Listeria spp. contamination prevalence in bulk-tank milk was notably low in this study. [7][12] reported that approximately 4–6% of Listeria spp. was prevalent in bulk tank milk. Contamination occurrence differs in each area. In Europe, the prevalence was as high as 45%, whereas prevalence was approximately 10% and 3% in North America and Asia, respectively [8][9][13]. Milk management and improper practices during milking and post-milking on the farm level (especially for small-size farms) could cause differences in these occurrences [8][14][15].
Additionally, bacteria could remain in the milk products in cases of improper cooling, cross-contamination during handling, packaging, or storage [16][17]. Listeria spp. can still exist in food even after refrigeration because the organism can survive at low temperatures [16]. L. monocytogenes appearance in raw milk was reported to spread from indigenous silage. Therefore, farmers should carefully consider hygiene practices and high-quality feeds [8][18].
The leukocytes in milk would degrade if the raw or improper pasteurized milk were stored at 4°C for over 3 days. As a result of this deterioration, the number of heat-resistant L. monocytogenes could increase [19]. Listeria spp. was not identified in the milk products from retailers. [10] reported approximately 18% Listeria spp. contamination in boiled milk. Listeria prevalence, even in pasteurized milk, ranged from 5 to 40% [20][21]. Raw milk consumption or improper processes was the cause of the contamination in ready-to-drink milk [10][22].
We cultured Staphylococcus spp. and Bacillus spp. in the milk from the vendor. The Bacillus spp. were heat-resistant species that could be killed in a wide range of temperatures and time [23] Bacillus spp. spores may resist heat treatment. Spores from some bacilli species can be isolated in milk, even after sterilization [23][24]. Additionally, spores are commonly found in the environment, including the soil, dust, air, and surfaces [25].
Staphylococcus spp. are normal microbiota on human skin and mucous membranes [26]; however, its contamination possibly resulted in cross-contamination. Many studies have elucidated the contamination of Staphylococcus spp., especially S. aureus, and its severity on human health [27][28][29]. Pasteurization products were identified as the source of S. aureus contamination. The incidence of S. aureus contamination in pasteurization was 4% in China, whereas that in South Africa was high up to 20% [30][31].
The location was associated with the milk volume of each lot that the vendor ordered and their pasteurization practices. The order volume is related to the number of dairy farms in the area (Fig. 2). Retailers in Bangkok may have a larger stock than other locations because of the transport limitations. The magnitude of milk treatment was unexpectedly related to location. People with awareness of the risk of milk-borne pathogens were twice less likely to contract abdominal illness than those who were unaware [32]. Consequently, the responsible organizations should spread knowledge on adequate milk treatment protocols.
We suspect that five milk-processing measures in the trial effectively destroyed L. monocytogenes because of the absence of identification after treatment. L. monocytogenes would be reduced to greater than log10 6.9 mL-1 after heating at 65.5°C for 15 s [19]. However, [33] reported that low temperature, long-time pasteurization at 63°C for 30 min and high temperature, short time could make a negligible log10 2 mL-1 reduction. This variation implied the possibility of failure in milk treatment. Low pathogen load following treatment could also be attributed to the lack of detectable Listeria spp. in the culture.
In the experiment, we isolated S. epidermidis, S. warneri, and E. coli, which may result from cross-contamination. Staphylococcus spp. is usually found on the skin and surfaces [25]. E. coli is a human pathogen that causes gastrointestinal illness. Typically, the source is the host intestine; however, E. coli can persist on surfaces [1][34]. Pasteurized milk was the source of up to 9% E. coli contamination [35]. Milk processed with inadequate measures presented approximately two-fold higher E. coli numbers than adequate treatment measures [36]. We, therefore, recommend that the post-pasteurization process is strictly hygienic [2][28][35][37].
However, our study has the following limitations. This study was a snapshot; therefore, the lack of Listeria spp. identification at a retailer level might be underestimated. The treating processes in trial were assumed from questionnaires. The temperature and time considered were not the exact practices followed by the sellers. Additionally, contamination could occur during inappropriate milk treatment and after the treatment process.