I. Technological Trends and Advances for the Dairy Research and Industry
Nonconventional Technologies: Promising Stabilization Methods for the Fresh Dairy Sector
Milk and fresh dairy products are important components of the food system, even though they are characterized by a very short shelf-life that limits their distribution to local borders. Due to their composition, these foods are considered ideal for microbial growth. Pasteurization is commonly used to improve the microbial safety; however, heat may also cause undesired biochemical and nutritional changes that can affect color, flavor, taste, and texture of final dairy products. The changing lifestyle, with the growing consumer demand for high-quality processed foods, both concur to promote research of new food preservation teclmiques. Furthermore, the competitiveness of the dairy chain and the need for extending shelf life pushed the daily industry to explore alternative processing measures to heat-based processes. In this context, considerable interest has been reserved for nonconventional technologies, as a valid alternative to thermal processing. Therefore, the current chapter provides an overview of the most relevant non-thermal technologies applied to the fresh dairy sector, with a state of the ait about the topic and specific infonnation on the mechanisms of action and application to the dairy sector of high-pressure processing, pulsed high-intensity light technology, continuous UV light irradiation, ionizing radiation, ultrasound, and pulsed electric field (PEF).
Today, the natures of the dairy sector are multifunctional which contributes to the development of sustainable agriculture. Milk is produced and consumed in most countries of the world; in particular, the global milk production is represented by the whole fresh cow milk 82.7%, while the remaining 17.3% is characterized by other milk such as buffaloes (13.3%), goats (2.3%), sheep (1.3%) and camels (0.4%) (FAOSTAT, 2016). In 2015, global consumption of the dairy per capita was expected at 111.3 kg, thus confirming that the dairy sector is growing fast. According to FAO, world milk production should increase until 2025 with an annual average growth rate of 1.8%. Consequently, is also expected an increase in per capita consumption of daily products between 0.8% and 1.7% per year in developing countries, and between 0.5% and 1.1% in developed ones (FAOSTAT, 2016). Milk and daily products are an important resource for the food industry, accounting for around 14% of world agricultural trade. The imports and exports in global trade with 62% and 92% respectively are dominated by developed countries (Manjunatha et al., 2013). As for milk processing, it has a long tradition as demonstrated by the wide variety of typical cheeses produced in different areas around the world. North America, followed by Europe, is the largest cheese producer across the globe, holding significant share, given the immense usage of cheese in fast food (PMFDC, 2014). Europe and the USA contributed 70% to the world cheese production in 2012, it is expected that the production will increase with dynamic growth up to 2020 (PMFDC, 2014). Cheese from cow’s milk represents more than 80% of the global natural cheese production, the rest is cheese made from other milk (sheep, goat, and buffalo). Some of the well-known products of cheese are mozzarella, Cheddar, feta, Roquefort, and others. The technical problem that afflicts the dairy industry since its inception is the search for techniques able to fully sterilize milk and fresh dairy products without perceptibly changing their sensory quality. The choice of appropriate methods faces to the conservation of the nutritional, chemical, and microbiological characteristics of food have become vital, allowing achieving a shelf-life prolongation in the safest and healthy way.
The perishability of milk and fresh dairy products is well known due to the content of water, proteins, carbohydrates, minerals, and vitamins, all important elements for the growth of many forms of bacteria (De Jonghe et al., 2011; Franciosi et al., 2011). Psychrotrophic bacteria (such as Pseudomonas spp., Enterobacter spp., Klebsiella spp., Escherichia spp., Bacillus spp.) are the most commonly isolated microorganisms that can cause spoilage of heat-treated products, due to possible recontamination (Samarzija et al., 2012). In general, these microorganisms are not common in raw milk, but a consequence of milk contamination due to long storage times or wrong refrigeration temperatures (De Jonghe et al., 2011; Franciosi et al., 2011; Martin et al., 2011). The genus Pseudomonas is the most common population of gram-negative in raw milk with the predominance of R fluorescens (Samarzija et al., 2012). Unlike psychrotrophic bacteria, Pseudomonas spp. are characterized by short generation tune, thus reaching high concentrations in a few days of storage at refrigerated temperatures (Samarzija et al., 2012). Frequently, sensory changes and generation of off-odors are related to the action of hydrolytic thermostable enzymes produced by psychrotrophic bacteria that are active even after the thennal treatment of milk (Samarzija et al., 2012). In general, in cooled raw milk, the contamination by psychrotrophic bacteria provokes composition changes that affect both milk quality and its processability (Koka and Weimer, 2001). Furthermore, heat treatment generally destroys psychrotrophic bacteria of raw milk but not then enzymes that can influence milk stability during storage or its transformation into cheese. For example, during cheese making, proteases, and lipases cause a significant yield loss (Mitchell and Marshall, 1989) and other defects, as the poor texture or the anomalous discoloration (Soncini et al., 1998; Cantoni et al., 2003, 2006; Franzetti and Scarpellini, 2007; Samarzija et al., 2012; Carascosa et al., 2015).
High-temperature short-time (HTST) pasteurization method is commonly used to guarantee for the safety of the product and proper shelf-life. Although thermal processing is a conventional preservation technique, applied heat may cause undesirable changes in terms of color, flavor, texture, and nutritional composition. Furthermore, thermal processing is unable to eliminate thermostable bacteria, spores, and enzymes that also can compromise final quality and safety (Samarzija et al., 2012).
Today, natural products are in growing demand which is particularly vulnerable to contamination by L. monocytogenes (Schoder et al., 2015) and can be considered shelf-stable even with minimal processing (Falguera et al., 2012). In order to satisfy consumer demand, the dairy sector is continually looking for an alternative treatment to heating that not only ensures product safety but also improves shelf-life, retains nutrients and maintains freshness and wholesomeness of food. Over the years, different approaches have been proposed to prolong the shelf life of daily products (Gammariello et al., 2008; Del Nobile et al., 2009a, b; Rodriguez-Aguilera et al., 2011a, b; Singh et al., 2012; Mastromatteo et al., 2015), mainly based on the adoption of active coatings combined to modified atmosphere packaging. An alternative strategy for fresh cheese preservation could be represented by the adoption of non-thermal technologies, as physical methods to slow or prevent microbial growth and consequently to retain sensory changes. The most interesting non-thermal methods are represented by high-pressure processing (HPP), gases-based teclmiques (ozone, chlorine dioxide, and cold plasma), light-based teclmiques (ultraviolet (UV), pulsed light (PL)), and ionizing radiation, ultrasound-based process and pulsed electric field (PEF). The studies earned out on non-thermal technologies increased rapidly. In particular, studies on non-thermal technologies are reported for fresh-cut fruit and vegetables (Fernandez et al., 2013; Tappi et al., 2014; Manzocco et al., 2016), raw meat and fish (Bolumar et al., 2011; McDonnell et al., 2014), egg (Manzzoco et al., 2013) and liquids such as fruit juice (Thairi et al., 2006; Franz et al., 2009) apple cider or milk (Evrendilek et al., 2004; Cilliers et al., 2014; Innocente et al., 2014). In the dairy sector, some examples of applications have been reported, demonstrating the ability of these technologies to destroy pathogens without affecting the sensoiy quality (Barbosa-Canovas et al., 1998; Ross et al., 2003).
The aim of this chapter is to report specific information about the most important nonthermal processes applied to the dairy sector, thus highlighting the general concepts, the mechanisms of microbial inactivation, main advantages, and limitation.