Abstract
The growth of Streptococcus thermophilus ST12 (ST12) in liquid milk, reconstituted from low-heat skim milk powder reconstituted skim milk (RSM) and in RSM with rennet addition (r-RSM) at 40°C was monitored by microcalorimetry. It was shown that the growth rate of bacteria decreased in renneted samples in comparison with liquid RSM starting from certain sizes of the colonies (“deviation moments”), which depended on the inoculation rates. The hydrolysis of lactose was delayed for about 1 h in the r-RSM in comparison with RSM but otherwise the metabolism of carbohydrates in the renneted and non-renneted milks was similar. The total free amino acids (TFAA) content by the end of fermentations was higher in r-RSM than in RSM presumably due to the enzymatic hydrolytic activity of rennet. The quantitatively dominating amino acids were remarkably different in the r-RSM and RSM indicating that the hydrolysis cascade of caseins and/or metabolism of amino acids by the bacteria functioned differently in the two cases. The data obtained showed potential of microcalorimetry to characterize quantitative differences of growth and metabolism of the bacteria in renneted and liquid samples of milk.
Highlights
Studies of growth of LAB in the liquid milk (Favrot and Maubois, 1994, 1996; Letort et al, 2002) and solid cheese are complicated because traditional microbiological methods are not well suited for the enumeration of bacteria in opaque and especially in solid media
The specific aim of the present study was to investigate the growth of Streptococcus thermophilus ST12 in renneted reconstituted skim milk (RSM) and compare the peculiarities of the growth with those in liquid milk samples
The growth curves of St. thermophilus ST12 in r-RSM and RSM were practically coinciding in the beginning, but starting from a certain number of the bacteria in the sample (N∗ = 0.63 × 107 CFU mL−1 at N0 = 105 CFU mL−1), at the “deviation moment” marked by the arrow in Figure 2 the maximal calorimetric growth rate in r-RSM
Summary
Studies of growth of LAB in the liquid milk (Favrot and Maubois, 1994, 1996; Letort et al, 2002) and solid cheese are complicated because traditional microbiological methods are not well suited for the enumeration of bacteria in opaque and especially in solid media. Studies of growth of LAB in solid coagulated milk samples are clearly less numerous (Favrot and Maubois, 1996; Floury et al, 2010, 2013; Jeanson et al, 2011). The most noticeable advantage of the calorimetric method in comparison with the other mostly invasive techniques is the possibility to follow the bacterial growth in opaque and/or solid environments like various food matrices monitoring the heat evolution without destroying or perturbing samples studied (Lobete et al, in press). The interactions of Streptococcus thermophilus and Lactobacillus bulgaricus were studied in milk using calorimetry in 1979 (Monk, 1979). Riva et al (1997) investigated shelf life of fresh milk using isothermal calorimetry for continuous monitoring of microbial growth. Gardea et al (2002) compared heat evolution by bacteria with the results of traditional plate counts assessing microbiological quality of milk undergone different treatments. Wadsö and Galindo (2009) compared the thermal power-time profiles of the fermentation of milk at two incubation temperatures using two different buttermilk cultures. Krišciunaite et al (2011) applied microcalorimetry to study the influence of H2O2 on the growth of thermophilic starter bacteria in UHT milk
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