Food products being composed of biological raw materials inherently deteriorate over time. Degradation of foods is a complex process which may involve many mechanisms. The main categories of food deterioration that can occur are physical (e.g., bruising of fresh fruits and vegetables, and chill injury), chemical (e.g., development of off-flavor, discoloration, and vitamin degradation), biochemical (e.g., enzymatic protein breakdown by proteases and enzymatic browning by polyphenol oxidase) and microbiological (e.g., growth of microorganisms and production of metabolites or toxins) [4, 23, 24]. Multiple factors influence deterioration rates, and hence, the shelf-life of foods. These factors can be classified into intrinsic factors, including water activity (aW), pH, use of preservatives, available
oxygen, and natural microflora; or extrinsic factors, including temperature and relative humidity control during storage, composition of atmosphere in the packaging, consumer handling, etc. In conjunction, all these factors can operate in an interactive and often unpredictable way, therefore shelf-life prediction is a difficult problem for processors and researchers [6, 10, 23]. From the safety point of view, the risk produced by growth of
R. N. Zúñiga and E. Troncoso 134
pathogenic microorganisms and formation of toxic substances during storage of MP foods is much important than the physical or chemical spoilage. For this reason the discussion in this point is focused on the growth of microorganisms and loss of food safety.
Foodborne disease outbreaks are one of the main health problems all over the world. Outbreaks have an extensive impact on human health (i.e., causing morbidity and mortality) and also are a significant burden on nation economies and public health [25]. Moreover, the increasing trend in infectious diseases produced by emerging pathogens has caused significant impact on global health in the last decades [26]. The Center for Disease Control and Prevention estimated that each year roughly 48 million people gets sick in USA, 128,000 are hospitalized and 3,000 die due to foodborne diseases. The main pathogens causing diseases and/or death were Salmonella spp, Listeria monocytogenes, Campylobacter spp., and E. coli O157:H7 [27]. In this context, a recent survey performed in urban Chilean areas between 2005 and 2010 showed that a total of 12,196 people were affected by foodborne diseases with an incidence rate of 32 cases per 100 inhabitants. The main etiologic agents found were Salmonella spp, Shigella spp and Vibrio parahaemolyticus [28]. The researchers concluded that the largest numbers of outbreaks happened in the households, which were due to bad handling and/or inappropriate storage of the foods, confirming that home storage is a critical point for consumer safety.
Growth of microorganisms during storage can strongly affect the food safety. On this regard, manufacturers must have the means to predict the end-point of shelf-life under a given set of storage conditions. Criteria based on the microbial limits of spoilage and pathogenic microorganisms for rejection of a food can be reasonably well set [6, 12]. Even though consumers find a product still acceptable it may be not acceptable anymore because of the presence of pathogens or toxins, unnoticeable to consumers [12]. The growth of food- poisoning organisms such as Salmonella species and Listeria monocytogenes will not necessarily be accompanied by changes in appearance, odor, flavor or texture that could be detected by the human senses, and consequently pose serious health concerns [6]. Foods may be considered to be microbiologically unsafe owing to the presence of microorganisms that may invade the body (e.g., Salmonella, Listeria monocytogenes, E. coli O157:H7 and Campylobacter) or those that produce a toxin ingested with a food (e.g., Clostridium botulinum, Staphylococcus aureus and Bacillus cereus) [16]. Growth of a specific microorganism during storage depends on several factors, the most important being:
1) the initial microbial loading at the start of storage; 2) the physicochemical properties of the food;
3) the processing methods used in the food production line; and
4) the external environment of the food, such as the surrounding gas composition and storage temperature [6].
Rates of deteriorative changes and microbial growth at normal food storage conditions often depend on aW. Food deterioration due to microbial growth is not likely to occur at aW <
0.6 [29], but to maintain the characteristics of fresh-like products, MPFV kept a similar aW of
the fresh tissue.
Microbiological changes are of primary importance for short-life products. Roughly speaking, food products can be classified as either fresh or processed, recognizing that there are levels within and between these classifications. MPFV fall between these two
Shelf-Life Calculation and Temperature-Time Indicators 135 classifications [30]. In general, microbial deterioration becomes the dominant process affecting safety and quality of fresh and MPFV, their shelf-life will be determined mainly by microbial and biochemical changes, whereas biochemical and physical deterioration become the dominant mechanisms in determining shelf-life of processed foods [12]. MPFV can also be contaminated with human pathogens, present from before, during or after harvest [5]. Safety of MPFV relies on a combination of preserving factors which, rather than inactivating microorganisms, inhibit their growth [14]. MP and ready-to-eat products are one of the major growing segments in food retail establishments. There has been an increased demand for ready-to-eat and MPFV mainly because of the health benefits associated with their consumption, but the risk associated with consumption of these products has also increased. In addition, MPFV are still under study because of the difficulties in preserving their fresh- like quality during prolonged periods [31, 32].
Growth of microorganisms sets the limit on storage of fresh or MPFV. However, if biochemical processes (e.g., enzymatic browning and rancidity development) occurring on a time scale shorter than microbial growth can determine the cut-off [30]. Processing operations used for preparing MPFV or ready-to-eat products (e.g., peeling, cutting or shredding) create practical problems regarding shelf-life, safety and packaging. Physicochemical properties and cut surfaces condition the microbiological invasion of MPFV, causing both microbiological and physiological mechanisms to be possible limitations for the shelf-life [5, 31, 32]. After minimal processing, a relatively stable agricultural product with a shelf-life of several weeks or months will become one that has only a very short shelf-life, generally no more than 7 to 10 days, but it should be preferably longer, up to 21 days, depending on the market [4, 24]. In general, total counts of microbiological populations in MPFV after processing range from 3 to 6 log CFU/g. When changes in sensory quality factors of MPFV result in rejection of the product, microbiological counts are in most cases high (> 7–8 log CFU/g) [5]. For this kind of products the goal is maximizing taste and fresh-like characteristics while minimizing the risk from foodborne pathogens.