A salt on the senses

Sineenath Sukkwai, Kongkarn Kijroongrojana, Napapan Chokumnoyporn, Sujinda Sriwattana, Damir D. Torrico, Kairy D. Pujols and Witoon Prinyawiwatkul report recent research on consumer responses to different sodium reduction approaches. They discuss the potential for using sensory science to achieve salt reduction in foods.

Introduction

Salt (NaCl) is essential for normal functioning in humans and the second most used food additive. However, high sodium (Na) consumption is a major contributor to high blood pressure. Globally, correlation exists between excessive sodium consumption and high blood pressure, which leads to possible coronary heart disease, stroke, heart and kidney failure. In the United States, the average daily Na intake is more than 3,400mg (equivalent to 8.5g salt), which exceeds the current maximum recommended intake level (2,300mg/d Na or 5.8g/d salt) established by the 2005 Dietary Guidelines for Americans. Individuals aged 51 years and older, and those of any age who are African American, or who have hypertension, diabetes, or chronic kidney disease should limit their Na intake to 1,500mg per day. In the US alone, more than 800,000 people die yearly from heart disease, stroke and other vascular diseases, costing a sum of $273 billion in healthcare dollars^[1]. Awareness of potential negative effects of high Na consumption is critical for public safety.

More than 40% of Na consumed originates from 10 types of foods, of which the top five foods are bread and rolls, cold cuts and cured meats, pizza, fresh and processed poultry, and soups.

Sodium reduction approaches

More than 40% of Na consumed originates from 10 types of foods, of which the top five foods are bread and rolls, cold cuts and cured meats, pizza, fresh and processed poultry, and soups. Moreover, 65% of Na consumed is present in retailed foods and about 25% comes from restaurants. Reducing NaCl is a challenge faced by the food industry. The most evident role of salt is to make food taste pleasantly salty; salt is a multifunctional ingredient that can also act as a preservative and a food flavour enhancer.

Busch et al.^[2] described the three recent major approaches to reducing sodium in foods as chemical mechanisms, cognitive mechanisms or modification of the food product structure (Figure 1). This article describes research work performed at the School of Nutrition and Food Sciences (SNFS) at Louisiana State University (LSU) and its collaborators, following these three approaches.

Figure 1 Sodium reduction approaches (Adapted from Busch et al., 20132)

I. Chemical approaches

Potassium chloride (KCl) has commonly been used to substitute for NaCl due to their similar chemical and physical properties.  Low-sodium products (<140 mg Na per serving) containing KCl are usually less desirable due to bitterness and metallic off-flavour. Kairy Pujols at LSU SNFS evaluated changes in overall-liking (OL), emotional response and purchase intent (PI) of low-sodium roasted peanuts as the added KCl concentration increased. Compared with the control (no KCl), adding 30-50% KCl did not significantly decrease OL scores (6.33 vs. 6.11-6.20) but the scores decreased (5.80-5.84) at 70-90% KCl.  A PI of 64% was observed at 0-30% KCl and about 55% at 50-90% KCl. No significant differences were observed for positive emotions (energetic, happy and pleased) among all treatments. Adding more than 70% KCl decreased the ‘satisfied’ response. No consumer rejection threshold of added KCl was reached under the conditions of this study. This work demonstrated that low-sodium roasted peanuts containing KCl (replacing up to 90% of NaCl) with 138.9 mg Na/50g peanuts were acceptable to consumers.

Using KCl can reduce the Na intake level and increase the K intake, which may help to reduce the problem of elevated blood pressure. However, KCl elicits salty taste but also imparts an unpleasantly bitter taste, hence a bitterness blocker may be needed.  5’-adenosine monophospate (AMP) was the first natural compound shown to block several bitter tastants. Pamarin Waimaleongara-ek evaluated sensory recognition thresholds of KCl in the presence of AMP (Table 1, unpublished data).

The bitterness recognition threshold of KCl was significantly lower (about 1.5 times) than that of KCl/AMP. This implies that the presence of AMP in the KCl solution lowered the bitter taste perception imparted by KCl. AMP may bind to bitter-responsive taste receptors or interfere with receptor-G protein coupling to serve as a naturally occurring taste modifier. For saltiness, the threshold concentration of KCl/AMP was slightly lower than KCl, implying that AMP may enhance the salty taste of KCl.

L-arginine (Arg) masks bitterness by binding at the receptor site as well as an interaction between the guanidinium side-chain of Arg and the sodium channel in the human taste bud. It may enhance saltiness of NaCl. Waimaleongara-ek found that at 0.3% w/v, L-arginine at a ratio of 15:2:1 (KCl:Arg:AMP) was effective at inhibiting bitterness of KCl/AMP. Other amino acids that show bitterness-masking ability are glycine and lysine.  In another study, Kennet Carabante performed sensory optimisation of low-sodium cheddar cheese containing a salt substitute and bitterness blockers. The overlay of RSM (Response Surface Methodology) contour plots (≥5.9 as a cut-off hedonic score) yielded an optimal formulation range (30% NaCl, 60% KCl and 10% glycine; Figure 2), indicating that up to 60% KCl could be incorporated without compromising sensory acceptability.

Table 1 Sensory threshold values (g/100 ml) ± standard deviations of KCl and KCl/AMP

II. Cognitive approaches

Several studies have evaluated the role of aroma in taste perception. Odour and taste interactions may be a result of physicochemical, physiological and psychological effects but the mechanism is unclear.  Some researchers have proposed that odour induces saltiness enhancement (OISE) via multisensory-integration mechanisms in the brain and not in the mouth. However, relatively few studies have dealt with salt-odour interactions using sensory threshold determination. Soy sauce is one of the most popular flavourings. Chokumnoyporn et al.^[4] demonstrated that soy sauce odour could induce and enhance salty taste perception in water with undetectable salty taste and salt solution, respectively (Figure 3).

Increasing soy sauce odour concentrations increased recognisable salty taste perception. However, OISE may become less effective at a higher salt concentration. This finding should be taken into account when utilising OISE in development of reduced-sodium food products. A combination of OISE and other approaches should be considered in an effort to reduce sodium intake.

Many studies have reported effects of colour on perception of tastes and their intensities, however, very few studies have focused on association of colour with bitter and salty taste qualities. Nowadays, consumers prefer natural colourants to synthetic counterparts. Sukkwai et al.^[5] evaluated the effects of colourant concentration and a colourant with a ‘natural source’ statement on expected saltiness intensity, sensory liking, emotion and purchase intent (PI) of dipping sauces containing no-colourant (NC), moderate-colourant (MC), high-colourant (HC) (Figure 4).

Increasing colourant concentrations significantly decreased overall liking scores, which, in turn, largely decreased positive PI. Positive emotion (good, interested, satisfied) scores decreased while the negative emotion (worried) increased with increasing colourant concentration. The ‘natural-source’ statement decreased guilty and unsafe scores but minimally affected PI of NC and MC. Saltiness expectation was not different between NC (off-white colour) vs. MC (lighter orange) and between NC vs. HC (darker orange), however, consumers expected HC to be saltier than MC. Sukkwai et al.[6] further reported that statements of ‘natural colourant’ and ‘sodium content claim’ (regular salt, RS = NaCl; reduced-sodium, ReS = KCl; no salt, NS) had minimal effects on elicited emotions. As salty taste may not be associated with a particular colour and this association is dependent on food types, more research is needed to further explore relationships between visual colour cues and salty taste expectation, which may be beneficial to the current effort of stepwise sodium reduction in retailed foods.

III.  Modification of the food product structure

Busch et al.^[2] stated that the modification of the food product structure by itself can improve the perception of saltiness. In this article, we investigate the following: (1) modifying the salt crystals through changing their size, shape and morphology and (2) modifying rheological properties of the food, such as oil-in-water emulsion.

Chokumnoyporn et al.^[7] investigated the combined effect of OISE and foam-mat salt on saltiness perception and sensory acceptability of oil roasted peanuts. In this study, foam‐mat drying was used. The salt solution containing a foaming agent was first whipped to a foam, then dried in a stream of warm air.The foam-mat salt had a bulk density about two-times lower than the commercial salt and had a smaller particle size (34.94-265.07 µm) compared to that of the commercial salt (170.78-529.55µm) (Table 2). A sensory descriptive panel evaluated roasted peanuts seasoned with the control (commercial salt, 100%), CS50 (commercial salt:soy sauce odour; 50:50) or FS50 (foam-mat salt:soy sauce odour; 50:50) and reported the perceived saltiness enhancement of FS50, which was likely due to the smaller particle size of the foam-mat salt as well as the OISE effect imparted by the soy sauce odour. FS50 roasted peanuts had 10mg of sodium per 100g of sample less than CS50 but were as acceptable as the control.

Modifying some emulsion characteristics including flavour, fat/oil content, viscosity, droplet size and the type of emulsifier may affect the sensory perception of emulsions^[8]. Saltiness of NaCl and KCl, and bitterness of KCl and caffeine were evaluated in emulsions prepared with varying oil (20/40/60%) and tastant [NaCl (0.50/0.75/1.00%) or KCl (0.50/1.00/1.50%)] concentrations by a sensory descriptive panel and an e-tongue[^{9, 10]}. At a given tastant concentration, NaCl had higher saltiness intensity compared to KCl in emulsions. Increasing oil concentration increased saltiness of both NaCl and KCl (Figure 5a,b), while it slightly increased bitterness of KCl in emulsions (data not shown). Saltiness enhancement of NaCl and KCl imparted by oil was more pronounced between 20 and 40% oil. E-tongue saltiness measurements corresponded to those of the descriptive data (Figure 5c,d).

Viscosity was the discriminating property among emulsions and possibly contributed to changes in salty and bitter perception.

Torrico and Prinyawiwatkul^[11] further reported that increasing oil concentrations affected consumers’ taste perception (saltiness and bitterness) of mayonnaise spreads. Cerrato Rodriguez et al.^[12] confirmed that the type of oil (e.g., olive, rice bran, and soybean oils) had significant effects on bitterness and overall taste liking of mayonnaise-type spreads.  These findings are useful for understanding consumers’ taste perception of oil-in-water emulsion products.

Conclusions

Excessive sodium intake is observed worldwide, hence there is an urgent need to reduce sodium in diets. Sodium reduction is an important public health priority that should be addressed by coordinated effort at multiple levels of government authorities, food industries, educational entities and other diverse stakeholders.

Reducing sodium in the food supply is feasible and will increase consumer choices. It will save thousands of lives and health-care expenses each year. This article demonstrates the importance of sensory science as a tool for salt reduction in foods.

Professor Witoon Prinyawiwatkul

Associate Editor – International Journal of Food Science and Technology, IFST, UK

Telephone: +82-25-578-5188

Emailwprinya@lsu.edu

Sineenath Sukkwai1, Kongkarn Kijroongrojana1, Napapan Chokumnoyporn2, Sujinda Sriwattana2, Damir D. Torrico3,4,

Kairy D. Pujols4and Witoon Prinyawiwatkul4

1 Department of Food Technology, Faculty of Agro-Industry, Prince of Songkla University, HatYai, Songkhla 90112, Thailand

2 Division of Product Development Technology, Faculty of Agro-Industry, Chiang Mai University, Chiang Mai 50100, Thailand

3 Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Parkville, VIC 3010, Australia

4 School of Nutrition and Food Sciences (SNFS), Louisiana State University (LSU) Agricultural Center, Baton Rouge, LA 70803-4200, USA

References

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3. ASTM E-679. 2004. Standard practice for determination of odor and taste thresholds by a forced-choice ascending concentration series method of limits. E-679-04. West Conshohocken, Pa: ASTM.
4. Chokumnoyporn, N., Sriwattana, S., Phimolsiripol, Y., Torrico, D. D., and Prinyawiwatkul, W.  2015.  Soy Sauce Odour Induces and Enhances Saltiness Perception: A Sensory Threshold Study.  International Journal of Food Science and Technology.  50(10):2215-2221.
5. Sukkwai, S., Chonpracha, P., Kijroongrojana, K., and Prinyawiwatkul, W.  2017. Influences of a Natural Colourant on Colour and Salty Taste Perception, Liking, Emotion and Purchase Intent: a Case of Mayonnaise-based Dipping Sauces.  International Journal of Food Science and Technology.  52(10): 2256-2264.
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11. Torrico, D. D. and Prinyawiwatkul, W.  2017.  Increasing Oil Concentration Affects Consumer Perception and Physical Properties of Mayonnaise-Type Spreads Containing KCl.  Journal of Food Science.  82(8):1924-1934.
12. Cerrato Rodriguez, W. A., Torrico, D. D., Osorio, L. F., Cardona, J., and Prinyawiwatkul, W.  2017.  Taste Perception and Purchase Intent of Oil-in-Water Spreads: Effects of Oil Types and Salt (NaCl or KCl) Concentrations.  International Journal of Food Science and Technology.  52(10): 2138-2147.