Considine, Kelly, Fitzgerald, Hill, & Sleator, 2008; Garriga, Grebol, Aymerich, Monfort, & Hugas, 2004; Hugas, Garriga, & Monfort, 2002; Patterson, 2005). Application of HPP in meat processing is widely reported (Considine et al., 2008; Hugas et al., 2002; Patterson, 2005). However, in some cases high-pressure treatment of raw and processed meat can induce lipid oxidation depending on processing parameters of pressure, temperature and time and subsequent storage conditions. Moreover, high pressure can as detrimental as heat treatment in terms of the oxidation level in cold-stored meat products (Simonin, Duranton, & de Lamballerie, 2012). The combination of HPP with biopreservation to enhance bacterial inactivation and reduce the recovery of sublethally injured cells during product storage has been demonstrated in cooked meat products (Aymerich, Jofré, Garriga, & Hugas, 2005; Jofré et al., 2007; Marcos, Jofré, Aymerich, Monfort, & Garriga, 2008).

Shelf-stability of fermented meat products (e.g. sausages) is due to a combination of hurdles, whose interaction inactivates or prevents the growth of undesired microorganisms present in the product (Leistner, 2007). HPP has been reported for various fermented meat products as shown in Table 4.1.

Table 4.1 Synergistic effect of HPP and different hurdles on meat products/effect of HPP on various fermented meat products
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In fermented sausages, high pressure has been proved to be a useful post-process intervention to decrease the levels of several food-borne pathogens (Garriga et al., 2005; Gill & Ramaswamy, 2008; Jofre, Aymerich, & Garriga, 2009; Marcos, Aymerich, Guardia, & Garriga, 2007; Porto-Fett et al., 2010) and has been recognized as a listericidal treatment by FDA and Codex Alimentarius (CAC, 2007; HHS, 2008). Marcos, Aymerich, Garriga, and Arnau (2013) indicated that inactivation of Enterobacteriaceae during the whole shelf life of the product could only be prevented with HPP. Pressurization (600 MPa, 5 min) of sliced fermented sausages induced an immediate reduction of Enterobacteriaceae counts. Non-pressurized batches showed a 1 log unit reduction of LAB population throughout storage. On the other hand, pressurized batches with or without nisin packed in PVOH films showed reductions of about 2.4 log units at the end of storage. Simon-Sarkadi, Pásztor-Huszár, Dalmadi, and Kiskó (2012) studied the effect of HPP on aerobic plate counts of Hungarian dry fermented sausage during 4 weeks of storage at 8 °C. The total viable counts of control samples of semi-dry sausage and extra thick sausage reduced after cold storage, which was followed by regrowth to about 7.5 log CFU/cm3 within 4 weeks. Treatment with 500 MPa HPP reduced the total viable count by 1–3 log depending on sample type. This difference was more pronounced by the end of storage in case of semi-dry sausage (3 log) and extra thick sausage (5 log). However, significant changes occurred in the organoleptic characteristics of semi-dry sausage. Texture seemed to be the most sensitive to high pressure treatment caused significant changes in firmness of dry and semi-dry sausages, and both samples became softer as a result of pressurization. Smell, firmness and taste of extra thick sausage remained practically unchanged after pressure treatment.

Ananou et al. (2010) demonstrated that in ripened control fuets (a dry-cured sausage ), HPP had a low effect on the viability of Staphylococci at room or refrigerated storage temperatures. The application of HPP treatment caused an immediate reduction of 1.06 log units in LAB population. However, viable counts recovered to similar values to those of the controls at 18 days storage (at 7 °C or room temperature). Until the end of storage, no significant differences in lactic acid bacteria or staphylococci counts were detected between the different types of fuets investigated. However, Rubio, Martín, Aymerich, and Garriga (2014) indicated that the application of high-pressure treatment (600 MPa for 5 min) at the end of ripening (day 14) produced an immediate reduction of L. monocytogenes to <1 log CFU/g, which was not detected after 35 days of storage at 4 °C.

Significant changes in volatile profile of high pressure processed fermented meat products have been reported. For example, Rivas-Canedo, Nunez, and Fernández-García (2009) investigated the changes in the volatile profile of Spanish dry fermented sausage “salchichon” when subjected to HPP (400 MPa, 10 min at 12 °C). HPP-treated samples had higher levels of aldehydes (both linear and branched chains), alcohols and some compounds e.g. 1-methoxy-2-propanol, carbon disulfide and benzaldehyde. Similar changes in volatile profile of pork meat have been reported for high-pressure processed samples (Rivas-Canedo, Ojeda, Nuñez, & Fernández-García, 2012).

Changes in biogenic profile of high-pressure processed fermented meat samples have been reported extensively. For example, Latorre-Moratalla et al. (2007) investigated the effects of high hydrostatic pressure (200 MPa) on meat batter just before sausage fermentation and the inoculation of starter culture to improve the safety and quality of traditional Spanish fermented sausages (fuet and chorizo). In batches without starter culture and no HPP treatment biogenic amine accumulation was much lower in fuet compared to chorizo sausages. Tyramine was the only biogenic amine detected in fuet, whereas cadaverine was the major amine in chorizo sausages, which is usually associated with lysine-decarboxylase activity of undesirable Gram-negative bacteria . In spontaneously fermented sausages, the application of HPP resulted in a strong inhibition of diamine accumulation, the levels of putrescine and cadaverine being up to 88 and 98% lower than the non-pressurized batch as shown in Fig. 4.1.

Fig. 4.1
figure 1

Variation in biogenic amine contents during the manufacture of fuet (a, c, e, left column) and chorizo (b, d, f, right column) through spontaneously (nS) and starter (S) mediated fermentation, without (nP) and with (P) high hydrostatic pressure treatment (From Latorre-Moratalla, M.L., Bover-Cid, S., Aymerich, T., Marcos, B., Vidal-Carou, M.C., Garriga, M. 2007. Meat Science, 75: 460–469. With permission )

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4.2.2 Fermented Dairy Products

The dairy industry is focusing on standardizing the properties and extending the shelf life of the milk products. Today, there has been a strong demand in the consumption of probiotic bacteria using food products , including probiotic dairy products due to increasing consumer awareness about the impact of food on health. Fermented milk is the most popular and most consumed probiotic food carrier throughout the world. Several studies about the development of probiotic fermented milks (Almeida et al., 2008; Kearney et al., 2009; Oliveira, Perego, Converti, & Oliveira, 2009) and probiotic fermented beverages (Castro, Cunha, Barreto, Amboni, & Prudêncio, 2009; Zoellner et al., 2009) have been reported. Pasteurization of milk for the destruction of pathogenic microorganisms and to kill the natural microflora has been traditionally carried out by heat treatment . The potential of HPP treatment as an alternative method to heat pasteurization of milk was proposed almost a century ago (Hite, 1899) and has been investigated for a range of dairy products Apart from pasteurization effects , HPP provides new opportunities for homogenization effects. Table 4.2 summarizes various studies highlighting the application of HPP in fermented dairy products. Patrignani et al. (2007) evaluated the effects of various factors like milkfat content, non-fat milk solids content, and high-pressure homogenization on rate of fermentation of the probiotic strain Lactobacillus paracasei BFE 5264 inoculated in milk. Loss of Lactobacillus paracasei strain during refrigerated storage, texture parameters, volatile compounds and sensorial properties of the coagula obtained were investigated. Patrignani et al. (2007) observed significant effect of independent variables on the measured attributes of fermented milks. The coagulation times were significantly affected by pressure and added milk fat, and the rheological parameters such as firmness, viscosity index and consistency of the fermented milk increased with the pressure applied to the milk. Rodriguez-Alcala et al. (2015) indicated that neutral and polar lipids remained stable in the pressure treated sample (250–900 MPa).

Table 4.2 Synergistic effects of HPP and different hurdles on dairy products
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Consumer demand for shelf-stable yoghurt is difficult due to problem of storing live microflora at ambient conditions. The use of heat treatments for extension of shelf life of yoghurt has resulted in syneresis and a decrease in viscosity during storage. The application of hydrostatic pressure directly to yoghurts has been proposed as an alternative to the use of additives, which can adversely affect the yoghurt taste, flavour , aroma and mouthfeel (De Ancos, Cano, & Gomez, 2000). Penna, Rao-Gurram, and Barbosa-Cánovas (2007) investigated the effect of HPP on microstructure of low-fat probiotic yoghurt. Microstructure of HPP (C and D) processed yoghurt has more interconnected clusters of densely aggregated protein of reduced particle size, on the other hand heat-treated (A and B) milk yoghurt had fewer interconnected chains of irregularly shaped casein micelles (Fig. 4.2).

Fig. 4.2
figure 2

Scanning electron microscopy micrographs of yoghurt fermented with starters YO MIX 236 (a, c, e) and DPL ABY 611 (b, d, f) with different treatments: (a, b) heat, (c, d) HPP, (e, f) heat + HPP. St, Streptococcus thermophilus ; Lb, Lactobacillus delbrueckii ssp. bulgaricus; La, Lactobacillus acidophilus ; B, Bifidobacterium longum ; v, void space; cs, casein (From Penna, A.L.B., Subbarao-Gurram, Barbosa-Canovas, G.V. 2007. Food Research International.40: 510–519. With permission)

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Cheese is one of the most versatile dairy products available and consumed in various forms. Application of HPP has shown to improve shelf life and safety profiles of cheese while inducing desirable changes in techno-functional and organoleptic properties . Evert-Arriagada, Hernández-Herrero, Guamis, and Trujillo (2014) demonstrated the commercial application of HPP to increase the starter-free fresh cheese shelf life. High-pressure treatment of cheese did not affect panellists’ preference for treated cheese over the non-treated cheese. Moreover, the preference mean score (6.5) for the pressurized cheeses stored during 22 days and for the freshly made cheese was almost the same. Bermúdez-Aguirre and Barbosa-Cánovas (2012) studied fortification of mozzarella cheese using selected sources of omega-3 and non-thermal approaches (PEF and HPP). The growth of coliforms was delayed using thermal pasteurization and pressurization of milk ; however, PEF did not delay the growth of coliforms, showing counts around 2.8 log after 8 days of storage. The mesophiles counts of cheeses processed with PEF and thermal treated milk were close to 7 log, while the pressurized milk showed a lower microbial growth of 5.8 log at the end of storage of 12 days. Similar profiles for psychrophiles were observed during storage, with HPP being the most effective treatment for milk processing before cheesemaking. Lopez-Pedemonte, Roig-Sagués, Lamo, Gervilla, and Guamis (2007) demonstrated reductions in Staphylococcus aureus counts on HPP treatments (300, 400 and 500 MPa at 5 °C and 20 °C). While reductions obtained for HPP treatments at 5 °C differed significantly between 300, 400 and 500 MPa in both Staphylococcus aureus strains, for 20 °C HPP treatments differences only became significant after applying 500 MPa pressure. Similarly, Lopez-Pedemonte, Roig-Sagues, Lamo, Hernandez-Herrero, and Guamis (2007) indicated that counts of control samples for both strains of L. monocytogenes did not significantly differ during storage. In contrast, counts of all treated samples diminished with storage time.

Serrano, Velazquez, Lopetcharat, Ramirez, and Torres (2005) indicated that short and moderate hydrostatic pressure (MHP) treatments accelerated the shredability of Cheddar cheese. Both MHP (345 MPa for 3 and 7 min) and higher pressure (483 MPa for 3 and 7 min) treatments applied to 1 day milled curd cheddar cheese induced a microstructure resembling that of ripened cheese as shown in Fig. 4.3. The reduction in the amount of crumbles as well as increase in desirable physical properties such as surface smoothness and shred mean length and uniformity in pressure-treated samples was reported. Further, the pH of the control cheese samples increased during storage. During storage, pH fluctuated and the pH difference on day 27 was lower than initial pH difference (day 0) but remained higher for pressure-treated samples with 483 MPa > 345 MPa > control. Dhakal et al. (2014) demonstrated that in case of high pressure treatment (450 MPa and 600 MPa) of almond milk, the amandin content, which is the major almond allergen could no longer be detected, whereas, thermally processed samples did not show significant reductions unless the samples were treated at a temperature higher than 85 °C.

Fig. 4.3
figure 3

Scanning electron microscope micrographs of control (a) and pressure-treated (b–f) milled curd cheddar cheese (From Serrano, J., Velazquez, G., Lopetcharat, K., Ramirez, J.A., Torres, J.A. 2005. J. Food. Sci. 70 (4): 286–293. With permission )

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4.2.3 Alcoholic Beverages

Currently a range of food products such as fruit juices , sea foods and meat products can be found on market shelves all around the world (Matser, Krebbers, van den Berg, & Bartels, 2004). However, few data have been reported about the use of HPP for beer and wine . Although, rice wine (nigori sake) is one of the earliest HHP-treated commercial products that appeared on the Japanese market (Suzuki, 2002), no HHP-treated alcoholic beverage such as beer and wine is introduced to date.

Wine cannot be treated with heat since its characteristics such as flavour , taste and colour are very sensitive to temperature (Mermelstein, 1998). Therefore, a common practice is the addition of sulphur dioxide (SO2) to wine to reduce the microbial population of the grape must and to preserve the final product for long period of time (Ribéreau-Gayon, Dubourdieu, Donèche, & Lonvaud, 2006). SO2 acts as an antimicrobial agent and antioxidant in wine (Amerine, Berg, & Cruess, 1967; Romano & Suzzi, 1993). However, SO2 may have negative effects on human health (Romano & Suzzi, 1993). Therefore, wine industry is challenged to meet consumers’ demands of reducing the levels of SO2 used in wine production (Du Toit & Pretorius, 2000). Tabilo-Munizaga et al. (2014) studied the effects of high hydrostatic pressure (HPP 400–500 MPa for 3–10 min) on the protein structure and thermal stability of Sauvignon blanc wine. It was observed that higher thermal stability and major structural changes observed in wine proteins were obtained by 450 MPa pressure for 3 and 5 min. With this pressure–time combination, the structural conformations achieved by the wine proteins could provide higher thermal stability and thus delay haze formation in wine during 60 days storage. Santos et al. (2013) studied the effect of high-pressure treatments on the physico-chemical properties of a sulphur dioxide-free red wine . The wine pressurized at 500 MPa presented more scents of cooked fruit and spicy aroma. The untreated wines presented less perceived fruity and floral aroma and had a more pronounced metallic and leather aroma than the other wines. Comparing the taste assessment of the different wine samples, the pressurized wines presented a similar taste assessment than the wine with SO2. The untreated wines showed a higher acidity and lower balance. In terms of colour, the pressurized wines presented higher values of brown and limpidity and lower values of violet than unpressurized wines. After 12 months of storage, pressurized wines showed a better sensorial assessment, with the pressure treatments imparting aged-like characteristics to the wines. The results demonstrated that HPP can influence long-term red wine physico-chemical and sensorial characteristics. HPP results in an increase of condensation reactions of phenolic compound. The compounds formed with higher degree of polymerization become insoluble in wine along storage. Mok et al. (2006) studied the pasteurization of low-alcohol red wine (ethanol 9 %, pH 3.27, acidity 0.068 %, total sugar 0.85 %) using HPP (100–350 MPa for 0–30 min at 25 °C). Corrales, Butz, and Tauscher (2008) applied HPP to wine from the dornfelder grape variety. A decrease in the concentration of malvidin-3-O-glucoside in pressurized (600 MPa at 70 °C for 1 h) samples was detected. Wherein, wine subjected to pressure treatment (600 MPa, 70 °C for 10 min) exhibited no significant differences in anthocyanin composition and antioxidant activity .

The aerobic bacteria decreased below the detection limit after 20 min pressurization at 300 MPa and 10 min pressurization at 350 MPa as well as initial yeast count (5.46 log) decreased to 2.46 and 1.15 log after HPP treatment at 300 MPa for 10 and 20 min, respectively. Puig, Vilavella, Daoudi, Guamis, and Minguez (2003) investigated the microbiological and biochemical stabilization of wines by use of HPP. A white wine (with 40–50 mg L−1 of total SO2) and a red wine (with 80–90 mg L−1 of total SO2) were used. Two yeasts: S. cerevisiae and Brettanomyces bruxellensis (107 CFU L−1 of each wine), two lactic acid bacteria: L. plantarum and Oenococcus oeni (109 CFU L−1 of each wine), and two acetic acid bacteria: A. aceti and A. pasteurianus (109 CFU L−1 of each wine) were inoculated into wines. HPP treatments were done at 400 or 500 MPa for 5 or 15 min with 4 °C or 20 °C of temperature. HPP treatments resulted 6 log reduction for yeasts and 8 log reduction for bacteria.

Commercially, beer is pasteurized to guarantee microbiological stability during its shelf life (Zufall & Wackerbauer, 2000). However, heat can cause protein denaturation , promoting the formation of new tannin-protein complexes, with consequent turbidity enhancement (Stewart, 2006). In addition, heat processing promotes the Maillard reaction , resulting in an alteration of beer colour to reddish (Castellari, Arfelli, Riponi, Carpi, & Amati, 2000) and the formation of undesirable flavours (Stewart, 2006). These are related to oxidation and staling, with the development of off-flavours (long chain aldehydes ) (Zufall & Wackerbauer, 2000). HPP may be used to increase the shelf life of special quality beers without altering the original characteristics of the untreated product and without heat treatments or filtration. Thus, HPP could be an alternative to the conventional pasteurization of beer. The first trial of HPP on brewing process of beer was carried out by Fischer, Schöberl, Russ, and Meyer-Pittroff (1998). Fischer et al. (1998) concluded that HPP treatment (300–700 MPa, 5 min) of bright lager beer samples packed in polyethylene naphthalate bottles did not significantly change the colour, foam durability and the spectrum of flavour materials. In contrary, Perez-Lamela, Reed, and Simal-Gándara (2004) indicated that HPP treatment (300–600 MPa, 20 min) of beer resulted in an increase in the foaming and haze characteristics of the beer . Gänzle, Ulmer, and Vogel (2001) investigated the effect of ethanol and hop extract on inactivation of L. plantarum in model beer system during and after HPP treatment. Addition of ethanol and hop extract accelerated HHP inactivation of L. plantarum in model beer. Buckow, Heinz, and Knorr (2005) investigated the combined effect of HPP (0.1–900 MPa) and temperature (30–75 °C) on the activity of β-glucanase from barley malt. Thermostability of β-glucanase was found to be highest at 400 MPa. Wherein, at temperatures above 55 °C and ambient pressure, as well as at (900 MPa, 40 °C), the inactivation of β-glucanase was higher resulting in complete loss of enzyme activity in less than 30 min.

Effect of HPP on quality parameters of lager beer was studied by Buzrul, Alpas, and Bozoglu (2005a). Unpasteurized lager beer samples from a commercial brewery were treated either by HPP (200–350 MPa, 3 and 5 min, 20 °C) or by conventional heat pasteurization (60 °C for 15 min). The colour, protein sensitivity and chill haze values increased as the pressure and pressurization time increased. Change in bitterness was higher in conventional heat pasteurization, but pressures up to 300 MPa had no significant effect on bitterness. In another study Buzrul, Alpas, and Bozoglu (2005b) investigated the effects of HPP on shelf life of lager beer. Filtered bright lager beer samples were either treated with HPP (350 MPa, 3 and 5 min, 20 °C) or by conventional heat pasteurization (60 °C for 15 min). However, HPP-treated samples had lower bitterness and protein sensitivity and higher chill haze values than the heat pasteurized samples at the end of the storage period. Fischer et al. (2006) examined the effect of pH (4.0, 5.0, 6.0 and 6.5) and HPP treatment (300 MPa, 20 °C for various holding times up to 120 min) on L. plantarum (moderately hop tolerant) in model beer system. It was observed that inactivation was more effective at lower pH values, where up to 5 log reductions could be achieved.

4.2.4 Bakery Products

Different scientific reports described the effect of HPP on specific cereal components properties or model systems, namely starch and gluten (Apichartsrangkoon et al., 1998; Gomes et al., 1998; Kieffer et al., 2007). HPP induced starch gelatinization , following different mechanism than thermally induced gelatinization (Gomes et al., 1998). HPP treatment provokes swelling of starch but keeping granule integrity; as a consequence HPP-treated starches modify their microstructure and rheological properties in a different way than thermally treated ones (Gomes et al., 1998; Stolt et al., 2000).

Scanning electron microscopy was used to determine the effect of the HPP on dough microstructure. Scanning electron micrographs of wheat doughs treated at 50, 150 and 250 MPa for 4 min are showed in Fig. 4.4. Untreated wheat dough was characterized by having a continuous structure with the intact starch granules embedded in the matrix structure of proteins and soluble solutes. Dough treated at pressure of 50 and 150 MPa showed well-defined starch granules with diverse size, and the surrounding structures (mainly of protein nature) were progressively reduced, being confined in the case of 150 MPa to agglomerates of starch granules. Drastic changes were observed in dough treated at 250 MPa where starch granules as individual structures disappeared adopting a discontinuous film-like organization similar to what happened after swelling and gelatinization (Barcenas, Altamirano-Fortoul, & Rosell, 2010). Barcenas et al. (2010) reported that the treatment of wheat dough with HPP induced a rapid reduction of the microbial population but sufficient mould and yeast survival, for ensuring bread dough fermentation at moderate pressure conditions (50–250 MPa, for 2 min at 20 °C). This study suggests that high hydrostatic processing in the range 50–200 MPa could be an alternative technique for obtaining novel textured cereal based products .

Fig. 4.4
figure 4

Scanning electron micrographs of wheat dough exposed to varying pressure levels (0–250 MPa/4 min) (From Barcenas, M.E., Altamirano-Fortoul, R., Rosell, C.M. 2010. LWT—Food Science and Technology.43: 12–19. With permission)

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4.3 Concluding Remarks and Future Outlook

HPP has attracted considerable interest in food industry due to its promising effects in food preservation . Consumers require safe and natural products without added preservatives. Consuming fermented foods such as probiotic drink (kefir , kombucha), sauerkraut, kimchi, as well as fermented meat products helps in maintaining proper balance of gut bacteria and digestive enzymes . Synergistic effect of HPP in combination with multiple hurdles such as low pH , aw and acidic environment in fermented foods helps in achieving an increased rate of inactivation of spoilage microbes and endogenous enzymes . The hygienic quality of raw materials may be improved by decreasing microbial load through pasteurization , in the dairy industry. However, conventional heat treatments in the case of fermented meat products cause detrimental changes. Hence, alternative mild preservation techniques like high pressure processing show promising opportunities. However, there are certain technological limitations. For example, bacterial spores are highly pressure resistant, since pressures exceeding 1200 MPa may be needed for their inactivation (Knorr, 1995). Sterilization of low-acid foods (pH > 4.6) requires combination of high pressure and other forms of relatively mild treatments like heating (Rastogi & Raghavarao, 2007). Nonetheless, HPP has promising applications to satisfy consumer demand for high-quality food products . HPP enables extended shelf life and safety of fermented meat and dairy products with improved sensory and organoleptic characteristics. Hence, HPP can be considered as an innovative technique that can be employed by the food industry to meet the raising consumer demands for safe and nutritious fermented food products .