EFFECT OF TEMPERATURE AND RATE OF ACIDIFICATION ON THE RHEOLOGICAL PROPERTIES OF ACID SKIM MILK GELS

Effect of atmospheric cold plasma treatment on acid gelation properties of skim milk: Rheology and textural studies

Cold plasma processing is a non-thermal food processing technique that has been shown to improve the gelling properties of plant proteins by altering their structure through oxidation and crosslinking. This study aimed to investigate the effects of cold plasma treatment on the rheological properties of skim milk under different conditions, focusing on the impact of feed gas and treatment time on skim milk's sulfhydryl content, flow properties, and acid gelling behavior. Results showed that free sulfhydryl content decreased with treatment time, with a notable reduction observed after 2 min of N2-O2 plasma treatment. Skim milk treated with N2 plasma experienced a more gradual decrease in free SH content. Cold plasma increased skim milk viscosity over time. N2-O2 plasma treatment significantly affected G'40 and G'4 storage moduli, with an increase observed after 2 min of exposure but no change beyond that time. Acid gels' greenness (a* value) decreased with increasing treatment time compared to the control. Acid gel firmness of milk treated with N2-O2 plasma for 1 min significantly increased from 1.804 N to 1.912 N, and further to 2.072 N after 2 min of treatment. However, longer exposure times led to lower firmness in gels. N2 plasma treatment also significantly impacted acid gel firmness. Syneresis in acid gels decreased from 63.4 % to 57.7 % and 58.7 % after 1 and 2 min of N2-O2 plasma treatment, respectively, but increased to about 70 % after 4 min. Acid gels made from milk treated with N2 plasma experienced considerably less syneresis. The cold plasma treatment under different conditions significantly affected the properties of skim milk, with various impacts on sulfhydryl content, flow properties, and acid gelling behavior. These findings demonstrate the potential applications of cold plasma processing in the food industry to improve product properties.

Effect of milk pH at heating on protein complex formation and ultimate gel properties of free-fat yoghurt

The effect of milk pH before heating on casein-whey protein interactions and ultimate gel properties of the free-fat yoghurt was investigated. Reconstituted skim milk at different pH values (6.4, 6.8 and 7.2) was heated at 80 °C for 30 min. The type of protein and size of casein micelle in milk were determined. The storage modulus (G'), loss tangent (tan δ), flow behaviour as well as microstructure, firmness and water holding capacity of the yoghurt samples were measured. Heating milk at pH 7.2 formed mostly soluble protein complexes whereas at pH 6.4 micelle bound complexes was dominant. However, heating milk at pH 6.8 resulted in a relatively compact protein network due to a balanced contribution from both soluble protein/κ-casein complexes and whey protein-casein micelle associated complexes. Yoghurt prepared with milk heated at pH 6.8 showed significantly higher G' values, shorter gelation times, higher water holding capacity, firmness and more compact protein network compared to those at pH 6.4, 7.2 and unheated milk. The obtained results demonstrated that milk pH adjustment before heating could be an important factor governing uniform quality yoghurt production.

Effects of using different O2 scavengers on the qualitative attributes of bifidus yogurt during refrigerated storage

Among the factors that adversely influence the viability of probiotics, the oxygen content of the product and the permeation of oxygen molecules through the packaging system have a noticeable role in the viability loss during the manufacture and storage of fermented milk products. The objective of this study was to examine the qualitative attributes of probiotic yogurt containing different O₂ scavengers, including the commercial O₂ absorber and cysteine-ascorbic acid. Bifidobacterium lactis BIA-7 and B. longum BIA-8 were used as probiotic strains for the production of bio-yogurts. The biochemical parameters, including the changes in pH, titratable acidity, redox potential and incubation time, were determined throughout the fermentation period at 30-min intervals. Also, the changes in viable count, pH, redox potential, titratable acidity, and dissolved oxygen were evaluated at 7-day intervals during the 28 days of refrigerated storage. In addition, the evaluation of rheological and sensory properties measured in the freshly made samples was carried out. The results showed that the utilization of different oxygen scavengers has an effective impact on the decrement of oxygen content and improvement of probiotic viability. As such, the population of B. lactis in the treatments containing various oxygen scavengers was maintained above 7 log CFU/mL throughout the refrigerated storage. Notwithstanding the effective function of cysteine-ascorbic acid in the enhancement of viability, the containing treatments had not only weaker gel structure probably due to short incubation time (360 min) and fast acidification [22.20-22.35 (˚D/min) × 10^-2], but also lower sensory acceptance. Overall, the yogurt treatment containing commercial O₂ scavenger and B. lactis indicated a great potential for the industrial applications. To the best of our knowledge, there is no study on the efficiency of commercial O₂ absorber as a potential factor to maintain the viability of probiotics in yogurt.

Gelation of milks of different species (dairy cattle, goat, sheep, red deer, and water buffalo) using glucono-δ-lactone and pepsin

Dynamic low-amplitude oscillatory rheology was used to study the gelation properties of skim milk gels made at 37°C, using glucono-δ-lactone alone (acid gels) or a combination of glucono-δ-lactone and porcine pepsin ("combination gels"). The protein contents of the skim milks increased in the order goat milk < cattle milk < buffalo milk < sheep milk < deer milk, whereas the average casein micelle diameters increased in the order cattle milk < buffalo milk < goat milk < sheep milk ≃ deer milk. The gelation pH (4.55-4.73) of all milks were close to the isoelectric pH (4.6) of casein, except for buffalo milk, which had a significantly higher gelation pH (5.72). The storage moduli (G') of the acid gels increased with time in the milks of all species except for buffalo milk, for which a double peak in G' was observed. The final storage moduli after 6 h (G'_final) increased in the order goat milk < cattle milk < sheep milk < deer milk < buffalo milk. In general, for the combination gels, the G'_final values and the gelation pH increased to variable extents, except for goat milk. Confocal scanning laser microscopy showed that goat milk and cattle milk formed gels with more open protein networks compared with the dense clustered protein networks of the milks with high protein content (buffalo, sheep, and deer milks). This study indicates that milks from different species respond differently under the action of an acid precursor and pepsin. These results can be used to provide a better understanding of curd making and the digestion properties of noncattle milks.

Effects of High-Pressure, Microbial Transglutaminase and Glucono-δ-Lactone on the Aggregation Properties of Skim Milk

The object in this study is to investigate the effects of high pressure and freezing processes on the curdling of skim milk depending on the presence of transglutaminase (TGase) and glucono-δ-lactone (GdL). Skim milk was treated with atmospheric freezing (AF), high pressure (HP), pressure-shift freezing (PSF) and high pressure sub-zero temperature (HPST) processing conditions. After freezing and pressure processing, these processed milk samples were treated with curdling agents, such as TGase and GdL. Pressurized samples (HP, PSF and HPST) had lower lightness than that of the control. In particular, PSF had the lowest lightness (p<0.05). Likewise, the PSF proteins were the most insoluble regardless of whether they were activated by TGase and GdL, indicating the highest rate of protein aggregation (p<0.05). Furthermore, the TGase/GdL reaction resulted in thick bands corresponding to masses larger than 69 kDa, indicating curdling. Casein bands were the weakest in PSF-treated milk, revealing that casein was prone to protein aggregation. PSF also had the highest G' value among all treatments after activation by TGase, implying that PSF formed the hardest curd. However, adding GdL decreased the G' values of the samples except HPST-treated samples. Synthetically, the PSF process was advantageous for curdling of skim milk.