By Hella Frenkel and Anat Kartaginer
One of humanity’s greatest challenges throughout the ages that has been more forcefully confronting us in the last few years and will follow us into the future, is concern about water in general and drinking water in particular. Of high priority are supply, availability and access to quality water to all of the world’s population. Water is the most consumed product on earth; there is no disputing that it offers a key consumer experience. Water taste, odor and appearance are the most important factors in the consumer experience and therefore, in addition to being safe, it must also be tasty, clear and inviting.
Water that is safe in terms of the chemical and microbiological parameters, but lacking in flavor, will be classified, first and foremost, as low quality; it will be difficult to convince people to drink it. So what determines the taste of water? Does it have taste or is it tasteless? On what does the distinction depend? What properties are tested when trying to determine taste? And how can producers set taste if it has no flavor at all? The realm of water tastes and odor is a world unto itself, in which many professionals are engaged with the goal of providing safe, drinkable and tasty water.
The tools at our disposal, which we can use to analyze water’s taste, are extremely limited. Fortunately, human senses are very sensitive, more so than any other analytical tool. Therefore, the human palette serves as a valid analytical tool in the process of determining water taste. Various tests have been conducted in an attempt to normalize and turn this subjective tool into a truly objective index, on the one hand, training people to become analytical tools, and on the other, using the ‘wisdom of the crowd’ to potentially provide the most precise answer. Questions related to various parameters, such as sense of freshness, smoothness, satiation and the existence of additional aftertastes (salty, sweet, spicy, plasticity, acidity, metallic taste and more) are the central tools used in taste tests. This obviously presents a great challenge.
In this article, we will deal with some of the factors that impact water taste and odor, review aspects related to the physiology of taste, examine possible sources of taste in water in the supply network and focus on the challenges facing POU system manufacturers.
Physiology of taste
The sense of taste and smell developed at a very early stage of evolution, as critical survival skills. The proximity between the senses of smell and taste created a mechanism for distinguishing between proper and improper; what we call taste is actually a combined product of these senses. There is a passage connecting the mouth cavity with the nose cavity, so that in the tasting process molecules move from the mouth to the sensory cells in the nose and activate the sense of smell. When we taste something while holding our noses hermetically closed so that no air flows, most flavors vanish.
The four primary taste sensations are bitter, sweet, salty and sour. There is a fifth taste known as umami (pleasant savory taste, in Japanese) whose source is glutamic acid, which can be found in meat, fish and hard cheeses. Another taste perception is spicy, which is not transmitted by the sense of taste, but rather by the somatic sensory system that detects bodily temperatures.
The tongue is covered with taste buds that absorb certain food molecules and send the information to the brain, which interprets them as taste. A human tongue has about 10,000 taste buds, a figure that differs between the genders. The taste buds are the receptors located on the tongue, the soft palate and the epiglottis (flap of cartilage that covers the entrance to the larynx). In every single taste bud (receptor), the process of conversion of chemical stimuli into neural stimuli occurs, which is in turn transmitted along bone fiber, through the brain stem to the brain. The stimuli are interpreted as information (both congenital and acquired), which is stored in the brain. The four types of receptors each specialize in one of the basic tastes. According to the accepted hypothesis, there are evolutionary reasons for each taste: sweet is responsible for the search for energy; salty preserves the balance of liquids and electrolytes in the body; bitter warns of poison and sour identifies spoiled food. Taste buds or receptors, unlike other senses, undergo a continual cycle of renewal; the lifespan of a taste bud, due to the intense wear to which it is exposed, is about 10 days. Anatomically, the tongue is divided into areas sensitive to different tastes. The back part of the tongue is more sensitive to bitter taste, while the front area is more sensitive to sweet and the sides to salty and sour. In the middle of the taste pores, there is a central space into which taste compounds penetrate, around which elliptically shaped taste buds are scattered, containing 30-100 sensory cells that create electrical discharges as a result of the binding of specific taste molecules. These cells, which are replaced by new cells every 10 days, have two sides: on one, the cell creates connections with the ends of nerve axons and on the other, the cell sends cilia extensions from the central space of the taste pore. All the proteins participating in the transformation process, from the receptors to the ion channels, are located here.
The human population is divided into three groups based on the density of taste pores on their tongues. The first group, composed of excellent super-tasters, is characterized by a concentration of more than 100 taste pores per square centimeter in the front of the tongue. The second group, composed of poor non-tasters, is characterized by a concentration less than 40 taste pores per square centimeter, while the third category (that includes most of the population) has concentrations ranging from 40-100 per square centimeter. These differences are genetic and impact the potency of the sense of taste felt by members of each group. Familiarity with a taste is a process of acquisition, in addition to the basic taste capabilities. The process of identification of both tastes and smells is acquired during a personal process of development. The amazing smell and taste of chocolate will be recognized by a person who has been exposed to chocolate and has learned to love it. Therefore, the tasting process has genetic, psychological and acquired aspects.
What determines the taste of water?
The taste of water is determined by the memory of the water, from its source (where it was born and what it accumulated where it existed) through the journey it takes to consumers. This definition includes, of course, our subjective experience as water consumers. It is possible to divide the major factors into two categories: inherent factors (i.e., tastes related to the water source) and external factors (i.e., aftertaste), which also determines the objective taste of the water. Both central water treatment companies and manufacturers of water purification devices are making intensive and extensive efforts to meet the challenge of attaining the right balance between these components and producing tasty water.
Water source is the primary factor in determining its taste. Water from difference sources contain varied combinations of dissolved minerals and salts and to a great extent, it is these mixes that determine its taste. Water rich in minerals (such as calcium or magnesium) create a sensation of water that is less smooth while water low in mineral content is described as being smoother. Concentration of salt solutions in water has a direct impact on taste. For example, water rich in chloride will have a saltier taste. (A famous example, Borjomi Water, a salty water that is rich in chloride, sulfur and bicarbonate originating from a well in Borjomi, Georgia, is considered by consumers to be tastier in comparsion to low-sulfur contact water and some believe it may have healing properties.)
It is also clear that water tastes are heavily influenced by the human factor, by level of taste sensitivity, which is a function of genetic factors, exposure and habit. People used to a certain type of water do not sense its taste, but when they taste different water, with a different composition, they will recognize a taste and perhaps experience a different level of satisfaction, depending on how acclimated they are to their regularly consumed water. Note that habit is closely linked to sensitivity. Frequently, a local consumer will not sense the taste of water he/she is used to, while a new consumer with different habits will sense a different taste. Habit is a determining factor in the end users’ taste satisfaction. Low TDS water, such as distilled or filtered through RO, is considered by some as less tasty due to removal of dissolved solids, giving it a bitter taste. In many countries, RO is used as a household purification method; therefore, those consumers are used to the taste of this type of water. They frequently report that they are not satisfied with the taste of mineral water (even the world’s leading brands), which is based on habit, rather than taste. This example emphasizes habit factor and choice by habit.
Water temperature is another factor that impacts taste. Cold water is often described as being tastier and more refreshing, since chemical compounds are less noticeable at lower temperatures. The sense of smell is also intensified in the case of very hot or boiling water. Odors carried by steam will be sensed quite a while before the taste. Remember, smell is a primary component of taste.
Oxygen concentration affects freshness and crispness. One of the reasons that standing water in a closed container is not tasty is the reduction in the amount of dissolved gas in the water. If, for example, we take standing water and pour it from one cup to another several times, its flavor will improve as a result of its contact with the air. In addition to all the inherent factors listed above, other factors, such as the level of acidity (pH) and biological composition of the source water, help create the overall taste.
In professional jargon, these factors are referred to as aftertaste; i.e., water’s additional taste as a result of purification, storage and shipping from the source until it reaches the consumer, regardless of where it is accessed (i.e., tap, device or bar). Purification additives, pipes and type of plastic used all leave their fingerprint on the water and add to the taste mixture. A prime example is the smell and taste of chlorine-based additives used to preserve water at a proper microbiological quality.
Tastes in the water supply network
What determines the taste of the water in the supply network? In general, there are organoleptic (acting on or involving the use of the sense organs) parameters that are specified in standards and regulations, which define permitted levels and sensory thresholds. It is clear that the maximal permitted contaminant levels provide high safety margins with respect to taste thresholds. One of the goals of POU treatment is to prevent reaching the smell and taste threshold. In the US, the organoleptic parameters are indicated in US EPA standards (see Table 1). In addition to the maximal permitted levels, there are specific thresholds for tastes and odors from sources that are organic and inorganic, according to the American Water Works Association (AWWA) (see Table 2).
Note that the threshold for identification of substances through smell or taste is higher than the permitted threshold for drinking water. Additional possible sources of tastes in drinking water may stem from technical faults or execution of works in the water treatment system, the presence of organic compounds in the surface water, problems stemming from various types of contamination or pollution (such as sanitary or industrial sewage), execution of works in the public network and problems in the private network.
A case in point is the taste of water in the network that supplies water to customers located near ocean shorelines. These customers may be supplied with high concentrations of chlorides due to uncontrolled infiltration of seawater, whose source is the seawater-freshwater interface. These cases constitute a challenge for selection of the specific technology for treating the taste originating from the network water.
All the challenges described above lead us to treatment of the consumption point: POU. One of the most significant factors that impact the purchase of POU devices is taste. Therefore, the primary purpose of these devices is to improve the consumer experience, with consumer expectations being first and foremost: tasty water that is also safe to drink. According to AWWA, there are various methods used to treat taste and odor problems stemming from organic, inorganic and microbial sources (see Table 3).
There are numerous sources for a wide range of tastes originating from the water supply network. Treatment in the field of organoleptics is very complex and challenging for POU systems that are responsible for the clarity and taste of the water at the final points of use. In addition to possible impact due to the tap water source, water passes through the device’s hydraulic system, which can itself constitute a source for various tastes. Unlike the inlet water supply, POU devices are capable of controlling these parameters very well. The hydraulic system is comprised of various components, such as: pipes, taps, gaskets, containers made of various materials, surface treatments using various substances, carbon filters that affect taste, seals, subcomponents, resins, active compounds and more. Each of these components has the built-in potential to contribute to aftertastes in every POU device. This complexity poses a challenge for POU manufacturers, from the development stage to ongoing production.
This issue requires continual control and testing, and intensive work both at the single-item and system levels, with the only tool at the manufacturer’s disposal being the human sense of smell and taste. The challenge is to turn this subjective tool into an objective and reliable one. For this purpose, a tasters’ panel is established, comprised of people whose ability to identify tastes and odors are tested using certification tests that include identification of the four basic tastes and their intensity. People who are diagnosed as having high identification abilities serve as analytical tools. Taste tests are conducted using pre-defined conditions under which the tests (including the types of cups, time intervals after meals, room and water temperatures and more) are all aimed at reducing the subjective influences as far as possible. They are adjusted to testing needs and to an understanding of the system, from testing the separate parts in a jar test (during which an item is placed in water at a required volume, with the tasting panel examining its contribution to aftertaste versus a control group) to taste traces compared to the entire system or parts of the hydraulic system.
At this point, it is important to discuss the controls and proper comparisons. What are the proper controls for the test: bottled, tap, water from the system without any change, etc.? Proper planning of the controls will maximize the information and will reinforce the validity of the findings. In the basic blind test, the panel is given several unidentified samples and asked to score them or indicate their satisfaction with the samples. The set of samples and testing cycle are important and the goal is to prevent the impact of one sample on another. The samples range from those without any aftertaste to those with a strong aftertaste. In the case of aftertastes, the panel is asked to try to identify the aftertastes (with the dominant aftertaste being determined out of a series of options) and label them according to the number of testers that identified a specific taste.
In summary, supplying tasty water at the point of use is a great challenge for all those engaged in this field and requires extensive and intensive work in order to ‘produce’ safe and tasty drinking water.
- Wagner, Shlomo, Sense of Taste, Second Appendix, Factors with Organoleptic Impact, Table A. Galileo, pp. 44-55. Department of Neurobiology, Hebrew University, 2006.
- Water Quality and Treatment: A Handbook of community water Supplies, Fourth Edition. American Water Work Association, 1990. McGraw-Hill Inc., New York.
About the authors
Hella Frenkel, Water Technologies Innovations & Compliance Director at Strauss Water, has more than 20 years of experience in the water treatment industry, with more than a decade of expertise in water innovation technologies, water quality and compliance and drinking water. She can be reached via email, firstname.lastname@example.org or phone, +972 3 5385727.
S Anat Kartaginer is Water Section Manager for Strauss Water. Previously, she served on the WC&P Technical Review Committee and as a contributing author. Kartaginer can be contacted via email, email@example.com or phone, +972 2 9900529.
About the company
Strauss Water has been a major force in the provision of drinking water systems for over 30 years and is one of the world’s most advanced developers and manufacturers of point-of-use systems. Leading-edge products are matched by a focus on customer service, which comes from long-lasting partnerships with clients. Visit the website www.strauss-water.com to learn more.