Abstract
Much research has been conducted to comprehend the mechanisms of high pressure (HP) inactivation of spores in aqueous systems but for food model systems these information are scarce. In these systems spores can interact with ingredients which then could possibly lead to retarded or reduced inactivation, which can cause a problem for the sterilization process. The protective mechanism of a reduced aw-value is still unclear. HP processing might prove valuable to overcome protective effects of solutes and achieve shorter process times for sterilization under HP. To gain insight into the underlying mechanisms five aw-values (0.9, 0.92, 0.94, 0.96, 1) were adjusted with two different solutes (NaCl, sucrose). Solutions were inoculated with spores of Bacillus amyloliquefaciens and treated at 105, 110, and 115°C at 600 MPa. Further a thermal inactivation was conducted at the same temperatures for a comparison with the HP data. Afterward, the influence of HP high temperature treatment on the inactivation, the dipicolinic acid (DPA)-release and membrane constitution was assessed by plate count, HPLC and flow cytometry (FCM). The results show that during HP treatments sucrose and salt both have a protective effect, in which the influence of sucrose on the retarded inactivation is higher. The threshold water activities (aw), which is 0.94, here salt and sucrose have a significant influence on the inactivation. The comparison of thermal (105–115°C) and HP and high temperature (600 MPa, 105–115°C) treated samples showed that the time needed to achieve a 4–5 log10 inactivation is reduced from 45 (aw = 1) to 75 (aw = 0.9) min at 105°C to 3 (aw = 1) to 15 (aw = 0.9) minutes at 600 MPa and 105°C. The release of DPA is the rate limiting step of the inactivation and therefore monitoring the release is of great interest. The DPA-release is slowed down in high concentrated solutions (e.g., sucrose, salt) in comparison to aw 1. Since there is a difference in the way the solutes protect the spore it could be seen as an inner spore membrane effect. Maybe as shown for vegetative microorganism the solutes can interact with membranes, e.g., the inner spore membrane. Flow cytometry (FCM) measurement data show a similar trend.
Highlights
One of the main aims of the food industry is the product safety but in the recent years due to consumer demand the product quality of the food has gained more importance (Ramirez et al, 2009; Olsen et al, 2010; Belletti et al, 2013; Reineke et al, 2013a)
Five different aw-values were used for each solute (NaCl and sucrose); the range was selected from 1 to 0.9 (1, 0.96, 0.94, 0.92, 0.90), since this represents the aw of many relevant food systems
For NaCl the analyses showed that with increasing temperature the time to release the same amount of dipicolinic acid (DPA) is getting shorter and the released amounts increase to 84–92 % at 115◦C in comparison to 72– 92% at 105◦C (Table 1)
Summary
One of the main aims of the food industry is the product safety but in the recent years due to consumer demand the product quality of the food has gained more importance (Ramirez et al, 2009; Olsen et al, 2010; Belletti et al, 2013; Reineke et al, 2013a). The mechanisms of spore inactivation under these servere conditions will not be discussed in detail since they are well described in literature elsewhere (Wuytack et al, 1998; Setlow, 2003; Margosch et al, 2006; Black et al, 2007; Barbosa-Canovas and Juliano, 2008; Reineke et al, 2012, 2013b) It should be mentioned, since it is of importance for this work, that the release of dipicolinic acid (DPA), which makes up 5– 15 % of the dry matter content of the spores, is thought to be the rate limiting step of the inactivation (Reineke et al, 2013b). For a rapid and sudden inactivation of spores under pressure it is important to apply pressures ≥600 MPa and temperatures above 60◦C to ensure the loss of heat resistance (Reineke et al, 2013b)
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