Taste and palatability of strawberry jam as affected by organic acid content

ABSTRACT

The aim of this research was to clarify the effects of organic acid content on the taste and palatability of strawberry jam. Prepared jam samples with different organic acid contents (0.8–2.0%) were assessed for color and texture, and then subjected to sensory evaluation of sweetness, sourness and preference attributes. For the sensory evaluation, assessors evaluated the sweetness intensity of the jam as decreasing with increasing organic acid content. In contrast, the sourness intensity increased as the organic acid content increased. Palatability did not change with the content of organic acids when the jam was evaluated alone. However, a difference was observed when eating jam in combination with yogurt or bread; moderately sour jam was preferred over those at either extreme of the sourness scale. Assessors who commonly consume jam with yogurt showed a preference when evaluating jam with yogurt, but no differences were observed when evaluating jam with bread. By contrast, assessors who commonly consume jam with bread did not show a preference when evaluating jam with yogurt, but showed a preference when evaluating jam with bread. Assessors who typically consume jam with yogurt or bread did not show any differences in preference when evaluating jam alone. These results indicate the importance of considering the eating habits of assessors when investigating the palatability of foods, such as jam, that are not eaten alone. Because the preference of them may change depending on their eating habits.

KEYWORDS:

• Strawberry
• jam
• sourness
• organic acid
• sensory evaluation

Introduction

The strawberry (Fragaria × ananassa Duchesne) is one of the world’s most popular fruits, with over 9 million tons grown globally every year.^[1] Strawberries are used not only as fresh fruits but also as processed products. A considerable amount of research has been conducted on strawberries,^[2,3] which are typically available as frozen berries or processed products (purees and jams). When eating raw strawberries, those with high concentrations of soluble solid content (SSC) and low titratable acidity (TA) in the juice are preferred.^[4] For strawberry jam color,^[5] the effects of processing method,^[6] storage^[7] and selection of raw materials^[8] have been previously studied. However, there has been no study on organic acid concentration or sugar/acid ratio with respect to taste. When making strawberry jam, lemon juice or citric acid is often added as a food additive. The main purpose of this is to lower the pH and assist in the hardening of jam by pectin.^[9] However, the addition of organic acids will have a major impact on taste and preference. To investigate this, we made jam using the strawberry variety ‘Kirapika’, which has a very low organic acid content.^[10] Additionally, citric and malic acids were added as food additives to the samples to generate a wide range of jams with very low to high organic acid contents. Jam samples differing only in organic acid content were evaluated for taste and palatability by sensory evaluation. However, as consumers are unlikely to eat jam alone, the sensory evaluation was performed in combination with yogurt and bread, in addition to jam alone. Therefore, the aim of this research was to clarify the level of organic acid content of strawberry jam that is preferred by consumers, and whether the result would change depending on the combination with other foods.

Materials and methods

Strawberry fruits

Fresh strawberries (cv. Kirapika) were cultivated in a greenhouse at the Shizuoka Prefectural Research Institute of Agriculture and Forestry (Iwata, Shizuoka, Japan). Seedlings were planted in September 2018, and the fruits were harvested in May of the following year. The plants were cultivated based on the standard method for Kirapika.^[10] Kirapika was chosen because of its low organic acid content among several strawberry varieties.^[10,11]

Jam preparation

Fully ripened strawberries were harvested and washed with water. After removing the calyx, the strawberries were crushed with a mixer (TMV1100, Tescom Denki Co., Ltd., Tokyo, Japan) for 20 s. The crushed strawberry was sieved (1-mm pore size) to remove large solids such as the hard part near the peduncle, making a paste. The method of making jam was based on Igual et al.^[12] and Ribes et al.^[13]. Specifically, strawberry paste (195 g) and sugar (105 g of granulated sugar, Dai-nippon Meiji Sugar Co., Ltd., Tokyo, Japan) were mixed at a ratio of 65:35 and cooked for 30 min to reach 60 °Brix. This process was carried out in an electric food processor equipped with a heater (SBC-1000J, Cuisinart, Inc., Stamford, CT, USA). The jam used in the study was processed in lots by repeating this procedure. All processed jams were mixed, and then portioned as 500-g aliquots and stored at −30 °C until used in experiments.

SSC, free sugar and organic acid contents of strawberry paste and jam

Prior to the sensory evaluation, SSC, free sugar and organic acid contents of the strawberry paste used as a material and the processed strawberry jam were analyzed. This was done to determine the range of organic acids in the jam prior to sensory evaluation. The SSC of the fresh strawberry paste and jam was measured using a sugar refractometer (PAL-S, Atago Co., Ltd., Tokyo, Japan) at 20 °C. The paste and jam were diluted 50- and 250-fold in water, respectively, and were then subjected to centrifugation for 10 min at 6,400 × g at 4 °C using a high-speed refrigerated microcentrifuge (MDX-310, Tomy Seiko Co., Ltd., Tokyo, Japan). Supernatants after centrifugation were filtered through a 0.45-µm filter. The levels of glucose, fructose and sucrose as the main free sugars in strawberries^[14] were determined in the filtered supernatant using high-performance liquid chromatography (HPLC). The analysis conditions were as follows. HPLC: Alliance 2695 (Waters, Milford, MA, USA); column: SC1011 (8 mm I.D. × 300 mm, Showa Denko K.K., Tokyo, Japan); eluent: water; flow rate: 1 mL min⁻¹; detector: differential refractometer (2414, Waters); standard: glucose, fructose and sucrose (Wako Pure Chemical Industries, Osaka, Japan).

The same paste and jam were diluted 5-fold in water, centrifuged and filtered in the same manner, and were then used for determining the levels of citric and malic acids, which are the main organic acids in strawberries^[15] along with succinic acid, using HPLC. The analysis conditions were as follows. HPLC: LC-10AD-Vp (Shimadzu Corporation, Kyoto, Japan); column: Shim-pack SCR102H (8 mm I.D. × 300 mm, Shimadzu); eluent: 5 mM p-toluenesulfonic acid aqueous solution; flow rate: 0.8 mL min⁻¹; detector: conductivity detector (CDD-6A, Shimadzu); standard: citric, malic and succinic acids (Wako Pure Chemical Industries).

Preparation of jam with different organic acid contents

The SSC, free sugar and organic acid contents of the strawberry paste used as a raw material and the processed strawberry jam in the previous section are shown in Table 1. Different amounts of citric and malic acids were added to the jam, taking care not to change the ratio of the acids. Specifically, the thawed jam was equally divided into 200-g portions and various amounts of food grade citric (Benisei Co., Ltd., Tokyo, Japan) and malic (Happo Shokusan Co., Ltd., Osaka, Japan) acids were added. In this way, jam samples I (1.2%), II (1.6%) and III (2.0%) with different amounts of organic acids were prepared, in addition to the no additive jam (control). The amounts of citric and malic acids added to each sample are shown in Table 2. The contents of free sugars and organic acids in each jam sample, as calculated from the added amounts, are shown in Table 3.

Table 1. SSC, free sugar and organic acid contents of strawberry jam and the raw material paste used for sensory evaluation

Sample	SSC (°Brix)	Free sugars (mg/g)				Organic acids (mg/g)
		Glucose	Fructose	Sucrose	Total	Citric	Malic	Total
Paste	9.7	53.9	49.4	3.6	107.0	7.3	1.3	8.6
Jam	60.0	206.3	157.0	424.0	787.2	6.5	1.7	8.2

SSC, soluble solid content. The raw material paste was mixed with sugar at a ratio of 65:35 and heated until the jam reached 60 °Brix. The jam before the addition of organic acids and the raw material paste used in the test were all mixed once and homogenized before use. These values indicate the samples after homogenization.

Table 2. Amounts of citric and malic acids added to strawberry jam

Sample	Organic acids added (mg/g)
	Citric	Malic	Total
Control	0.00	0.00	0.00
I (1.2%)	3.18	0.82	4.00
II (1.6%)	6.36	1.64	8.00
III (2.0%)	9.54	2.46	12.00

Table 3. Free sugar and organic acid contents of strawberry jam samples used for sensory evaluation

Sample	Free sugar (mg/g)				Organic acids (mg/g)
	Glucose	Fructose	Sucrose	Total	Citric	Malic	Total
Control	206.3	157.0	424.0	787.2	6.5	1.7	8.2
I (1.2%)	205.5	156.3	422.3	784.1	9.7	2.5	12.2
II (1.6%)	204.6	155.7	420.6	781.0	12.8	3.3	16.1
III (2.0%)	203.8	155.1	419.0	777.9	15.9	4.1	20.0

Values were calculated from the analytical values of the prepared jam and the amounts of citric and malic acids added. The values of the control are the same as those of the jam in Table 1.

Color and texture of jam samples

The control and I, II and III jams prepared in the previous section were placed in a 10 mm-thick quartz cell and measured with a colorimeter (CR-13, Konica Minolta Japan, Inc., Tokyo, Japan). The color was expressed using the L*a*b* color system formulated by the International Commission on Illumination (CIE 1976, CIELAB). The color measurement was performed 5 times for each sample. Aliquots of the same jam samples were added to a petri dish with a diameter of 35 mm and a height of 15 mm, and the three texture properties (hardness, adhesiveness and cohesiveness) were measured with a universal testing machine (Rheoner II RE2-3305B, Yamaden Co., Ltd., Tokyo, Japan) equipped with a 20 mm-diameter cylindrical plunger.^[16] The measurement conditions were as follows: measurement speed of 5 mm/s, and distortion ratio of 50%. Hardness, adhesiveness and cohesiveness were calculated using the supplied software. The texture measurement was performed 5 times for each sample.

Sensory evaluation

Questionnaire about common jam usage of assessors

The taste of jams was evaluated by a panel of 37 assessors from various age groups (1 19-year-old, 10 in their 20s, 5 in their 30s, 9 in their 40s and 12 in their 50s) and both genders (17 female and 20 male) with various educational backgrounds. Prior to the sensory evaluation, we surveyed the assessors about their most common usage of jam, i.e., the assessors were asked to indicate the most common way they consume jam.

Sensory evaluation of jams

Jam samples (5 g) were adjusted to a temperature of 20 °C and coded with 3 random digits.^[17,18] All samples were provided at the same time and no instructions were given on the order of assessment. The assessors evaluated ‘sweetness intensity,’ ‘sourness intensity’ and ‘overall preference’ based on a previously described scoring method.^[18] Sweetness and sourness intensities were evaluated on a 9-point unstructured scale from 1 (absolutely no sweetness/sourness) to 9 (extremely sweet/sour). Overall preference was evaluated on a 9-point unstructured scale from 1 (extremely undesirable) to 9 (extremely desirable).

Sensory evaluation of jams with other foods

We conducted a sensory evaluation of jams in combination with other foods. First, we evaluated the jams with yogurt. A 7.5-g portion of plain yogurt (Bulgaria yogurt, Meiji Co., Ltd., Tokyo, Japan) was placed in a small dish and accompanied by 2.5 g of the jam sample. Next, we evaluated the jams in combination with bread. Each jam sample (2.5 g) was spread thinly onto a slice of soft bread (40 × 50 × 7.5 mm, Cho-juku for sandwich, Pasco Shikishima Corporation, Aichi, Japan). The method of sensory evaluation was the same as in the previous section. The ratio of jam to yogurt or bread was determined in a preliminary study, and the average value of the preferred jam to yogurt or bread ratio of 6 consumers of different ages and genders was used.

Statistical analyses

Each value is expressed as the mean ± standard deviation (SD). Results were evaluated using analysis of variance (ANOVA), and the means were compared using Tukey’s multiple range test (p < .05).

Results

pH, color and texture of strawberry jam

The pH, color index and texture of jams with different contents of organic acids are shown in Table 4. The pH decreased as the content of organic acids increased (p < .05). Regarding the color of jam, a* and b* were increased by the addition of organic acids (p < .05). However, the change was constant regardless of the amount of organic acids added. Jam texture, hardness and cohesiveness did not change with the content of organic acids (p > .05). In contrast, the adhesiveness increased with 1.6% (sample II) or greater organic acids (p < .05).

Table 4. Changes in pH, color index and texture of strawberry jam with different organic acid contents

Sample	pH			Color index									ΔE*ab	Texture
				L*			a*			b*				Hardness (N/m^2)			Cohesiveness			Adhesiveness (J/m^3)
Control	3.50	±	0.02^a	12.9	±	0.1	10.3	±	0.2^b	2.4	±	0.5^b	-	369.4	±	17.4	0.89	±	0.02	93.6	±	1.3^b
I (1.2%)	3.18	±	0.02^b	13.7	±	0.5	12.6	±	0.5^a	3.8	±	0.7^a	2.8	375.8	±	14.2	0.88	±	0.01	93.8	±	5.9^b
II (1.6%)	2.93	±	0.03^b	13.2	±	0.6	12.9	±	0.5^a	4.8	±	0.8^a	3.5	356.7	±	14.2	0.89	±	0.02	104.2	±	6.6^a
III (2.0%)	2.75	±	0.04^b	13.1	±	0.5	12.4	±	0.7^a	4.3	±	0.8^a	2.8	363.1	±	28.5	0.89	±	0.01	105.6	±	1.6^a

The results are presented as the mean ± S.D. (n = 5). Values marked with different letters within the same column are significantly different by Tukey’s multiple range test at p < 0.05.

Sensory evaluation

The survey results for the most common jam consumption habit of assessors are shown in Table 5. The most common response (19/37 assessors) was ‘eat with yogurt’. The next most common response (15/37 assessors) was ‘spread on bread’. The results of sensory evaluation are shown in Table 6. The results are presented for jam alone as well as in combination with yogurt or bread, and are subdivided by the preference of the assessors who indicated either ‘eat with yogurt’ or ‘spread on bread’.

Table 5. Assessors’ most common usage of jam

Response	Number of assessors
Eat with yogurt	19
Spread on bread	15
Eat with cheese and crackers	1
Almost never eat jam	2

The assessors selected the most common way they consume jam.

Table 6. Sensory evaluation of strawberry jam with different organic acid contents alone and in combination with yogurt or bread

Evaluation of jam alone
Assessor attributes	Total (n = 37)									Eat with yogurt (n = 19)									Spread on bread (n = 15)
Item	Sweetness			Sourness			Preference			Sweetness			Sourness			Preference			Sweetness			Sourness			Preference
Control	7.5	±	1.2^a	2.6	±	0.9^d	5.3	±	2.0	7.7	±	0.9^a	2.8	±	1.1^b	5.6	±	2.0	7.2	±	1.4^a	2.4	±	0.5^d	4.9	±	2.0
I (1.2%)	6.5	±	1.0^b	4.1	±	1.1^c	5.8	±	1.4	6.3	±	1.0^b	3.8	±	1.2^b	6.1	±	1.5	6.7	±	1.1^ab	4.5	±	0.8^c	5.4	±	1.2
II (1.6%)	5.0	±	1.0^c	6.3	±	1.3^b	6.0	±	1.5	4.5	±	1.3^c	6.3	±	1.5^a	6.1	±	1.7	5.7	±	1.4^bc	6.2	±	0.9^b	6.0	±	1.4
III (2.0%)	4.6	±	1.0^c	7.1	±	1.4^a	5.4	±	1.5	4.6	±	1.5^c	6.9	±	1.7^a	5.3	±	1.2	4.6	±	1.4^c	7.4	±	0.8^a	5.6	±	1.9
Evaluation with yogurt
Assessor attributes	Total (n = 37)									Eat with yogurt (n = 19)									Spread on bread (n = 15)
Item	Sweetness			Sourness			Preference			Sweetness			Sourness			Preference			Sweetness			Sourness			Preference
Control	6.6	±	1.6^a	3.4	±	1.5^c	5.5	±	1.7^ab	6.6	±	1.8^a	3.4	±	1.4^c	5.4	±	1.9^ab	6.5	±	1.3^a	3.6	±	1.6^c	6.1	±	0.9
I (1.2%)	6.1	±	1.0^ab	5.3	±	1.5^b	5.9	±	1.0^a	6.2	±	0.9^a	5.3	±	1.3^b	5.9	±	1.0^a	5.8	±	1.1^ab	5.4	±	1.6^ab	5.8	±	0.9
II (1.6%)	5.4	±	1.2^bc	5.4	±	1.5^b	5.9	±	1.5^a	5.5	±	1.4^ab	5.4	±	1.5^b	5.9	±	1.5^ab	5.4	±	0.9^bc	5.1	±	1.5^b	5.7	±	1.5
III (2.0%)	4.4	±	1.3^d	6.9	±	1.2^a	4.7	±	1.4^b	4.4	±	1.4^b	7.2	±	1.1^a	4.6	±	1.5^b	4.3	±	1.1^c	6.6	±	1.0^a	4.9	±	1.5
Evaluation with bread
Assessor attributes	Total (n = 37)									Eat with yogurt (n = 19)									Spread on bread (n = 15)
Item	Sweetness			Sourness			Preference			Sweetness			Sourness			Preference			Sweetness			Sourness			Preference
Control	6.1	±	1.4^a	3.9	±	1.6^c	5.0	±	1.7	5.8	±	1.3^a	4.2	±	1.7^bc	5.6	±	1.6	6.5	±	1.4^a	3.7	±	1.5^b	4.7	±	1.4^b
I (1.2%)	6.1	±	1.3^a	4.2	±	1.2^c	5.8	±	1.5	6.2	±	1.1^a	4.0	±	1.2^c	5.6	±	1.3	6.1	±	1.4^a	4.5	±	1.0^b	6.4	±	1.3^a
II (1.6%)	4.9	±	1.2^b	5.6	±	1.2^b	5.7	±	1.2	5.2	±	1.3^ab	5.4	±	1.2^b	5.8	±	1.2	4.8	±	1.2^b	6.1	±	0.9^a	5.5	±	1.1^ab
III (2.0%)	4.2	±	1.3^b	6.8	±	1.3^a	4.9	±	1.4	4.4	±	1.5^b	6.9	±	1.3^a	5.3	±	1.7	4.1	±	1.1^b	6.8	±	1.2^a	4.5	±	0.7^b

Each item was evaluated on a 9-point unstructured scale from 1 (absolutely no sweetness/sourness/undesirable) to 9 (extremely sweet/sour/desirable). The results are presented as the mean ± S.D.. Values marked with different letters within the same column are significantly different by Tukey’s multiple range test at p < 0.05.

Sensory evaluation of jams

We first evaluated the jams alone (Table 6). As the amount of added organic acids increased, there was a decrease in sweetness intensity and an increase in sourness intensity. However, there was a difference in the degree of distinction between sweetness and sourness. For sweetness, differences between samples II and III were not recognized (p > .05). For sourness, differences in all samples were recognized by the assessors (p < .05). There was no difference in overall preference regardless of the content of organic acids (p > .05).

Sensory evaluation of jams with yogurt

For sweetness and sourness intensity of jams in combination with yogurt, the same tendency was observed as for the evaluation of jam alone, i.e., a high content of organic acids reduced the sweetness and increased the sourness. However, the difference in scores between samples was smaller than when evaluating jam alone. In addition, a difference in preference was observed. Samples I and II, with modest additions of organic acids, were highly rated, while sample III with high organic acids was rated low (p < .05).

Sensory evaluation of jams with bread

For the sweetness and sourness intensity of jams in combination with bread, the results showed the same tendency as for the evaluation of jams alone. That is, a high organic acid content reduced the sweetness and increased the sourness, and the difference in scores was smaller than when jam was evaluated alone. In regards to preference, no difference was observed for assessors as a whole (p > .05). However, differences were observed when the results were categorized according to the most common usage of jam by assessors. Assessors who typically spread the jam on bread rated sample I significantly higher than the control and sample III (p < .05).

Discussion

This study was conducted to investigate the effects of organic acid content on the quality of strawberry jam, in particular the effects on taste. First, prior to the experiment, we examined the appropriate range of organic acid contents in the jam. When producing strawberry jam, citric acid in the form of a food additive or lemon juice is often added. The main purpose of this is to lower the pH, to assist in the hardening of jam by pectin.^[9] Pectin is an important factor in jam quality, and affects color and palatability, as well as physical properties.^[19,20] We referred to many strawberry jam recipes throughout the world and found that there were considerable differences in the amount of organic acids added. Some recipes had no added organic acid, while others contained large amounts of added organic acids, about 1% of the strawberry and sugar mixture. Further, these jams were assumed to contain organic acids at approximately 2%, which represents the amount added and the amount originally contained in the strawberries. In this study, since the content of organic acids in the jam before the addition was about 0.8% (control), the amount added was set to 0.4, 0.8 and 1.2%. The majority of strawberry jam recipes only add citric acid. However, in this study, malic acid was also added, in reference to the ratio of citric and malic acids in strawberry jam prior to the addition of organic acids. Importantly, organic acids are characterized by different degrees of tartness.^[21] The ratio of citric and malic acids was the same in all samples to control for the effect of differences in the ratio of organic acids. Further, the timing of the addition of citric acid to the jam was observed both before and after heating. We chose to add citric and malic acids after heating, to control for changes in these concentrations by heating.^[22] The heating conditions during jam production greatly affect the jam color, the amount and composition of the main components, as well as palatability.^[23] When preparing the samples used in this study, the composition of free sugars and organic acids was altered by heating. The strawberry paste mixed with sugar was reduced 34.5% by weight during heating to produce the 60 °Brix jam; thus, the glucose and fructose contents were projected to be 82.3 and 75.4 mg/g, respectively (Table 1). Despite adding only sucrose to the strawberry paste during the making of jam, the resulting jam also had increased glucose and fructose. For organic acids, the citric acid ratio decreased, and the malic acid ratio increased.

The addition of organic acids to the strawberry jam significantly increased the color index a* and b* values. The changes in these values indicate that the redness and yellowness of the jam have increased, and it is thought that the addition of organic acids brightened the color of the jam. However, there was no difference in color, even when the content of organic acids was increased from 1.2% (sample I) to 2.0% (sample III). Therefore, to produce strawberry jam with a vivid color, the required amount of organic acid is approximately 4 mg/g or less.

As for texture, the addition of organic acids had no effect on hardness and cohesiveness. Adhesiveness increased at an organic acid level of 1.2% (sample I) or more. In order for pectin to form a gel, the pH should be 3.5 or less, and 2.6 to 3.3 is particularly suitable.^[9] The pH of jam was lowered from 3.5 by the addition of organic acids to a level more suitable for gelation. However, the hardness of the jam did not change. There are many unknowns about the function of pectin, and the reason for this change remains to be identified.

Next, changes in the taste of jam due to the content of organic acids were examined. When evaluating jams alone, sensory evaluation revealed that the sweetness intensity of the jam decreased with increasing organic acid addition. On the other hand, there was no difference in sweetness intensity between a 1.6% (sample II) or 2.0% (sample III) organic acid content. In contrast, the sourness intensity increased as the content of organic acid increased, and differences due to the content of organic acids were clearly identified. These results suggest that changes in the taste of jam due to the content of organic acids are greater for sourness than sweetness. It has been reported that the sweetness of sugar is inhibited by the addition of organic acids,^[24] and our results clearly demonstrate this phenomenon.

Regarding palatability, no differences were identified according to the amount of added organic acids. The results indicated that the amount of organic acids has an effect on the sweetness and sourness of jams. However, there were significant individual differences in the sourness intensity that assessors found most favorable. In addition, it has been reported that gender and age impact the perception of taste.^[25,26] Thus, we examined the results by gender and age, but no differences were identified.

On the other hand, jam is typically consumed in combination with other foods. We anticipated that jam would be mainly eaten with yogurt or bread, and aimed to investigate the effect of these combinations on taste. In this test, assessors were asked about their common jam consumption habits. The assessors were revealed to typically eat jam with yogurt or bread. The assessors were able to recognize differences in the sweetness and sourness intensities with varying contents of organic acids even when the jam was eaten with yogurt or bread. However, the differences were more ambiguous than when evaluating the jam alone. The reason for this may be the effect of the sweetness, sourness and flavor of yogurt or bread. It is especially interesting that although there were no differences in preference when jams alone were consumed, in combination with yogurt, samples I and II with moderate additions of organic acids were highly rated, and sample III with a high organic acid addition was rated low. The reason why the strongly sour jam was rated low is probably because yogurt is inherently sour, and the combination produced an even more sour taste. However, the preference for the control with low sourness and strong sweetness was not high, even though it was consumed in combination with the sour yogurt. Further, the preference score for the control had the highest standard deviation. The control sample was often evaluated with the highest and lowest scores. Thus, the jam with low sourness showed large individual differences in palatability. From these results, it is expected that many consumers will prefer a jam with moderate sourness, even when eaten with sour foods like yogurt.

In addition, novel observations were made by analyzing the results according to the assessors’ typical usage of jam. Assessors who consume jam with yogurt showed a preference when evaluating jam with yogurt. They rated sample I with a moderate organic acid content higher, while sample III with a high organic acid content was rated lower. By contrast, no difference in preference was observed with respect to organic acid contents when assessors who typically consume jam with bread evaluated jam with yogurt. However, these assessors were found to have a preference when evaluating jam with bread. They rated sample I with a moderate organic acid content higher, while the control and sample III (low and high organic acid content, respectively) were rated lower. Neither the assessors who eat jam with yogurt nor those who eat jam with bread were observed to show any differences in preference when evaluating the jam alone. These results suggest that when investigating the preference of foods that are not eaten alone, such as jam, it is essential to investigate the eating habits of assessors. The age and gender of assessors are known to affect the results of sensory evaluation.^[27–30] The present study revealed that eating habits should also be considered an important factor. Preferences are established by training the palate through repeated consumption of foods with characteristic tastes.^[31] Since some individuals usually eat jam with yogurt, it is possible that the palatability of jam with yogurt is established. By extension, those who do not eat jam with yogurt may have no established preference for jam with yogurt. Therefore, even when differences in taste can be detected, preferences may not be easily evaluated. This is also the case when eating jam with bread. Although much research has been conducted on the effects of materials and processing methods on jam palatability,^[32] the sensory evaluation did not take into account the combination with other foods. Foods and beverages are also evaluated for their palatability in isolation.^[33,34] Thus, it is desirable to consider evaluating these in combination with other foods.

These results clarified that the color, texture and taste of strawberry jam change with the addition of organic acids. However, the added amount should be optimized according to the purpose, and a small amount may be used to brighten the color of jam. Under the present study conditions, the addition of organic acids had no effect on jam hardness. Meanwhile, organic acids addition greatly changed the taste of the jam. By contrast, the preference for jam varies greatly from person to person. However, the overall trend seems to favor moderately sour jams over very sweet or sour jams. When producing and selling jams in large quantities, the taste should be designed so that the jam is moderately sour, ensuring that more consumers will find it palatable. In addition, a certain demand is expected for products that support people who like very sweet jam. On the other hand, these results suggest that the preference for jam may change depending on how consumers want to use the jam. When designing the taste of foods that will be consumed in combination with other ingredients, it is important to consider how consumers will use them.

Conflict of interest

The authors declare no conflicts of interest or relationships, financial or otherwise. This study was approved by the Institutional Review Board of Shizuoka Prefectural Research Institute of Agriculture and Forestry.

Informed consent

Informed consent was obtained from all individual participants included in the study.

Acknowledgments

We thank Mr. Tomoaki Kawata and Mr. Masahiro Saito of Shizuoka Prefectural Research Institute of Agriculture and Forestry for their assistance in growing the strawberry plants.