From sporeformers to sensory: Measures of bulk tank raw milk quality are affected by dairy farm characteristics and management practices.

The production of high-quality milk is key to the economic benefits of the dairy industry. It influences consumer consumption habits, enables the distribution of products to external markets, and reduces loss due to premature spoilage. Whereas processing milk through pasteurization and filtration can help improve quality, the microbial and chemical processes occurring within the raw milk before it reaches those steps will still have an effect on the acceptability of the finished products. Thus, improving the microbial, chemical, and sensory quality of raw milk direct from the farm is essential in manufacturing high-quality dairy products.

Milk quality has mostly been defined using somatic cell count (SCC) and total bacteria count (TBC). These parameters have well-defined regulatory standards which assist dairy farm management decisions and responses to elevated levels. Increased SCC is an inflammatory reaction in the udder, often resulting from infection. High SCC is correlated with higher levels of heat-stable proteases and lipases, which can produce off-flavours and cause protein and fat degradation. Whereas higher levels of TBC are not necessarily direct indicators of reduced product quality, they can be indicative of poor hygiene practices at the farm level. Additionally, some bacteria shown to be responsible for driving higher levels of TBC are also those that can produce heat-stable enzymes responsible for reduced quality, specifically Pseudomonas; milk flocculation being an example. Other microbial tests such as laboratory pasteurization count (LPC), coliform count (CC), and preliminary incubation count (PI) can also be used to determine the microbial quality of raw milk. For example, laboratory pasteurization counts act as a measure of thermoduric (heat resistant) bacteria capable of surviving vat pasteurization, coliform counts can be used for assessment of milking hygiene, and preliminary incubation counts to select for psychrophilic/psychro-tolerant (cold resistant) organisms that can proliferate under mild temperature–abused storage conditions. Other underused measures of quality include sensory judgment, for a more comprehensive perspective of quality. Sensory defects, however, are still underused to evaluate quality and to address issues on the dairy farm itself.

For further investigation, there were three primary objectives of the study cited: The first was to evaluate the quality of New York State bulk tank raw milk, both through traditional and novel measures, and establish benchmarking data that can be used by the dairy industry to make improvements in quality. The second was to develop models that can identify farm characteristics and management practices that may assist in improving spore levels as well as the sensory qualities of raw milk. The third was to help support guidance for the selection of specific testing parameters to assess raw milk quality and make management decisions.

The study was conducted over a 15-month period, with 100 conventional dairy farms enrolled, each sampled six times. The farms represented a variety of sizes, milking systems, and other farming practices. Samples were evaluated for 24 different quality parameters, covering both traditional (e.g., TBC, SCC, and chemical composition) and novel measures of quality (e.g., sensory defect evaluation, mesophilic spore counts [MSC], thermophilic spore counts [TSC], psychro-tolerant spore counts [PSC], and butyric acid bacteria [BAB]). Overall, 593 bulk tank raw milk samples were collected and analysed for microbial, physico-chemical, and sensory quality parameters. Alongside sample collection, a survey was done among farm owners and herd managers to obtain comprehensive data, including farm and milking characteristics, and parlour practices. Models were developed to identify factors that may influence milk quality, such as spore-formers and sensory characteristics

The results showed that New York State raw milk is of exceptional quality, with a mean and standard deviation (SD) for total bacteria count of 3.52 ± 0.70 log cfu/mL and a geometric mean for SCC of 133 000 cells/mL. Overall sensory scores were also high, with a mean and SD of 8.6 ± 1.4 on a 0 to 10 scale.  For spore-former levels, the mean and SD were 0.61 ± 0.60 log cfu/mL, 0.32 ± 0.60 log cfu/mL, 1.70 ± 0.60 log MPN/L, and 2.22 ± 0.60 log MPN/L for MSC, TSC, PSC, and BAB, respectively. For spore-former levels, the models most commonly identified herd size along with factors associated with udder health and hygiene (pre-dip usage, frequency of udder clipping or flaming, and the vacuum of the milking system) as variables of importance. For sensory parameters, herd size, time spent on pasture, and measures of milk composition, including overall butterfat, percentage of preformed fatty acids, and percentage of de novo fatty acids, were identified as variables of importance. Given the financial and time burden associated with quality testing, parameters must be carefully selected to maximize utility of data. Thus, correlation analysis between test results was performed to identify quality parameters that could signal issues in other parameters. Relationships of note included log TBC and log preliminary incubation count (r = 0.79), log MSC and log TSC (r = 0.71), as well as overall sensory score and proportion of oxidized samples (r = −0.77). The correlation results imply that only one of the parameters can be measured as the correlated one will respond accordingly.

Overall, the study provides information to establish a baseline that can be used by the dairy industry, not only in New York State but worldwide, to evaluate improvements in raw milk quality This will help to identify farming practices that have potential to impact the finished product, and to support the selection of parameters that may be used for quality monitoring. The outcomes of this study are certainly useful for quality improvement in the SA dairy industry going forward.