EFFECT OF COLD STORAGE ON CULTURE VIABILITY AND SOME RHEOLOGICAL PROPERTIES OF FERMENTED MILK PREPARED WITH YOGURT AND PROBIOTIC BACTERIA

ABSTRACT

We examined the effect of storage time on culture viability and some rheological properties (yield stress, storage modulus, loss modulus, linear viscoelastic region, structural recuperation and firmness) of fermented milk made with Lactobacillus delbrueckii ssp. bulgaricus, Lactobacillus acidophilus (LA) and Bifidobacterium animalis ssp. lactis in coculture with Streptococcus thermophilus (ST). Acidification profiles and factors that affect viability (postfermentation acidification, acidity and dissolved oxygen) were also studied during 35 days at 4C. Fermented milk prepared with a coculture of ST and Bifidobacterium lactis gave the most constant rheological behavior and the best cell viability during cold storage; it was superior to ST plus LA for probiotic fermented milk production.

PRACTICAL APPLICATIONS

Probiotic cultures should grow quickly in milk, provide adequate sensory and rheological properties to the product, and remain viable during storage. Commercially, it is very common to use yogurt starter culture (i.e. Streptococcus thermophilus[ST] and Lactobacillus delbrueckii ssp. bulgaricus) in combination with the probiotic bacteria in order to reduce fermentation time. However, LB tends to post acidify fermented milk, which reduces the viability of the probiotic bacteria; thus, it is recommended to use starter cultures devoid of this species. We found that the technological properties and the viability of the probiotic bacterium Bifidobacterium animalis ssp. lactis BL O4 in coculture with ST make it suitable for probiotic fermented milk production; it produces rheological characteristics similar to those of yogurt.     

Fatigue behaviour of tubular AlMgSi welded specimens subjected to bending–torsion loading

This paper is concerned with an experimental and numerical study of the fatigue behaviour of tubular AlMgSi welded specimens subjected to biaxial loading. In‐phase torsion–bending fatigue tests under constant amplitude loading were performed in a standard servo‐hydraulic machine with a suitable gripping system. Some tests in pure rotating bending with and without steady torsion were also performed. The influence of stress ratio R and bending–torsion stress ratio were analysed. Correlation of the fatigue lives was done using the distortion energy hypothesis (DEH), based on the local stresses and strains. The applicability of the local strain approach method to the prediction of the fatigue life of the welded tubular specimens was also investigated. Static torsion has only a slight detrimental influence on fatigue strength. The DEH (von Mises criterion) based on local stresses in the weld toes was shown to satisfactorily correlate fatigue lives for in‐phase multiaxial stress–strain states. The stress–strain field intensity predictions were shown to have less scatter and are in better agreement with the experimental results than the equivalent strain energy density approach.     

Comparative analysis of fatigue life predictions in central notched specimens of Al–Mg–Si alloys

The fatigue behaviour of an Al–Mg–Si alloy was studied using notched specimens. Fatigue tests were conducted at two stress ratios R= 0 and R= 0.4 on thin plates with a central hole. Constant and block variable loading amplitudes were applied to the specimens using a servo‐hydraulic machine. The applicability of the local strain approach method to the prediction of the fatigue life was investigated for this type of discontinuity. Two methods, the equivalent strain energy density approach and a modified stress–strain intensity field approach, were used to predict the fatigue strength. For the second one an elastic–plastic finite element analysis was carried out in order to obtain the local strain and stress distributions near the notch root. Based on Miner's rule an equivalent stress was used to correlate the fatigue lives for the variable amplitude histories. The experimental results were compared with the predicted results obtained by the two methods investigated and better agreement was found with the stress–strain field intensity approach, while the strain energy approach gave more conservative results. Miner's rule gives a good correlation between the variable amplitude and constant amplitude results.     

MANIOC FLOUR AS A METHIONINE CARRIER TO BALANCE COMMON BEAN-BASED DIETS

A study was carried out to evaluate (a) the possibility of utilizing maniac flour as a methionine carrier to supplement common beans protein which is deficient in the amino acid; (b) effects of variable amounts of methionine in different maniac-beans mixtures; and (c) possible undesirable effects of large amounts of methionine. Results showed that fortification of maniac flour with 0.2–0.4% methionine would improve the protein value of different maniac flour: beans mixtures. Undesirable effects of high amounts of methionine in the mixture were demonstrated, but these levels were higher than those expected to be consumed under normal conditions. Final analysis of results points up the possibility and advantage of utilizing maniac flour as a carrier of methionine and/or other nutrients lacking in the diets of maniac-eating populations.     

Detection of Enterotoxigenic Escherichia coil in Foods by DNA Colony Hybridization

A DNA colony hybridization procedure was used to identify and enumerate a heat-labile toxin-producing strain of Escherichia coli (H10407) in various types of foods. Foods were seeded with H10407 cells and examined by DNA hybridization on nitrocellulose filters with ³²P- labeled heat-labile toxin gene fragments. The number of cells recovered on plate count agar and eosine methylene blue agar was compared. With nitrocellulose filters, recoveries were about 81% on plate count agar and about 76% on eosine methylene blue agar. No significant differences in recovery were observed with ten different food types.     

The viability of three probiotic organisms grown with yoghurt starter cultures during storage for 21 days at 4°C

This study investigated the viability of probiotic ( Lactobacillus acidophilus LA5, Lactobacillus rhamnosus LBA and Bifidobacterium animalis subsp . lactis BL-04) in milk fermented with Lactobacillus delbrueckii subsp . bulgaricus LB340 and Streptococcus thermophilus TAO (yoghurt – Y). Each probiotic strain was grown separately in co-culture with Y and in blends of different combinations. Blends affected fermentation time(s), pH and firmness during storage at 4°C. The product made with Y plus B. animalis subsp . lactis and L. rhamnosus had counts of viable cells at the end of shelf life that met the minimum required to achieve probiotic effect. However, L. acidophilus and L. delbrueckii subsp . bulgaricus were inhibited.     

Evaluation of physicochemical,microbiological and sensorial characteristics of fermented milk beverages with buttermilk addition

This study evaluated three fermented milk beverages to which had been added sugar strawberry puree post‐fermentation. The base was composed of 70% of milk, with whey and buttermilk in the concentrations of 30% and 0%, 15% and 15%, and 0% and 30%, respectively. The starter culture developed well with all formulations reaching pH 4.7–4.9 in 180 min of fermentation. Lactic acid bacteria in the products were above 8 log cfu/mL throughout the study. The beverages presented similar pH, acidity and viscosity. Buttermilk and whey can be interesting ingredients to be added into fermented milk beverages, because the consumers liked all the products equally, which had an average acceptance score exceeding ‘liked moderately’.     

H2-optimal control of linear systems with tracking/disturbance rejection constraints

This paper considers the design of two-degree-of-freedom, linear multivariable controllers for four-block matrix transfer functions with objectives pertaining to both random and deterministic signals. The former are taken into account in the cost functional whereas the latter entail that the controller is constrained to solve a servomechanism problem involving a class of reference signals and two classes of disturbances. The cost functional is set up by adding quadratic terms to the usual LQG/Wiener-Hopf cost in such a way as to ensure the existence of (unique) solutions even when the signals to be tracked/rejected, as well as models, have poles on the imaginary axis. The solutions of the servomechanism problem are explicitly parametrized by free, stable, rational and proper parameters under two sets of assumptions: one which is necessary and sufficient for the existence of solutions and another, stronger set which avoids Kronecker products and the associated higher dimensional factorizations involved in the first case. On the basis of this parametrization the servomechanism constraint is incorporated into an unconstrained problem which is then quite explicitly solved.