When determining whether baked goods are fresh, people often rely on whether the surface is soft and whether mold spots or mycelium have appeared.
For example, suppose there are two bags of toast: one has no mold spots on the surface but feels hard when squeezed; the other still feels soft to the touch, but mold spots have already appeared on the edges. Clearly, these are two bags of non-fresh toast, but why is there such a difference?
The main difference between the two is that one is caused by microbial growth, while the other is caused by textural changes.
The working principle of an oxygen absorber is to reduce the oxygen content inside the packaging to a very low, almost negligible level, and release other required gases (such as carbon dioxide), thereby inhibiting microbial growth inside the packaging and delaying the time to mold.
Whether baked goods can remain soft depends on their bread formula, manufacturing process, and moisture management. However, a soft bread does not necessarily mean it is fresh, nor does the absence of surface mold spots guarantee freshness.
The Principle of Bread Molding
First, let’s clarify the standard production process for packaged bread: baking, cooling, slicing, conveying, and bagging.
For example, during the baking and cooling process, the bread is not cooled under vacuum conditions. During assembly-line production, as it comes into contact with the air, it has the opportunity to re-encounter mold spores from the environment.
Completing the sealing process is merely the act of enclosing the bread inside a bag; it does not mean there are no microorganisms left inside the bag, nor does it mean that oxygen has been completely purged.
Research shows that oxygen-absorbing measures and the oxygen barrier properties of the packaging must be considered together, rather than just looking at whether an oxygen absorber has been added.
No Mold, Yet the Surface Still Becomes “Hard”
Usually, the principle behind bread hardening is simple: moisture loss is the main reason, but not the only one.
During baking, starch undergoes a series of reactions: water absorption, heating, and gelatinization. After the bread is cooled and stored, some starch molecules gradually rearrange to form a more ordered structure, a process commonly referred to as starch retrogradation. Bread staling is closely related to amylopectin retrogradation and the redistribution of moisture among different internal components. Therefore, even if the packaging is well sealed, the bread may still gradually harden.
Figure 2: Schematic comparison of anti-mold and softening mechanisms. Icons in the figure are used only to explain the action stages and are not experimental micrographs.
What an oxygen absorber can achieve is to lower the oxygen concentration inside the packaged bread, but it cannot prevent the aforementioned reactions from occurring. This is why the surface of some bread has already hardened without showing any mold spots. Similarly, due to process issues, some bread may remain relatively soft, but mold spots still appear on the surface under the action of the oxygen absorber, indicating that the microbial content inside the packaging has exceeded the limit, meaning the bread has definitely deteriorated and is inedible.
A Bag of Bread Requires Attention to Two Preservation Lines
| Focus Areas | Mainly Involves | The Role of Oxygen Absorbers |
| Surface mold spots appear | Microbial contamination, oxygen, and storage conditions | Lower oxygen content, helping inhibit common aerobic molds |
| Bread crumb gradually hardens | Starch retrogradation, moisture redistribution | Cannot directly prevent; requires formula and process cooperation |
| Water loss and dry texture | Water vapor barrier performance, sealing, and environment | Cannot replace moisture-retaining packaging |
How Baked Food Enterprises Choose Oxygen Absorbers
Nanjing Tianhua has been deeply involved in food preservation for many years, and our products are applied in many internationally renowned food brands. Bakery products are mostly porous foods, and when choosing an oxygen absorber model, the important metric is the volume of oxygen that needs to be treated within the entire package.
Figure 3: Schematic diagram of oxygen sources in bread packaging. Bread pores, intra-bag space, and oxygen penetration during storage must all be factored into model selection.
The oxygen that needs to be treated = the volume of oxygen the package can contain + oxygen in the bread pores + oxygen penetrating through packaging materials during the storage period. In non-vacuum packaging, nitrogen flushing can reduce the initial oxygen content, but it does not create an oxygen-free environment.
Equally important is the oxygen absorption speed. Model capacity solves “how much volume of oxygen can be absorbed,” while oxygen absorption speed solves “how soon the oxygen concentration can be reduced after sealing.” The oxygen absorbers manufactured by Nanjing Tianhua can reduce the oxygen concentration inside the packaging to 0.01% within 48 hours.
At the same time, merchants are reminded that packaging materials and sealing for food packaging are particularly important. If oxygen continues to enter the package in large quantities, then even the largest model of oxygen absorber cannot guarantee that the oxygen concentration inside the package remains at a very low level.
Anti-Mold Does Not Equal Sterilization
Oxygen absorbers cannot compensate for poor hygiene in the cooling zone, contamination of slicing equipment, or sealing air leaks, nor can they restore moldy bread to an edible state. Lowering the oxygen content is only one part of the preservation system and cannot be used as a direct basis for making food safety or shelf-life commitments.
Indicator-Type Products Provide Clues But Cannot Replace Shelf-Life Verification
Nanjing Tianhua’s 504A indicator-type oxygen absorber is one of the company’s best-selling products. Its packaging design with an indicator strip allows merchants to visually see whether oxygen is present inside the package. When using this product, we provide model instructions, the meaning of color changes, observation times, and applicable periods together. However, the oxygen indicator strip reflects the oxygen status inside the package; it cannot be directly interpreted as “this food is definitely safe,” nor can it be used to judge the texture of baked goods. Oxygen testing, microbial testing, and texture evaluation cannot all be determined solely by indicator strips.
How to Verify Mold Inhibition and Texture Softness.
Use a comparative method: package bread from the same batch using two different methods—one without an oxygen absorber and one with the proposed oxygen absorber—under consistent storage conditions. Continuously monitor residual oxygen levels, mold growth, and bread hardness, and combine these observations with necessary microbial testing.
Oxygen Parameters: Record the oxygen reduction process after packaging, as well as changes in residual oxygen during storage.
Microbiological Parameters: Record visible mold growth and conduct microbiological testing according to applicable methods; do not rely solely on visual inspection.
Edible Quality: Compare hardness, springback, and texture at the same time points; do not use “no visible mold spots” as a substitute for softness evaluation.
Use experimental data to determine whether the bread’s shortcomings lie in the packaging or the production process.