Effect of the lactoperoxidase system on proteolysis and physicochemical changes in ultra high temperature milk during storage

Abstract

Indirect semi-skimmed ultra high temperature (UHT) milk was made from refrigerated milk preserved by the lactoperoxidase system (LPS). The effect of the LPS on the physicochemical and biochemical properties of UHT milk during storage period of 6 months at 30°C was assessed. The levels of soluble nitrogen at pH 4.6 and non protein nitrogen in all UHT milk samples increased during storage. However, control UHT milk, manufactured with refrigerated milk which LPS was not activated; showed the highest values (p < 0.01). Less hydrophobic amino acids concentrations, such as found in nitrogen soluble fraction, were lower in activated LPS UHT milk. Also, bacterial proteinase activities, measured using ophthaldialdehyde (oPA) as substrate, increased during storage. They were found higher in control UHT milk than in treated milk (p < 0.01). However, a decrease of total fatty acids in UHT milk, dependent on time of storage and LPS activation, was found during UHT milk storage.

Nutritional, Physicochemical, and Endogenous Enzyme Assessment of Raw Milk Preserved under Hyperbaric Storage at Variable Room Temperature

“…Storage under 75 and 100/RT maintained SP concentration after 7 days ( p > 0.05), which tended to increase as the storage period increased, with a significant variation being observed from day 14 to 39 ( p < 0.05) of storage, under 75 and 100/RT, respectively. Especially, from day 39 until the end of the study, the SP concentration had a pronounce increase of almost 4- to 6-fold under 75 and 100/RT, respectively, which can be attributed to the enzymatic activity of proteases such as plasmin and microbial proteases . The SP increment in milk can occur in prolonged storage at RT, with Olfa et al reporting an increase of 140% from day 0 to 60 days at 30 °C in the SP content of UHT milk, possibly as a result of thermoresistant microbial proteases (mainly fromPseudomonas spp.)…”

“…This enzyme might be more susceptible to the changes observed under these storage conditions, AP/RT, AP/RF, and 50/RT, namely, high microbial activity, decrease in pH, and increase in acidity, which all together can promote LPO denaturation and/or reduce its activity. 22 Storage under 75 and 100/RT presented overall a much better maintenance in LPO residual activity throughout storage, compared to the other storage conditions performed. At day 7, both storage conditions presented an increased LPO activity, compared to the initial one, to values around 113% (p < 0.05) and 123% (p < 0.05) for 75 and 100 MPa, respectively.…”

“…Storage under 50/RT presented a behavior alike AP/RT, with a more pronounced reduction ( p < 0.05) in the LPO activity up to 33 and 5% of the initial value after 7 and 14 days, respectively. This enzyme might be more susceptible to the changes observed under these storage conditions, AP/RT, AP/RF, and 50/RT, namely, high microbial activity, decrease in pH, and increase in acidity, which all together can promote LPO denaturation and/or reduce its activity …”

“…Milk stored under 75 and 100/RT had a similar FAAs profile, but compared to the initial FAAs, a greater variance (p < 0.05) was observed especially for tyrosine > methionine > leucine > tryptophan > serine > isoleucine > phenylalanine > threonine > cystine > valine > and histidine, with these FAAs being associated with plasmin and microbial protease enzymatic activity on caseins. 22 Since in this study, raw milk without any kind of processing was employed, a variety of active microbial proteases can be present initially, alongside with endogenous plasmin, which can promote casein proteolysis into small peptides or amino acids. 35 The information regarding the effect of HPP in milk plasmin and other proteases usually employed higher pressures (200− 400 MPa) than the ones used in this study, and for shorter periods of time, and thus, it is difficult to make a straightforward comparison of the results obtained in these different conditions.…”

Laktoperoksidaz Sistem Aktivasyonunun Çiğ Sütün Mikrobiyolojik Kalitesi Üzerine Etkileri

“…No significant change was observed in the control-raw milk. The results indicate that activation of the lactoperoxidase system occurs within the first three h. A study reported that activation of the lactoperoxidase system in raw milk is quite effective against psychrotroph bacteria, since it increases the shelf life of cold stored milk for several days [26] . As reported by another study, the initial psychrotroph bacterial load in the raw cow milk was 5.5×10 3 /mL, and in the activated group, it reduced to 2.6×10 6 /mL on 6 th day.…”