Next Article in Journal
Systematic Review of Usnic Acid Extraction from Wild-Grown Lichen Biomass
Previous Article in Journal
A High-Speed 4-Tensor Computational Framework for the Solar Energy Prediction of Curved HAPS Photovoltaic Modules
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Applied Sciences
Volume 16
Issue 5
10.3390/app16052184
applsci-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Investigation of Microbiological Quality and Chemical Properties of Ready-to-Eat Milk Jam †

by
Selçuk Alan
1,*,
Gönül Damla Büyük
1,
Betül Adem
2 and
Gökçenur Çetin
2
1
Department of Food Hygiene and Production, Faculty of Veterinary Medicine, Kafkas University, 36100 Kars, Türkiye
2
Department of Food Hygiene and Production, Institute of Health Sciences, Kafkas University, 36100 Kars, Türkiye
*
Author to whom correspondence should be addressed.
†
This article is a revised and expanded version of a paper entitled “Investigation of Microbiological Quality and Chemical Properties of Ready-to-Eat Milk Jam”, which was presented at the Van Yüzüncü Yıl University 4th International Health Sciences Congress, held in Van, Türkiye, in 15–16 December 2025.
Appl. Sci. 2026, 16(5), 2184; https://doi.org/10.3390/app16052184
Submission received: 24 January 2026 / Revised: 12 February 2026 / Accepted: 20 February 2026 / Published: 24 February 2026
(This article belongs to the Section Food Science and Technology)
Browse Figures
Versions Notes

Abstract

Milk jam (dulce de leche) is a dairy product obtained by boiling milk with sugar, characterized by high sugar concentration and low water activity. Consumption of this product, which has unique color, taste, and consistency, has been increasing in Türkiye in recent years. However, scientific studies examining the microbiological and chemical quality of milk jam are quite limited. This study aimed to determine quality parameters of milk jams on the Turkish market, evaluate results against relevant standards and provide comprehensive information about current product quality. Twenty packaged milk jam samples from different brands were analyzed. Chemical analyses included pH value, acidity in terms of lactic acid, dry matter content, fat content, reducing sugar, total sugar, and sucrose. Microbiological analyses included counts of Total Mesophilic Aerobic Bacteria (TMAB), psychrophilic bacteria, yeast-mold, lactic acid bacteria, Enterobacteriaceae, coliform bacteria, and proteolytic bacteria. Relationships between obtained data were evaluated by Pearson correlation analysis. Chemical results showed pH 7.54 ± 0.63 (6.70–8.99), acidity 0.49 ± 0.19%, dry matter 62.42 ± 8.85%, and fat 1.51 ± 1.50%. Sugar analyses revealed reducing sugar 17.25 ± 6.16 g/100 mL, total sugar 37.88 ± 8.67 g/100 mL, and sucrose 19.59 ± 8.74 g/100 mL. Microbiological analyses detected TMAB in 50% of samples (4.51 ± 1.91 log CFU/g), yeast-mold in 45% (3.92 ± 1.26 log CFU/g), Enterobacteriaceae in 30% (3.15 ± 1.26 log CFU/g), coliforms in 25% (3.63 ± 1.06 log CFU/g), and proteolytic bacteria in 55% (4.21 ± 1.57 log CFU/g). Positive correlation was found between pH and sucrose (r = 0.466, p ≤ 0.05), and strong negative correlation between dry matter and proteolytic bacteria (r = −0.732, p ≤ 0.05). Significant standardization deficiencies and microbiological quality problems were identified in milk jam samples. High pH values and the presence of hygiene indicator microorganisms indicate the need for HACCP system implementation and increased hygiene controls in production.

Graphical Abstract

1. Introduction

Milk is a valuable food that plays a critical role in human nutrition, containing almost all the nutrients necessary for growth, development, and the maintenance of life. Many methods have been developed in food technology to benefit from the superior nutritional value of milk for longer periods and in more diverse ways. Among these methods, dairy products produced to increase the durability of milk and obtain different flavors hold an important place [1].
One of these products, milk jam (dulce de leche), is a traditional dairy product obtained by slowly boiling and thickening milk with sugar, which is very popular in South American countries such as Argentina, Brazil, and Uruguay [2,3]. With its unique color, taste, and consistency formed by the effect of Maillard reaction and caramelization, milk jam is consumed as a breakfast item and also has a wide range of use as a filling material in pastry products and as an ice cream sauce [3].
The production of milk jam is based on the principle of evaporating some of the water contained in the milk and increasing its durability by adding sugar. In traditional production, about 20% sugar is added to the milk, and the mixture is boiled under atmospheric pressure until it reaches a dry matter content of 68–70%. During production, alkalizing agents such as sodium bicarbonate can be used to prevent protein coagulation and to promote the Maillard reaction to achieve the desired color formation [3,4]. This production method extends the shelf life of milk, a perishable raw material, while also reducing storage costs. In addition, being richer in protein than other types of jam and its high energy content are important features that increase its nutritional value [1].
The quality of a food product is directly related to its nutritional value and consumer acceptability, as well as its microbiological and chemical safety. Although the high sugar concentration and low water activity (aw) value of milk jam prevent the growth of many microorganisms, factors such as inadequate hygienic conditions in the production process, the quality of the raw material, or improper storage conditions can lead to the presence of pathogenic or spoilage microorganisms in the product. Indeed, in a study conducted by [5], it was reported that pathogens such as Salmonella typhimurium, Listeria monocytogenes, Escherichia coli O157:H7, and Staphylococcus aureus, which were experimentally inoculated into milk jam, could survive in the product for up to 30 days. This situation poses a potential risk to public health.
Although the consumption of milk jam has been increasing in Türkiye in recent years, the number of scientific studies examining the microbiological and chemical quality of this product is quite limited. Therefore, revealing the current situation of milk jams sold on the market is of great importance both for the protection of consumer health and for creating a guide for producers to make quality production in accordance with standards.
The aim of this study is to determine the microbiological and chemical quality parameters of milk jams available on the Turkish market, to evaluate the results considering relevant standards and literature data, and to provide comprehensive information about the current quality of the products.
A preliminary version of this study was previously presented in conference form [6].

2. Materials and Methods

2.1. Materials

In this study, 20 original packaged milk jam samples of different commercial brands, obtained from various supermarkets and online sales stores in Türkiye, were used as material. The samples were brought to the laboratory under aseptic conditions and stored in a refrigerator at +4 °C until analysis.

2.2. Microbiological Analyses

For microbiological analyses, 25 g of each sample was weighed into sterile stomacher bags (Seward, Worthing, UK) in accordance with the procedures specified by the International Organization for Standardization (ISO). A total volume of 225 mL of 0.1% peptone water (Merck, Darmstadt, Germany) was added and homogenized in a stomacher (Bagmixer®, Interscience, Saint-Nom-la-Bretèche, France) for 2 min. From this initial 10−1 dilution, decimal dilutions up to 10−6 were made using the same diluent. Inoculations were made from each dilution onto the respective media using standard pour or spread plate methods. Total Mesophilic Aerobic Bacteria (TMAB) Count: Inoculation was performed by the pour plate method on Plate Count Agar (PCA) (Neogen Corporation NCM0010A, 620 Lesher Place, Lansing, MI, USA), and the plates were incubated at 30–35 °C for 48 h [7]. Psychrophilic Bacteria Count: Inoculation was performed by the pour plate method on PCA, and the plates were incubated at 7 °C for 10 days. Yeast and Mold Count: Inoculation was performed by the spread plate method on Dichloran Rose Bengal Chloramphenicol Agar (DRBC) (Chemsolute, 9677.0500, Th. Geyer GmbH & Co. KG, Renningen, Germany), and the plates were incubated at 25 °C for 5 days. Lactic Acid Bacteria Count: Inoculation was performed by the pour plate method on De Man, Rogosa and Sharpe Agar (MRS) (Chemsolute, 8761.0500, Th. Geyer GmbH & Co. KG, Renningen, Germany), and the plates were incubated at 30 °C for 72 h. Enumeration of Enterobacteriaceae Members: Inoculation was performed by the double-layer pour plate method on Violet Red Bile Glucose Agar (VRBG) (Neogen Corporation NCM0041A, 620 Lesher Place, Lansing, MI, USA), and the plates were incubated at 37 °C for 24 h [8]. Coliform Group Bacteria Count: Inoculation was performed by the double-layer pour plate method on Violet Red Bile Agar (VRB) (Biokar Diagnostics BK152HA, Rue des 40 Mines, Allonne, France), and the plates were incubated at 37 °C for 24 h [9]. Proteolytic Bacteria Count: Inoculation was performed by the spread plate method on Calcium Caseinate Agar (Condalab 1069.00, Torrejon de Ardoz, Madrid, Spain), and the plates were incubated at 30 °C for 28–72 h.

2.3. Chemical Analyses

Following the microbiological analyses, pH measurements, acidity in terms of lactic acid, dry matter (%), fat (%), reducing sugar (g/100 mL), total sugar (g/100 mL), and sucrose (g/100 mL) determinations were performed for the samples. pH Value: The pH value of the samples was measured at 25 °C using a digital pH meter (Hanna Instruments, Woonsocket, RI, USA) after homogenizing 10 g of the sample in 90 mL of distilled water. Titration Acidity: The acidity of the samples was calculated in terms of % lactic acid by titrating 10 g of the sample mixed with 10 mL of distilled water with 0.1 N NaOH in the presence of phenolphthalein indicator until a pink color was formed [10]. Dry Matter Content: Dry matter determination was carried out by the gravimetric method by drying in an oven (Memmert GmbH + Co. KG, Schwabach, Germany) at 102 ± 2 °C until a constant weight was reached [11]. Fat Content: Fat determination was determined by the modified Gerber method for dairy products [12]. Sugar Determinations (Reducing Sugar, Total Sugar, Sucrose): Sugar analyses were performed by the Lane-Eynon general volumetric method using Fehling’s solution. For the determination of total sugar, titration was performed after an inversion process with hydrochloric acid (HCl), and the amount of sucrose was calculated from the difference between total sugar and reducing sugar, multiplied by a factor of 0.95 [13].

2.4. Statistical Analyses

The microbiological data obtained were converted to logarithms, and the mean and standard deviation values were determined. The mean and standard deviation values were also determined for the results of the chemical analyses. In this study, a correlation analysis was performed to reveal the relationships between the groups of microorganisms because of the microbiological analyses, and the relationships of the chemical data with each other and with the microbiological data. Pearson correlation analysis was used to determine the correlation coefficients. The SPSS 26 package program was used for statistical analyses (SPSS statistics software, version 26.0, IBM Corp., Armonk, NY, USA).

3. Results

The chemical analysis results of the 20 milk jam samples examined are summarized in Table 1, the Pearson correlation matrix of chemical parameters is shown in Figure 1, the box-plot graph in Figure 2, and the histogram graph in Figure 3. The average pH value of the samples was determined to be 7.54 ± 0.63, the acidity value (in terms of % lactic acid) was 0.49 ± 0.19, the dry matter content was 62.42 ± 8.85%, and the fat content was 1.51 ± 1.50%. As a result of the sugar analyses, the average amount of reducing sugar was found to be 17.25 ± 6.16 g/100 mL, the total sugar amount was 37.88 ± 8.67 g/100 mL, and the sucrose amount was 19.59 ± 8.74 g/100 mL.
The microbiological analysis results of the milk jam samples are shown in Table 2. TMAB was detected in 10 (50%) of the 20 samples examined, and the average count in the positive samples was found to be 4.51 ± 1.91 log CFU/g. Psychrophilic bacteria were not detected in any of the samples (<1.00 log CFU/g). Yeast-mold was found to be positive in 9 samples (45%) and was detected at an average level of 3.92 ± 1.26 log CFU/g. Lactic acid bacteria gave positive results in 3 samples (15%) and were detected at an average of 4.33 ± 1.86 log CFU/g. Among the hygiene indicator microorganisms, Enterobacteriaceae members were found to be positive in 6 samples (30%) with an average of 3.15 ± 1.26 log CFU/g, and coliform group bacteria were found to be positive in 5 samples (25%) with an average of 3.63 ± 1.06 log CFU/g. The number of proteolytic bacteria was detected in more than half of the samples, in 11 samples (55%), with an average of 4.21 ± 1.57 log CFU/g.
The distribution analysis of the microorganism counts detected in the milk jam samples is shown in Table 3, and the positivity distributions are shown in Figure 4. When evaluated in terms of TMAB, no microorganisms were detected in 50% of the samples (<1.00 log CFU/g), while microorganism counts were found in the range of 2.00–3.99 log CFU/g in 20%, 4.00–5.99 log CFU/g in 20%, and 6.00–7.99 log CFU/g in 10%. In the yeast-mold distribution, they were not detected in 55% of the samples, while they were found in the range of 2.00–3.99 log CFU/g in 25% and 4.00–5.99 log CFU/g in 20%.
While the absence of Enterobacteriaceae in 70% of the samples is considered a positive finding, their detection in the range of 2.00–3.99 log CFU/g in 25% is noteworthy. Coliform group bacteria showed a similar distribution and were not detected in 75% of the samples. Proteolytic bacteria were not detected in 45% of the samples, while they were detected in the range of 2.00–3.99 log CFU/g in 30%, 4.00–5.99 log CFU/g in 15%, and 6.00–7.99 log CFU/g in 10%.
The results of the Pearson correlation analysis performed to determine the relationships between the microbiological and chemical parameters of the milk jam samples are shown in Table 4. According to the analysis results, a significant positive correlation was found between pH and sucrose (r = 0.466, p ≤ 0.05). A significant negative correlation was found between acidity and total sugar (r = −0.509, p ≤ 0.05).
A significant negative correlation was found between dry matter and fat (r = −0.447, p ≤ 0.05), a significant positive correlation between dry matter and total sugar (r = 0.497, p ≤ 0.05), and a significant negative correlation between dry matter and proteolytic bacteria (r = −0.732, p ≤ 0.05).
Among the sugar parameters, a strong positive correlation was found between total sugar and sucrose (r = 0.764, p ≤ 0.01). When evaluated in terms of microbiological parameters, strong positive correlations were found between reducing sugar and lactic acid bacteria (r = 0.824, p ≤ 0.05) and coliform bacteria (r = 0.908, p ≤ 0.05).
Microbiological parameters showed strong statistical correlations among themselves. Very high correlation coefficients were observed between yeast-mold and lactic acid bacteria (r = 0.997, p ≤ 0.05), lactic acid bacteria and Enterobacteriaceae (r = 1.000, p ≤ 0.01), Enterobacteriaceae and coliform bacteria (r = 1.000, p ≤ 0.01), and lactic acid bacteria and proteolytic bacteria (r = 1.000, p ≤ 0.01). However, these coefficients should be interpreted with caution. Microbial counts were generally low and narrowly distributed among samples; this may have resulted in very high (excellent) correlation values. Therefore, these relationships are likely to reflect parallel distribution patterns under similar hygienic conditions rather than direct biological or mechanistic relationships.

4. Discussion

4.1. Evaluation of Chemical Quality

The pH value of milk jam has a significant effect on the stability, shelf life, and sensory properties of the product. In our study, the pH values of the samples varied over a wide range, from 6.70 to 8.99, with an average of 7.54 ± 0.63. In the literature, the pH values of milk jams are generally reported to be between 5.70 and 6.28 [2,14]. Gaze et al. (2015) [15], in their study characterizing samples of commercial milk jam (Dulce de Leche) in the Brazilian market, reported that the pH values of the products ranged from 6.14 to 6.37. The researchers emphasized that this wide variation observed in physicochemical parameters stemmed from the lack of standardization in operational procedures and production parameters at milk processing plants. The study concluded that these operational parameters should be strictly monitored and controlled during manufacturing to improve the intrinsic quality and safety of the product. Martin et al. (2021) [16] reported that Bacillus weihenstephanensis causes coagulation in milk, Clostridium tyrobutyricum causes gas formation (delayed swelling) in cheeses, and that low pH values limit these types of microbial spoilage. In our study, the fact that the pH value was above 8.0 in 30% of the samples constitutes a critical risk factor for the proliferation of these microorganisms. Sulejmani et al. (2021) [17] reported that pH values in milk jam samples ranged from 6.23–6.82, and that samples with a pH level of 6.5–6.8 were particularly appealing sensorially, offering the desired characteristic dark color. The pH values observed in this study (mean 7.54, maximum approaching 9.0) are markedly higher than those commonly reported for concentrated dairy products. Although alkalization is used in dulce de leche manufacture to promote the Maillard reaction and improve color development, excessive pH modification may result in important physicochemical and technological consequences. Higher pH alters the ionization of milk proteins, including caseins and whey proteins, thereby affecting stability, aggregation, and texture. In highly concentrated systems such as milk jam, where heating and evaporation are intensive, deviations from the optimal range may further influence protein interactions and the final structure of the product.
In addition, acidity–alkalinity balance is a critical intrinsic factor influencing microbial growth. While the high sugar content of milk jam lowers water availability and creates an inhibitory osmotic environment, more alkaline conditions weaken the protective effect of acidity. Therefore, when hygienic control is insufficient, a shift toward alkalinity may favor the survival of certain microorganisms. For these reasons, pH values exceeding 8.0 should not be interpreted as inherently acceptable technological conditions but may indicate excessive use of alkalizing agents or insufficient process control. These findings underscore the importance of carefully regulating alkalization practices and implementing continuous pH monitoring during production to ensure product stability, sensory quality, and microbiological safety. The positive correlation between pH and sucrose (r = 0.466, p ≤ 0.05) reflects the effect of the use of alkalizing agents on the sugar composition (Figure 5). The increase in sucrose concentration at high pH values suggests that the use of sodium bicarbonate in the production process affects sugar hydrolysis.
Acidity is a parameter that provides information about the degree of fermentation and freshness in dairy products. The acidity value of our samples, in terms of lactic acid, was found to be 0.49 ± 0.19 on average. The negative correlation between acidity and sucrose (r = −0.509, p ≤ 0.05) indicates that as the sugar concentration increases, the acidity decreases (Figure 6). This can be explained by the buffering effect of sugar and the change in the acid–base balance during the production process. Akal et al. (2018) [14] similarly reported that as the sugar ratio increased, the titration acidity decreased. The values in our study vary depending on the ratio of milk and sugar used in production.
Dry matter content directly affects the consistency, texture, and water activity of milk jam. In our study, the dry matter content was found to be in a very wide range, from 49.07% to 78.47%, with an average of 62.42 ± 8.85%. Considering that the target dry matter content in traditional milk jam production is approximately 70% [1,18], there is a severe lack of production standardization among the examined samples. A low dry matter content increases the risk of microbiological spoilage by increasing the water activity of the product, while a very high dry matter content can cause undesirable textural changes such as crystallization (sandiness). The negative correlation between dry matter and fat (r = −0.447, p ≤ 0.05) indicates that the use of skim or low-fat milk increases the dry matter concentration. This finding reveals that producers use milk with different fat contents and that this affects the composition of the final product. This relationship shows that a high dry matter content reduces water activity, thereby inhibiting the growth of proteolytic bacteria. Reducing water activity in foods effectively limits microbial proliferation by disrupting the essential physiological activities required for cell division in microorganisms. Beuchat et al. (2013) [19] found that the absolute minimum water activity threshold for microbial growth is 0.60; this limit is approximately 0.87 for most bacterial species and can drop to 0.83–0.85 for xerotolerant pathogens such as Staphylococcus aureus.
The average fat content was found to be 1.51 ± 1.50%. This value may vary depending on the fat content of the milk used in production. The use of skim or semi-skimmed milk by some producers may be a reason for this wide variation in fat content. Akal et al. (2018) [14] reported higher fat contents in jams produced from milk with 3.65% fat in their studies.
Sugar concentration is one of the most important properties of milk jam. A high sugar content not only gives the product its sweetness but also plays a key role in ensuring microbiological stability by reducing water activity. In our study, the average total sugar content was found to be 37.88 g/100 mL. While this value contributes to the durability of the product, it also indicates a high calorie content from a nutritional perspective.
The relationships observed between sugar parameters and microbiological counts (Table 4) should be interpreted cautiously. Although positive correlations were detected between reducing sugars and certain microbial groups, this does not indicate a direct growth-promoting effect of sugars in milk jam. In high-solids dairy systems, microbial behavior is primarily influenced by dry matter content and processing conditions rather than sugar concentration alone. While simple sugars may act as metabolic substrates, overall microbial proliferation depends on the combined effects of formulation, concentration level, and hygiene control. Considering the limited sample size and the relatively low microbial counts, these associations should be regarded as exploration rather than causal.

4.2. Evaluation of Microbiological Quality

The Total Mesophilic Aerobic Bacteria (TMAB) count is a general indicator of the overall microbiological quality of a food, as well as the production and storage conditions. The detection of TMAB in 50% of the samples and the average count of 4.51 log CFU/g in positive samples suggest that hygienic conditions were not adequately followed during the production process or that the initial microbial load of the raw material (raw milk) may have been high. The microorganism distribution analysis presented in Table 3 provides a detailed profile of the microbiological quality of the milk jam samples. According to the dairy product quality criteria proposed by Martin et al. (2023) [20], the TMAB count is the most important indicator of the overall microbiological quality of the product. In particular, the high TMAB levels detected in 2 samples (>6.00 log CFU/g) point to serious hygiene deficiencies. Furthermore, in the study by Fusco et al. (2020) [21] state that in the dairy industry, microbiological quality is assessed as a standard using total mesophilic counts (TMAB) and enterobacteria, and the upper legal limit set for high-quality raw milk is 100.000 CFU/mL (5.0 log CFU/mL). Although the long boiling process in milk jam production destroys a large portion of vegetative microorganisms, heat-resistant spore-forming microorganisms or contamination that occurs after heat treatment (cooling, packaging) can increase this count. The fact that 50% of the TMAB distribution is below the detectable level indicates that half of the samples are at a microbiologically acceptable level.
Yeasts and molds are the main microorganisms that can cause spoilage in foods with high sugar and low water activity. The detection of microorganisms at a detectable level in 45% of the samples in the yeast-mold distribution shows that milk jam provides a suitable environment for these microorganisms despite its high sugar content. In particular, the detection of yeast-mold in the range of 4.00–5.99 log CFU/g in 20% of the samples poses a risk to the shelf life and sensory quality of the product (Table 2). This situation may be a result of post-packaging contamination or improper storage conditions. In a comprehensive review by Shi and Maktabdar (2022) [22], the main causes of yeast-mold contamination in dairy products include insufficient heat treatment, post-packaging contamination, and improper storage conditions. Yeast-mold contamination can lead not only to sensory quality degradation but also to the risk of mycotoxin production. In particular, Aspergillus and Penicillium spp. can produce toxic metabolites such as aflatoxins and ochratoxins in environments with low water activity.
The detection of Enterobacteriaceae at a rate of 30% and coliform bacteria at a rate of 25% in our study indicates significant hygiene deficiencies in the production and processing stages. According to the data reported by Hervert et al. (2017) [23], these microorganism groups are among the most reliable hygiene indicators in dairy products. These microorganisms are generally indicators of fecal contamination, inadequate sanitation, or cross-contamination. Although parameters such as low water activity and heat treatment limit microbial proliferation in dairy products, high levels of indicators such as TMAB, E. coli, and yeast-mold directly reflect the risk of pathogens such as Listeria monocytogenes due to inadequate sanitation or cross-contamination after packaging, as well as general production hygiene deficiencies [24]. What is particularly alarming is the detection of these hygiene indicators in the range of 2.00–3.99 log CFU/g. Enterobacteriaceae are widely recognized as process hygiene indicators in dairy products and are particularly useful for identifying post-heat treatment contamination associated with inadequate sanitation, equipment hygiene, or handling practices. According to the Algerian microbiological specifications for foodstuffs (Official Journal No. 39, 2017), the acceptable limit for Enterobacteriaceae in pasteurized milk and other pasteurized dairy products is ≤10 CFU/mL (1.0 log CFU/mL) [25]. Although milk jam differs from pasteurized milk in terms of formulation, owing to its high sugar content and reduced water activity, specific microbiological criteria for milk jam products are currently lacking. In addition to Enterobacteriaceae, Total Mesophilic Aerobic Bacteria (TMAB) and coliform counts are widely used as general indicators of microbiological quality and hygienic performance in dairy products. According to Regulation (EC) No. 2073/2005 [26] and the Turkish Food Codex Microbiological Criteria Regulation [27], Enterobacteriaceae is defined as a process hygiene criterion for certain pasteurized dairy products, whereas no product-specific numerical limits are currently established for TMAB or coliforms in ready-to-eat milk jam. Therefore, the elevated levels observed in some samples were interpreted as indicators of inadequate hygienic control and process variability rather than as direct evidence of statutory non-compliance specific to this product category.
In the present study, Enterobacteriaceae were detected in 30% of the analyzed samples, with concentrations ranging from 2.00 to 3.99 log CFU/g, clearly exceeding the reference limit. This level of contamination suggests that the thermal processing step alone was insufficient to ensure microbiological hygiene, most likely due to post-processing contamination occurring during cooling, filling, or packaging stages. Although high sugar content and low water activity may limit microbial growth, the presence of Enterobacteriaceae at these levels indicates inadequate hygienic control and may represent a potential microbiological safety concern, particularly in the context of ready-to-eat products. A strong positive correlation (r = 0.908, p ≤ 0.05) was observed between reducing sugar content and coliform counts (Figure 7). Coliform bacteria can utilize simple sugars as metabolic substrates. However, in high-solids dairy systems, microbial growth is primarily influenced by water activity and processing conditions rather than sugar concentration alone. Given the limited sample size and the narrow distribution of microbial counts, this association should be interpreted cautiously and considered exploratory rather than causal.
In the study by Ahansaz et al. (2023) [28], it was reported that lactic acid bacteria used under controlled conditions showed a biopreservative effect, but their uncontrolled growth negatively affected product quality. The positive correlation between lactic acid bacteria and reducing sugar (r = 0.824, p ≤ 0.05) indicates that these bacteria produce acid through sugar metabolism. This can lower the pH of the product, inhibiting the growth of some pathogenic microorganisms. However, excessive acid production can negatively affect sensory quality.
The detection of proteolytic bacteria in 55% of the samples is a significant risk factor for the protein quality of milk jam. Proteolytic bacteria hydrolyze proteins by secreting protease enzymes, leading to the formation of undesirable odors and tastes in products [29]. This not only spoils the sensory quality but also negatively affects the nutritional value of the product. The negative correlation between dry matter and proteolytic bacteria (r = −0.732, p ≤ 0.05) (Figure 8) demonstrates the effectiveness of water activity control in limiting the growth of these bacteria. Proteolytic activity increases, especially during storage. The proteases produced by these bacteria are heat-resistant and can remain active even after pasteurization. Therefore, the prevention of proteolytic bacteria contamination is directly related to raw material quality and production hygiene.
Hentges et al. (2010) [5] stated that milk jam can be a suitable environment for pathogens, and therefore spoilage microorganisms can also grow. The strong positive correlations between microbiological parameters indicate that the sources of contamination are common and that hygiene deficiencies affect more than one group of microorganisms.

5. Conclusions

This study has revealed that milk jam samples sold on the Turkish market show significant differences and deficiencies in terms of their chemical and microbiological qualities. The lack of standardization in chemical composition, especially in dry matter and pH values, and the inadequacy in microbiological quality indicate that production processes are not sufficiently controlled and that hygienic conditions are not fully ensured.
According to the WHO (2015) report [30], the global and regional burden of foodborne illnesses is quite significant and seriously affects individuals of all ages, especially children under 5 and those living in low-income areas. Considering the increasing consumption of milk jam in Türkiye, the potential cost of quality control deficiencies could reach significant dimensions.
In line with the results obtained, the following recommendations can be made:
Based on the specific quality defects identified in this study, targeted and operable improvement measures can be proposed. Regarding the elevated pH values observed in some samples (pH > 8.0), it is recommended that producers strictly control the addition of alkalizing agents, such as sodium bicarbonate, and establish a real-time pH monitoring mechanism during production to prevent excessive alkalization and ensure product consistency.
The substantial variation in dry matter content (49.07–78.47%) indicates insufficient control of the concentration process. To address this issue, unified production process parameters, including boiling temperature and boiling time, should be defined and standardized. These measures would help maintain the dry matter content of milk jam products within a technologically reasonable and stable range (e.g., 68–70%).
In addition, the high detection rate of proteolytic bacteria (55%) suggests potential deficiencies in raw milk quality and sanitation practices. Strengthening raw material selection, improving cleaning and disinfection procedures, and implementing Good Manufacturing Practices (GMP) and Hazard Analysis and Critical Control Point (HACCP) systems are recommended to minimize proteolytic activity and associated quality deterioration in ready-to-eat milk jam products.
Raw Material Quality: The microbiological quality of the raw milk to be used in production should be high and regularly checked. The use of milk with a high bacterial load will negatively affect the quality and safety of the final product.
Inspection and Legislation: There is no specific food legislation for milk jam in Türkiye. The creation of a standard for this product, including chemical and microbiological limits, will both guide producers and protect consumers. For the protection of food safety and public health, it is of great importance to inspect the compliance of milk jams sold on the market with the relevant general food legislation and to increase training for producers.
In conclusion, improving the quality and safety of traditional products such as milk jam will both protect consumer health and increase the economic value of these products. Further studies on this subject will contribute to the improvement of production technologies and the development of quality standards.

Limitations

This study has several limitations that should be considered when interpreting the results. First, the relatively limited sample size may have influenced the robustness of some statistical analyses, particularly the reliability of correlation relationships among chemical and microbiological parameters. Increasing the sample size in future studies would improve statistical power and strengthen the validity of such analyses. Second, the samples were collected from a limited number of production sources, which may not fully represent the variability of ready-to-eat milk jam products available across different regions of Türkiye. Future research should expand geographical coverage and monitor batch-to-batch variations within the same brands to better evaluate product consistency and quality stability. Finally, while this study focused on indicator and spoilage microorganisms, specific foodborne pathogens were not included in the microbiological analyses. In future studies, the inclusion of pathogens such as Salmonella spp. and Listeria monocytogenes would contribute to a more comprehensive assessment of the microbiological safety of ready-to-eat milk jam products.

Author Contributions

Conceptualization, S.A.; methodology, S.A.; software, S.A.; validation, S.A.; formal analysis, S.A.; investigation, S.A., G.D.B., B.A. and G.Ç.; resources, S.A. and G.D.B.; data curation, S.A., G.D.B., B.A. and G.Ç.; writing—original draft preparation, S.A.; writing—review and editing, S.A. and G.D.B.; visualization, S.A.; supervision, S.A.; project administration, S.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

We would like to express our heartfelt gratitude to the experts who supported the creation of the study’s evaluative framework and to all participants who contributed to this research.

Conflicts of Interest

Authors declare no competing financial interests or personal relationships that could potentially affect outcomes reported in this manuscript.

References

  1. Tuna, C.; Arslan, S. Production Methods and Properties of Milk Jam. Akad. Gıda 2016, 14, 204–208. [Google Scholar]
  2. Ranalli, N.; Andrés, S.C.; Califano, A.N. Physicochemical and Rheological Characterization of ‘Dulce de Leche’. J. Texture Stud. 2012, 43, 115–123. [Google Scholar] [CrossRef]
  3. Tavares Filho, E.R.; Pimentel, T.C.; Silva, R.; Praxedes, C.I.S.; Oliveira, J.M.S.; Prudêncio, E.S.; Felix, P.H.C.; Neta, M.T.S.L.; Silva, P.H.F.; Mársico, E.T.; et al. Inulin or Xylooligosaccharide Addition to Dulce de Leche Affects Consumers’ Sensory Experience and Emotional Response. Food Res. Int. 2025, 200, 115492. [Google Scholar] [CrossRef] [PubMed]
  4. Rodríguez, A.; Lema, P.; Bessio, M.I.; Moyna, G.; Panizzolo, L.A.; Ferreira, F. Isolation and Characterization of Melanoidins from Dulce de Leche, a Confectionary Dairy Product. Molecules 2019, 24, 4163. [Google Scholar] [CrossRef] [PubMed]
  5. Hentges, D.; da Silva, D.T.; Dias, P.A.; da Conceição, R.D.C.S.; Zonta, M.N.; Timm, C.D. Pathogenic Microorganism Survival in Dulce de Leche. Food Control 2010, 21, 1291–1293. [Google Scholar] [CrossRef]
  6. Alan, S.; Büyük, G.D.; Adem, B.; Çetin, G. Investigation of microbiological quality and chemical properties of ready-to-eat milk jam. In Proceedings of the Van Yüzüncü Yıl University 4th International Health Sciences Congress, Van, Türkiye, 15–16 December 2025; Available online: https://webportal.yyu.edu.tr/vusak2025 (accessed on 11 February 2026).
  7. Maturin, L.; Peeler, J.T. Bacteriological Analytical Manual: Chapter 3, Aerobic Plate Count; U.S. Food and Drug Administration: College Park, MD, USA, 2001. [Google Scholar]
  8. Cirolini, A.; Miotto, M.; Machado, F.M.; Silva, H.S.D.; Ogliari, P.J.; Vieira, C.R.W. Evaluation of the Petrifilm™ EB and TEMPO® EB Systems with ISO 21528-2:2004 Method for the Count of Enterobacteriaceae in Milk. Braz. J. Microbiol. 2013, 44, 771–775. [Google Scholar] [CrossRef]
  9. Bird, P.; Bastin, B.; Klass, N.; Crowley, E.; Agin, J.; Goins, D.; Bakken, H.; Lingle, C.; Schumacher, A. Evaluation of the 3M™ Petrifilm™ Rapid E. coli/Coliform Count Plate for the Enumeration of E. coli and Coliforms: Collaborative Study, First Action 2018.13. J. AOAC Int. 2020, 103, 513–522. [Google Scholar] [CrossRef]
  10. Alan, S.; Akgöl, M.; Öksüztepe, G. Some quality parameters of unpackaged raw milk sold in Elazığ. FU Sağ. Bil. Vet. Derg. 2023, 37, 209–216. [Google Scholar]
  11. Williams, S. (Ed.) Official Methods of Analysis of the Association of Official Analytical Chemists, 14th ed; Association of Official Analytical Chemists: Rockville, MD, USA, 1984. [Google Scholar]
  12. Andrade, E.H.P.; da Fonseca, L.M.; de Souza, M.R.; Penna, C.F.D.A.M.; Cerqueira, M.M.O.P.; de Oliveira Leite, M. Fat Content in Fermented Milk Beverages: Determination by the Gerber Method. Semin. Ciênc. Agrár. 2022, 43, 441–448. [Google Scholar] [CrossRef]
  13. Da Silva, L.C.A.; Junior, J.A.L.; Leite, M.O.; Fontes, E.A.; Coimbra, J.S. Comparative Appraisal of HPLC, Chloramine-T and Lane-Eynon Methods for Quantification of Carbohydrates in Concentrated Dairy Products. Int. J. Dairy Technol. 2020, 73, 795–800. [Google Scholar] [CrossRef]
  14. Akal, C.; Buran, İ.; Delialioğlu, R.A.; Yetişemiyen, A. The Effect of Different Sugar Ratios on the Quality Properties of Milk Jam. GIDA 2018, 43, 865–875. [Google Scholar] [CrossRef]
  15. Gaze, L.V.; Costa, M.P.; Monteiro, M.L.G.; Lavorato, J.A.A.; Júnior, C.C.; Raices, R.S.L.; Cruz, A.G.; Freitas, M.Q. Dulce de Leche, a Typical Product of Latin America: Characterisation by Physicochemical, Optical and Instrumental Methods. Food Chem. 2015, 169, 471–477. [Google Scholar] [CrossRef]
  16. Martin, N.H.; Torres-Frenzel, P.; Wiedmann, M. Invited Review: Controlling Dairy Product Spoilage to Reduce Food Loss and Waste. J. Dairy Sci. 2021, 104, 1251–1261. [Google Scholar] [CrossRef] [PubMed]
  17. Sulejmani, E.I.; Boran, O.S.; Coşkun, A.; Hayaloglu, A.A. Physicochemical, Sensorial and Rheological Characterisation of Whole-Fat or Low-Fat Milk Jams as Influenced by Calcium Chloride, Sodium Bicarbonate and Sucrose Content. Int. J. Food Sci. Technol. 2021, 56, 4455–4464. [Google Scholar] [CrossRef]
  18. Giménez, A.; Ares, G.; Gámbaro, A. Consumer Reaction to Changes in Sensory Profile of Dulce De Leche Due to Lactose Hydrolysis. Int. Dairy J. 2008, 18, 951–955. [Google Scholar] [CrossRef]
  19. Beuchat, L.R.; Komitopoulou, E.; Beckers, H.; Betts, R.P.; Bourdichon, F.; Fanning, S.; Joosten, H.M.; Kuile, B.H. Low-Water Activity Foods: Increased Concern as Vehicles of Foodborne Pathogens. J. Food Prot. 2013, 76, 150–172. [Google Scholar] [CrossRef]
  20. Martin, N.H.; Evanowski, R.L.; Wiedmann, M. Invited Review: Redefining Raw Milk Quality—Evaluation of Raw Milk Microbiological Parameters to Ensure High-Quality Processed Dairy Products. J. Dairy Sci. 2023, 106, 1502–1517. [Google Scholar] [CrossRef]
  21. Fusco, V.; Chieffi, D.; Fanelli, F.; Logrieco, A.F.; Cho, G.S.; Kabisch, J.; Böhnlein, C.; Franz, C.M. Microbial Quality and Safety of Milk and Milk Products in the 21st Century. Compr. Rev. Food Sci. Food Saf. 2020, 19, 2013–2049. [Google Scholar] [CrossRef]
  22. Shi, C.; Maktabdar, M. Lactic Acid Bacteria as Biopreservation Against Spoilage Molds in Dairy Products—A Review. Front. Microbiol. 2022, 12, 819684. [Google Scholar] [CrossRef]
  23. Hervert, C.J.; Martin, N.H.; Boor, K.J.; Wiedmann, M. Survival and Detection of Coliforms, Enterobacteriaceae, and Gram-Negative Bacteria in Greek Yogurt. J. Dairy Sci. 2017, 100, 950–960. [Google Scholar] [CrossRef]
  24. Aleksic, B.; Udovicki, B.; Kovacevic, J.; Miloradovic, Z.; Djekic, I.; Miocinovic, J.; Tomic, N.; Smigic, N. Microbiological Assessment of Dairy Products Produced by Small-Scale Producers in Serbia. Foods 2024, 13, 1456. [Google Scholar] [CrossRef]
  25. Souad, R.; Mossadak, H.T.; Leila, B. Assessing Hygiene Indicators in Two Dairies in Algeria in Producing Pasteurized Milk. Vet. World 2021, 14, 2317–2322. [Google Scholar] [CrossRef]
  26. European Commission. Commission Regulation (EC) No 2073/2005 of 15 November 2005 on Microbiological Criteria for Foodstuffs. Off. J. Eur. Union 2005, L338, 1–26. Available online: https://eur-lex.europa.eu/eli/reg/2005/2073/oj (accessed on 11 February 2026).
  27. Republic of Türkiye; Ministry of Agriculture and Forestry. Turkish Food Codex Microbiological Criteria Regulation. Off. Gaz. Repub. Turk. 2025, 32812. 13 February 2025. Available online: https://www.resmigazete.gov.tr/eskiler/2025/02/20250213-1.htm (accessed on 11 February 2026).
  28. Ahansaz, N.; Tarrah, A.; Pakroo, S.; Corich, V.; Giacomini, A. Lactic Acid Bacteria in Dairy Foods: Prime Sources of Antimicrobial Compounds. Fermentation 2023, 9, 964. [Google Scholar] [CrossRef]
  29. Alan, S.; Öksüztepe, G.; Güran, H.Ş. Effect of potassium sorbate, nisin and lysozyme singly and in combinations on the quality of pickled white cheese during storage. Rev. Cient. 2025, 35, 9. [Google Scholar] [CrossRef]
  30. World Health Organization (WHO). WHO Estimates of the Global Burden of Foodborne Diseases: Foodborne Diseases Burden Epidemiology Reference Group 2007–2015. 2015. Available online: https://www.who.int/publications/i/item/9789241565165 (accessed on 11 February 2026).
Applsci 16 02184 g001
Figure 1. Pearson correlation matrix of chemical parameters.
Figure 1. Pearson correlation matrix of chemical parameters.
Applsci 16 02184 g001
Applsci 16 02184 g002
Figure 2. Box plot graphs of chemical parameters of milk jam samples. The red line represents the median and the green dashed line represents the mean value for each parameter.
Figure 2. Box plot graphs of chemical parameters of milk jam samples. The red line represents the median and the green dashed line represents the mean value for each parameter.
Applsci 16 02184 g002
Applsci 16 02184 g003
Figure 3. Histogram distributions of chemical parameters of milk jam samples.
Figure 3. Histogram distributions of chemical parameters of milk jam samples.
Applsci 16 02184 g003
Applsci 16 02184 g004
Figure 4. Microorganism positivity distributions in milk jam sample.
Figure 4. Microorganism positivity distributions in milk jam sample.
Applsci 16 02184 g004
Applsci 16 02184 g005
Figure 5. Positive correlation between pH and sucrose. Dots represent data points, the red line shows the linear regression (y = 6.89 + 0.03 × x), and R2 = 0.217 indicates that sucrose explains 21.7% of pH variation.
Figure 5. Positive correlation between pH and sucrose. Dots represent data points, the red line shows the linear regression (y = 6.89 + 0.03 × x), and R2 = 0.217 indicates that sucrose explains 21.7% of pH variation.
Applsci 16 02184 g005
Applsci 16 02184 g006
Figure 6. Negative correlation between acidity and sucrose. Dots represent data points, the red line shows the linear regression (y = 0.71 − 0.01 × x), and R2 = 0.259 indicates that sucrose explains 25.9% of acidity variation.
Figure 6. Negative correlation between acidity and sucrose. Dots represent data points, the red line shows the linear regression (y = 0.71 − 0.01 × x), and R2 = 0.259 indicates that sucrose explains 25.9% of acidity variation.
Applsci 16 02184 g006
Applsci 16 02184 g007
Figure 7. Positive correlation between reducing sugar and coliforms. Dots represent data points, the red line shows the linear regression (y = 3.06 + 6.02 × x), and R2 = 0.824 indicates that coliforms explain 82.4% of reducing sugar variation.
Figure 7. Positive correlation between reducing sugar and coliforms. Dots represent data points, the red line shows the linear regression (y = 3.06 + 6.02 × x), and R2 = 0.824 indicates that coliforms explain 82.4% of reducing sugar variation.
Applsci 16 02184 g007
Applsci 16 02184 g008
Figure 8. Negative correlation between dry matter and proteolytic bacteria. Dots represent data points, the red line shows the linear regression (y = 81.59 − 4.44 × x), and R2 = 0.530 indicates that proteolytic bacteria explain 53% of dry matter variation.
Figure 8. Negative correlation between dry matter and proteolytic bacteria. Dots represent data points, the red line shows the linear regression (y = 81.59 − 4.44 × x), and R2 = 0.530 indicates that proteolytic bacteria explain 53% of dry matter variation.
Applsci 16 02184 g008
Table 1. Chemical analysis results of milk jam samples.
Table 1. Chemical analysis results of milk jam samples.
AnalysisMinimumMaximumMean ± SD
pH6.708.997.54 ± 0.63
Acidity (% lactic acid)0.251.000.49 ± 0.19
Dry matter (%)49.0778.4762.42 ± 8.85
Fat (%)0.004.601.51 ± 1.50
Reducing sugar (g/100 mL)8.1230.3717.25 ± 6.16
Total sugar (g/100 mL)18.5252.0037.88 ± 8.67
Sucrose (g/100 mL)8.3435.0419.59 ± 8.74
Table 2. Microbiological analysis results of milk jam samples (log CFU/g).
Table 2. Microbiological analysis results of milk jam samples (log CFU/g).
AnalysisNumber of Positive Samples (%)Mean ± SD (For Positive Samples)Range (Min–Max) (For Positive Samples)
TMAB10 (50.00%)4.51 ± 1.912.30–7.54
Psychrophiles0 (0.00%)--
Yeast-Mold9 (45.00%)3.92 ± 1.262.30–5.94
Lactic acid bacteria3 (15.00)4.33 ± 1.862.30–5.95
Enterobacteriaceae6 (30.00)3.15 ± 1.262.30–5.65
Coliforms5 (25.00%)3.63 ± 1.062.87–5.46
Proteolytic bacteria11 (55.00%)4.21 ± 1.572.30–7.00
Table 3. Number (n) and percentage (%) distribution of microorganism counts detected in milk jam samples.
Table 3. Number (n) and percentage (%) distribution of microorganism counts detected in milk jam samples.
Log CFU/g
Microorg.<1.001.00–1.992.00–3.994.00–5.996.00–7.99>8.00
n%n%n%n%n%n%
TMAB1050--420420210--
Psychrophiles------------
Yeast-Mold1155--525420----
Lactic acid bacteria1785--15210----
Enterobacteriaceae1470--52515----
Coliforms1575--42015----
Proteolytic bacteria945--630315210--
Table 4. Pearson correlation coefficients between microbiological and chemical parameters of milk jam samples (* p ≤ 0.05; ** p ≤ 0.01).
Table 4. Pearson correlation coefficients between microbiological and chemical parameters of milk jam samples (* p ≤ 0.05; ** p ≤ 0.01).
AcidityDry MatterFatReducing SugarTotal SugarSucroseTMABYeast-MoldLactic Acid BacteriaEnterobac.ColiformProteolytic Bacteria
pH−0.2470.1890.086−0.2580.3110.466 *−0.497−0.403−0.278−0.493−0.4550.402
Acidity −0.3930.3580.392−0.261−0.509 *0.2920.091−0.5190.4230.3760.433
Dry matter −0.447 *0.0550.497 *0.432−0.6170.0010.184−0.438−0.455−0.732 *
Fat 0.019−0.058−0.0680.2350.330−0.585−0.191−0.3440.545
Reducing sugar 0.267−0.4180.3570.5720.4110.824 *0.908 *−0.492
Total sugar 0.764 **−0.3600.4981.000 *0.3350.016−0.459
Sucrose −0.573−0.040−0.102−0.502−0.679−0.241
TMAB 0.3150.8570.9490.9630.809 *
Yeast-mold 0.999 *0.997 *0.544−0.312
Lactic acid bacteria 1.000 **0.6871.000 **
Enterobac. 1.000 **0.688
Coliform −0.527
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Alan, S.; Büyük, G.D.; Adem, B.; Çetin, G. Investigation of Microbiological Quality and Chemical Properties of Ready-to-Eat Milk Jam. Appl. Sci. 2026, 16, 2184. https://doi.org/10.3390/app16052184

AMA Style

Alan S, Büyük GD, Adem B, Çetin G. Investigation of Microbiological Quality and Chemical Properties of Ready-to-Eat Milk Jam. Applied Sciences. 2026; 16(5):2184. https://doi.org/10.3390/app16052184

Chicago/Turabian Style

Alan, Selçuk, Gönül Damla Büyük, Betül Adem, and Gökçenur Çetin. 2026. "Investigation of Microbiological Quality and Chemical Properties of Ready-to-Eat Milk Jam" Applied Sciences 16, no. 5: 2184. https://doi.org/10.3390/app16052184

APA Style

Alan, S., Büyük, G. D., Adem, B., & Çetin, G. (2026). Investigation of Microbiological Quality and Chemical Properties of Ready-to-Eat Milk Jam. Applied Sciences, 16(5), 2184. https://doi.org/10.3390/app16052184

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop