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Smell is one of our most important senses. Developed through millennia of evolution, our ability to smell the world around us provides important cues about our surroundings. In fact, the human nose is reportedly able to detect thousands of different odors, even if we consciously do not recognize the broadness of the range. An entire $75 billion fragrance and aromas industry exists because of this sense, powering everything from cosmetics and home products to food and beverage.1,2

But as many homebound consumers during the past year can attest, smell can also have a dirtier side.  Our emotional relationship with smell means that malodors, whether in the home or outside of it, can have an adverse effect on our outlook and opinions. In the context of the cleaning industry, smell can play a direct role in how consumers evaluate cleaning efficacy, whether in the home, workplace, doctor’s office or retail locations. In the post-COVID era where consumers are more attuned to the importance of wellbeing and cleanliness, such opinions can be quite salient. Understanding and employing strategies to deliver the “smell of clean” to consumers is an important consideration for cleaning product formulators.

The Science of Malodor

Our sense of smell is closely linked with memory, possibly more so than any of our other senses. This is why a smell can act as a trigger for memory recall, such as of a barbecue bringing one back to good times or the smell of a dental office bringing emotional anxieties.3

The science behind smell was demystified in good measure though groundbreaking work by Dr. Richard Axel and Dr. Linda Buck, who won the 2004 Nobel Prize in Physiology and Medicine for their delineation of the olfactory system and its odor receptor system. Perhaps most relevant from a product development perspective are findings that incoming smells, which are first processed by the olfactory bulb inside the nose and run along the bottom of the brain, has a direct connection to two brain areas that are strongly implicated in emotion and memory: the amygdala and hippocampus (see Figure 1). The visual, auditory and tactile senses do not pass through this brain area, conferring the olfactory sense with an advantage in triggering memory association.4

It is estimated that 75% of the emotions we generate on a daily basis are affected by smell.5 This is in part due to the fact that memory and smell are intertwined; it’s through memory that we learn to remember smells, and smells directly trigger memories that can be associative to emotions. Disorders that take away memory also take away the ability to distinguish scents.6  The sensitivity of olfactory nerves along with such strong connection to memory allows the human nose to discern nearly 50,000 different scents. And, smell has been shown to itself impact other senses, such as the flavor of foods.

Malodors are often recognized as such from an early age, pointing to the evolutionary basis of some odor recognition.6,7  Malodors likely served a primordial purpose, such as indicating spoilage of foods or other threats.  In the modern world, this function is often much less critical, but malodors are still found nearly everywhere and are recognized as such (see Figure 2).

Despite its munificent nature, many malodors seem to have common chemical constituents.  In other words, what humans often associate with malodor is grounded on common chemical classes.   Prototypically, such malodors involve a mix of chemicals that are nitrogen-based, sulfur-based, organic acid-based or aldehyde/keytone-based.  As suggested by Figure 3, common malodors can be decomposed to specific chemical species of these chemical classes.

The commonality in the chemical classes of these malodors is suggestive that malodor control may be possible through strategies that specifically address these chemical classes.

Malodor Control Strategies

Over the years, five generalized strategies have emerged for controlling malodor: masking, killing, capturing, decomposing and eliminating. Each presents its own set of benefits and drawbacks, making selection particularly dependent upon the application that one is targeting.

Masking. The oldest approach, predating recorded history, is odor masking. Early historical record from Egypt famously recounts the use of perfumes to mask body malodors, a practice later adopted by ancient Greece and Rome and passed down to this day.9  Masking is the most common approach in part because it is notionally the most simple to execute: often the goal is simply to “hide” malodors through the dominance of more pleasant odors. This dominance-strategy is still common, but some modern masking also takes a more nuanced approach, often through selection of odors that offset or recharacterize malodors rather than only overwhelming them.

Kill. As many malodors are generated as byproducts from mold, mildew, and bacteria, the advent of anti-bacterial or biocidal agents in the mid to late nineteenth century opened the door to a kill-based strategy.10 By killing the biological generators of malodors, oxidizing agents and other chemistries stop odor at its source, often as a side-benefit to their primary purpose of achieving high pathogen kill rates. Despite the appropriateness of this strategy in some applications, concerns have been raised regarding the effect of chronic exposure to antimicrobials for human health and the environment.11  An increased awareness of the commensal role some bacteria play in the skin microbiome is spawning a rethink in appropriateness of the daily use of kill-based products.

Capture. A more popular malodor control strategy involves the physical capture of odor molecules, thereby sequestering them from sensorial perception by consumers. Several chemistry options exist including activated carbon fiber and chitin, but the most commercially successful example of this strategy are cyclodextrin-based products, such as those marketed by Procter & Gamble under the Febreze brand. Cyclodextrin traps odorous molecules in their hollow, tube-like molecular structures, thereby stopping them from permeating through the air.12 This strategy addresses some of the deficits posed by the kill strategy, and has been found to be generalizable to applications as diverse as water purification and soil remediation. In this strategy, an important consideration, and in some cases limitation, is the selectivity of odor molecule captured. In some contexts, where use of fragrance is desired, care should be taken to ensure that these odor molecules are not themselves captured.  Additionally, as cyclodextrins relies on odor binding as a formulation dries, there is also some risk of odor release upon exposure to water or humidity.

Decompose. Oxidative decomposition of odor species in the presence of catalysts or other biological enzymes can provide another means for odor removal. In this approach, the odor molecule itself is in some way altered or deconstructed, thereby removing its olfactory impact. Several industry standard solutions like titanium dioxide and other specific metal ions have been shown to provide deodorizing benefits, such as in smart textile applications.13  Enzymatic odor removal, leveraging biological enzymes like protease, amylase and lipase, are popular owing to their good toxicology and sustainability profile.14  Nonetheless, the more sensitive nature of such enzymes can limit the scope of applications where this strategy can be employed.

Eliminate. The Eliminate strategy removes malodor by chemically complexing malodor molecules. In some respect, it is a compromise solution combining elements from both capture and decompose strategies. The chemical reaction that takes place is often irreversible, thereby effectively and permanently neutralizing malodor. At the same time, such solutions are often more chemically resilient for use across several application contexts. As the mechanism relies on chemical complexing, there is a natural range of selectivity as to what type of odor molecule can be eliminated. As such, this selectivity can allow positive fragrances to be unaffected while selectively eliminating malodor species. Among the most proven chemistries in this category are based on zinc ricinoleate, represented by Evonik’s Tego Sorb products.

Relevance of Malodor 

Malodor has always been a sore-point for consumers. But its relative importance as a claim had been trending lower prior to the pandemic. According to Mintel’s Global New Product (GNPD) database, the number of new product instances with “odor neutralizing” as a claim fell globally by 5.9% CAGR between 2016 and 2019. This decline was seen across household, beauty, health and hygiene, and even pet care categories. The claim declined in all regions, except for the Middle East which registered a modest gain during that period.

But the COVID-19 pandemic has changed attitudes and daily realities in ways that make malodor control more relevant. At the most basic level, the pandemic helped foment cleaning habits that are expected to persist for some time. As much as regular cleaning has been an unwelcome chore, it has also become a source of security and comfort for the consumer. By fulfilling both physical and psychological needs, the cleaning habit has become more integrated into consumers’ lives. Mintel’s data shows that new product launches during 2020 using “odor neutralizing” claims finally stopped declining from previous years, as formulators reassessed the claim’s relevance.

The pandemic has created a shift in the workplace routine of many workers; many companies expected to allow, if not encourage, increased levels of home-based work in the future. More time spent at home may further accentuate the cleaning habit, as workers seek the cleanliness levels they took for granted from regular janitorial services. The pandemic lockdown also led to an increase of different home-based activities. People are increasingly exercising at home and cooking at home, and there is a rise in pet adoption rates. Some of these activities are likely to persist post-pandemic, further increasing the need for products that help to eliminate malodors.

As previously noted, odor can be an important factor in consumer’s assessment of cleanliness.  The proverbial “smell of clean” can impact the “feel of clean.” This is true both in the home context as well as in commercial spaces, such as retail, hotels and airports.  Since much cleaning is invisible to the consumer in the industrial or institutional context, odor may contribute even more heavily to consumer assessment of clean.

Lastly, wellness has emerged as a broad-based consumer theme that is likely to persist. The wellness trend was evident prior to the pandemic, but 2020 helped consumers reprioritize how they spend their time. Odor’s close connection with emotional wellbeing already manifests itself through applications such as aromatherapy. Similarly, malodor’s negative emotional connotations suggest that its removal may be more relevant for consumers going forward. Along with wellness for one’s self, notions of wellness for the planet through sustainability have become widespread. These criteria can influence the strategies employed for malodor removal.

Elimination Strategy in Focus

The performance and sustainability requirements of the post-COVID consumer suggests that for several applications, malodor reduction through the elimination strategy could be attractive. As a case study, we can look to Evonik’s portfolio of Tego Sorb products, which are based on a zinc rinciolate chemistry with enhanced performance properties.15

Evonik’s Tego Sorb line contains products that are non-toxic, readily biodegradable and possess high renewable carbon index scores, making them suitable options for sustainable formulations.  At the same time, they are shown to be effective in malodor removal while being compatible with most fragrances. Their compatibility with aqueous systems, solubility in hard water, and ability to address different pH ranges, make them easy to formulate with and suitable for a broad range of applications. The Tego Sorb range consists of three products, optimized for performance needs of different applications based on pH.

To demonstrate the malodor elimination effectiveness of these products, a head-space analysis of different malodor sources was performed.  Malodor sources were placed into a closed container containing a Tego Sorb-based formulation. After some minutes, the gaseous headspace of the container was pumped through analytical measurement. These experiments were conducted using other benchmarks as well. Malodor sources included hydrogen sulfide, ammonia, and formaldehyde. Results from all tests showed that the TEGO Sorb products were effective in the removal of malodor molecules.

To complement these analytical tests, in vivo two-sided paired expert panel studies were conducted to compare the olfactory smell of a towel which had been exposed to cigarette smoke after being subjected to formulations with Tego Sorb and the same formulation without Tego Sorb.  Formulations consisted of an HDL laundry system in one test and a rinse cycle fabric softener in a second test. In both cases, results showed a statistically significant preference for the towel treated with the formulation containing Tegp Sorb in the HDL case (p<0.05) and fabric softener (p< 0.1).

Both analytical and in vivo test results suggest that an elimination strategy employing Tego Sorb products can enable formulators to reduce malodors. At the same time, these formulations have the potential to fulfill consumer sustainability requirements.

The Smell of Clean

What is the “smell of clean?” That is obviously a subjective answer, particularly given smell’s emotional connection. For some fragrance-free oriented consumers, clean might mean a lack of any odors, while for others, it may mean a brand-affiliated or experiential-related fragrance. In all cases, however, the smell of clean involves a lack of malodors. For the wellness-seeking, sustainably-conscious and more home-bound consumer, malodor removal is as important as ever. Luckily, formulators have several strategies and several options within each strategy to address malodor concerns. In particular, the elimination strategy with solutions like Evonik’s Tego Sorb products can enable the smell of clean sustainably, thereby giving consumers the security that seek with the feel of clean.

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16. Psychology Today.  [Online]

Source: Delivering The Smell Of Clean To Consumers – HAPPI

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Random sensory quotes

Mushy food is a form of sensory deprivation. In the same way that a dark, silent room will eventually drive you to hallucinate, the mind rebels against bland, single-texture foods, edibles that do not engage the oral device.

— Mary Roach