Standard practices for indirectly assessing the pasteurization status of milk products are primarily based on the thermal inactivation kinetics of the endogenous milk enzyme, alkaline phosphatase (ALP). This assessment provides an invaluable, if not required, tool for both regulatory and in-house process control and validation. Endogenous milk ALP manifests a slightly higher heat resistance than the pathogenic microflora upon which pasteurization time and temperature requirements are based. Hence, ALP activity is recognized and accepted as the method of choice for the rapid validation of milk product pasteurization. However, ALP assays have notable limitations that must be understood if they are to be administered and interpreted correctly and the results are to be applied judiciously. Issues such as the reactivation of heat-denatured ALP and the presence of both heat-stable and -labile microbial ALP are addressed. A discussion of ALP in the milk of nonbovine species is presented based on the limited literature available. Some discussion of research involving alternative pasteurization indicators also is presented. This article is intended to summarize the pertinent details of the ALP assay for dairy products (noting the basis and limitations of various methods) and the processing, handling, and known compositional factors that influence the assay results.
Aims: To identify potential pathways for citrate catabolism by Lactobacillus casei under conditions similar to ripening cheese. Methods and Results: A putative citric acid cycle (PCAC) for Lact. casei was generated utilizing the genome sequence, and metabolic flux analyses. Although it was possible to construct a unique PCAC for Lact. casei, its full functionality was unknown. Therefore, the Lact. casei PCAC was evaluated utilizing end‐product analyses of citric acid catabolism during growth in modified chemically defined media (mCDM), and Cheddar cheese extract (CCE). Results suggest that under energy source excess and limitation in mCDM this micro‐organism produces mainly l‐lactic acid and acetic acid, respectively. Both organic acids were produced in CCE. Additional end products include d‐lactic acid, acetoin, formic acid, ethanol, and diacetyl. Production of succinic acid, malic acid, and butanendiol was not observed. Conclusions: Under conditions similar to those present in ripening cheese, citric acid is converted to acetic acid, l/d‐lactic acid, acetoin, diacetyl, ethanol, and formic acid. The PCAC suggests that conversion of the citric acid‐derived pyruvic acid into acetic acid, instead of lactic acid, may yield two ATPs per molecule of citric acid. Functionality of the PCAC reductive route was not observed. Significance and Impact of the Study: This research describes a unique PCAC for Lact. casei. Additionally, it describes the citric acid catabolism end product by this nonstarter lactic acid bacteria during growth, and under conditions similar to those present in ripening cheese. It provides insights on pathways preferably utilized to derive energy in the presence of limiting carbohydrates by this micro‐organism.
The objective of this study was to characterize variation and interrelatedness among primary functional and compositional parameters of commercially available sweet whey powders. Samples representing different plants/processes and cheese types were assayed for foaming capacity, foam stability, pH, protein content, soluble protein, turbidity, color, particle size distribution, lipid, and moisture. Data were analyzed using principal component analysis. Foaming capacity and stability varied from 10 to 220% and 0.1 to 14 min, respectively. Protein content and solubility ranged from 8.5 to 17.6% and 3.7 to 14.1%, respectively. Lipid content of sweet whey powder varied from 0.03 to 2.00%. The two main functional properties, foaming and protein solubility, did not show significant correlation with each other. Foaming properties showed a positive correlation to particle size and L* or lightness value, and negative correlation to lipid content. Protein solubility showed positive correlation with protein content and negative correlation with turbidity of the sample. Foaming behavior, protein, and particle size attributes were the main variables responsible for grouping of samples. Sweet whey powders from the same dairy plants were grouped together. The direct or indirect significance of these relationships to processing is detailed in this study.
A fully computer-controlled apparatus was designed. It combines a glass reactor with a temperature-controlled hood, in which headspace volatiles are captured. Flavored liquids can be introduced into the reactor and exposed to conditions of temperature, air flow, shear rate, and saliva flow as they occur in the mouth. As the reactor is completely filled before measurements are started, creation of headspace just before sampling start prevents untimely flavor release resulting in real time data. In the first 30 s of flavor release the concentrations of the volatiles can be measured up to four times by on-line sampling of the dynamic headspace, followed by off-line trapping of the samples on corresponding Tenax traps and analysis using GC-TDS-FID. Flavor compounds from different chemical classes were dissolved in water to achieve concentrations typically present in food (micrograms to milligrams per liter). Most of the compounds showed constant release rates, and the summed quantities of each volatile of three 10 s time intervals correlated linearly with time. The entire method of measurement including sample preparation, release, sampling, trapping, thermodesorption, and GC analysis showed good sensitivity [nanograms (10 s)(-1)] and reproducibility (mean coefficient of variation = 7.2%).
Catabolism of sulfur-containing amino acids plays an important role in the development of cheese flavor. During ripening, cystathionine -lyase (CBL) is believed to contribute to the formation of volatile sulfur compounds (VSCs) such as methanethiol and dimethyl disulfide. However, the role of CBL in the generation of VSCs from the catabolism of specific sulfur-containing amino acids is not well characterized. The objective of this study was to investigate the role of CBL in VSC formation by Lactobacillus helveticus CNRZ 32 using genetic variants of L. helveticus CNRZ 32 including the CBL-null mutant, complementation of the CBL-null mutant, and the CBL overexpression mutant. The formation of VSCs from methionine, cystathionine, and cysteine was determined in a model system using gas chromatography-mass spectrometry with solid-phase microextraction. With methionine as a substrate, CBL overexpression resulted in higher VSC production than that of wild-type L. helveticus CNRZ 32 or the CBL-null mutant. However, there were no differences in VSC production between the wild type and the CBL-null mutant. With cystathionine, methanethiol production was detected from the CBL overexpression variant and complementation of the CBL-null mutant, implying that CBL may be involved in the conversion of cystathionine to methanethiol. With cysteine, no differences in VSC formation were observed between the wild type and genetic variants, indicating that CBL does not contribute to the conversion of cysteine.Lactobacillus helveticus CNRZ 32 is used as a starter and an adjunct bacterium to enhance cheese flavor development and reduce bitterness (15). Catabolism of amino acids, including sulfur-containing amino acids, by lactic acid bacteria is a major contributor to the development of flavor compounds in cheese during ripening (8). Sulfur-containing amino acids, namely, methionine, are precursors of aroma-active volatile sulfur compounds (VSCs) such as methanethiol, dimethyl disulfide, and dimethyl trisulfide (12, 16). There are two different microbial pathways potentially leading to amino acid conversion into flavor compounds: one is initiated by a transamination reaction while the other is initiated by an elimination reaction (29). The transamination pathway is catalyzed by aminotransferases, which transfer the amino acid amino group to an ␣-keto acid, while the elimination reaction-based pathway is catalyzed by the activity of amino acid lyases which cleave amino acid side chains (29).Cystathionine -lyase (CBL) (EC 4.4.1.8) is a pyridoxal-5Ј-phosphate (PLP)-dependent enzyme and was purified and cloned from Lactococcus lactis (1,14). CBL is involved in the ␣,-elimination of cystathionine to form homocysteine, pyruvate, and ammonia. CBL from L. lactis can also catalyze the conversion of methionine into methanethiol (1). A CBL overexpression variant of L. lactis has been shown to produce higher quantities of VSCs with methionine as a substrate compared with a wild strain (14), suggesting a possible mechanism to increase VSC product...
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