Postpartum Stage and Lactic Acid Bacteria Strain Determine Fermentation Kinetics and Refrigerated Stability of Local Gorontalo Goat Colostrum

Milk is more than just a white liquid; it is a living, breathing suspension of tiny particles that constantly interact with one another. When milk is fresh, proteins, fats, and minerals float in a delicate balance, held apart by electrical charges that keep them from clumping together. This balance is what gives milk its smooth texture and stability. However, when bacteria are introduced to turn milk into yogurt or other fermented foods, they eat the sugars and produce acid. This acid changes the electrical charges on the protein particles, causing them to lose their repulsion and stick together, forming a gel. This transformation is the heart of fermentation, but the process is incredibly sensitive to the starting conditions. If the milk is too rich in certain nutrients or if the bacteria are too aggressive, the resulting gel can be weak, watery, or prone to falling apart over time. Understanding exactly how these microscopic changes happen is crucial for food scientists who want to create new, high-quality dairy products from unique sources, such as the colostrum produced by goats immediately after they give birth.

Colostrum is the first milk a mother produces, and it is biologically distinct from the milk that follows. It is packed with proteins and immune-protective compounds designed to help a newborn survive. Because of this rich composition, it is a promising candidate for making value-added fermented products, but it is also much harder to process than regular milk. Its high protein content and changing chemical makeup in the first few days after birth mean that a batch collected today might behave very differently from one collected tomorrow. Researchers in Gorontalo, Indonesia, set out to understand exactly how this changing environment interacts with different types of bacteria to determine if local goat colostrum could be reliably turned into a stable, fermented food. They focused on three specific questions: how the timing of the milking affects the fermentation speed, how different bacterial strains change the structure of the milk, and whether the final product could stay stable in a refrigerator for weeks.

The team collected colostrum from local Gorontalo goats on the first, second, and third days after the animals gave birth. They pasteurized the milk to remove unwanted bacteria and then inoculated separate batches with one of three common starter cultures: Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus acidophilus, or Lactococcus lactis subsp. cremoris. These bacteria were allowed to ferment the colostrum for a full day at a controlled temperature. After fermentation, the researchers did not stop there; they stored the products in a refrigerator at 4 degrees Celsius and monitored them for 28 days. During this time, they measured how the acidity changed, how the size of the protein particles shifted, how the electrical charge on those particles evolved, and how the texture held up. They also tracked the number of living bacteria to ensure the product remained safe and active, and they used advanced imaging and light-spectrum analysis to see what was happening to the molecules inside the milk.

The results showed that both the day the colostrum was collected and the specific type of bacteria used were critical factors in determining the final product. All three bacterial strains successfully lowered the acidity of the colostrum, turning it from a neutral pH of around 6.4 down to a sour pH between 4.6 and 5.0. However, the speed and final limit of this acidification depended heavily on the strain. The Lactococcus lactis subsp. cremoris strain was the most aggressive, driving the acidity to the lowest point, while the other two strains stopped at slightly higher levels. The day of collection also mattered; colostrum from the second day after birth tended to reach a lower final acidity than samples from the first or third days. This suggests that the changing chemical environment of the goat's udder in the first few days creates a unique substrate that influences how the bacteria work.

As the products sat in the refrigerator, the researchers observed a slow but steady transformation in the physical structure of the milk. The electrical charge on the protein particles, which helps keep them apart, decreased by about 15 percent over the four weeks. At the same time, the particles grew larger, increasing in size by between 8 and 20 percent. This indicated that the proteins were slowly clumping together into larger groups, a process known as aggregation. Despite this structural reorganization, the actual amount of protein and fat in the milk remained remarkably stable. The protein content dropped only slightly, and the fat content did not change significantly at all. This finding is important because it proves that the changes in texture were not caused by the bacteria eating away the nutrients or the fat breaking down. Instead, the milk was simply rearranging its existing components into a new, slightly looser structure.

The living bacteria also behaved differently depending on which strain was used. The Lactobacillus delbrueckii subsp. bulgaricus strain actually increased in number during the four weeks of storage, growing by about half a log unit, which represents a significant rise in population. In contrast, the Lactococcus lactis subsp. cremoris and Lactobacillus acidophilus strains saw a gradual decline in their numbers, though they remained well above the levels required for a healthy, active product. Even after 28 days, all samples contained millions of living bacteria per milliliter, ensuring that the fermented colostrum remained a viable probiotic food.

When the researchers looked at the molecular level using infrared spectroscopy, they found that the basic chemical building blocks of the milk had not been destroyed. The proteins, fats, and sugars were still there, but their internal arrangements had shifted. The way the molecules held onto water and connected to one another had changed, confirming that the fermentation and storage process had altered the molecular environment without breaking the food down. This molecular rearrangement was mirrored in the texture of the product. The fermented colostrum started as a firm, elastic gel, but over the four weeks, it became slightly softer and more fluid. The stiffness of the gel decreased, and the product became a bit more prone to flowing, though it never turned into a liquid. This weakening was most noticeable in samples made with certain bacteria on specific days, showing that the initial conditions of the milk dictated how well the final product could hold its shape over time.

Microscopic images provided a visual confirmation of these changes. The samples showed a mix of tiny protein clusters and larger fat droplets. In some batches, the bacteria had created a tight, uniform network, while in others, the structure was more uneven, with larger clumps forming. These visual differences aligned perfectly with the measurements of particle size and texture, proving that the microscopic world of the milk was directly responsible for the macroscopic qualities of the food. The study concluded that the success of making fermented goat colostrum products depends on a precise match between the day the milk is collected and the specific bacteria chosen to ferment it. By understanding these interactions, producers can select the right combination of raw material and starter culture to create a stable, nutritious, and high-quality fermented dairy product from this unique local resource.