Abstract
Four cultivars of English walnut (Juglans regia) were evaluated by a trained taste panel after 6 and 12 months of storage. English walnuts were stored at 5, 15, or 25 °C, and at 40%, 60%, or 80% relative humidity within each temperature. Principal component analysis was used to compare taste, texture, and aroma attributes evaluated by the taste panel to objective indicators of English walnut quality including water activity, moisture content, free fatty acids, peroxide value, hexanal content, and kernel color. Temperature was found to significantly impact English walnut oxidation and perceived rancidity, whereas storage at high relative humidity affected English walnut texture and accelerated quality loss. Water activity was more strongly correlated to textural changes than moisture content. The effect of relative humidity was more pronounced at lower temperatures, leading to increased hydrolytic rancidity and free fatty acids. Peroxide value had higher and more significant correlation to sensory attributes related to rancidity than hexanal. Free fatty acids were not correlated to the rancid sensory attribute, but were significantly correlated to bitter. English walnuts stored at 5 °C with 40% or 60% relative humidity were associated with the sweet sensory attribute and L* value (light color). Kernel darkening was associated with bitter and rancid, but a causal relationship is unknown. Sensory quality of English walnuts is complex and requires further study to establish thresholds for chemical indices of English walnut quality loss based on organoleptic perception.
English walnuts (Juglans regia) are valued for their taste and nutritional quality. English walnuts contain a higher amount of α-linolenic acid, an essential omega-3 fatty acid, than other tree nuts and also have a 4:1 ratio of linoleic acid to α-linolenic acid, an ideal ratio for decreasing cardiovascular disease (Simopoulos, 2004). In studies, diets rich in English walnuts have resulted in decreased total cholesterol and improved lipoprotein profile (Sabate et al., 1993), and increased high-density lipoprotein cholesterol (Lavedrine et al., 1999).
The polyunsaturated fatty acids that make English walnuts an attractive health choice also make them prone to oxidation in storage, leading to the development of off-flavors and rancidity. Development of rancidity shortens shelf life, restricts consumer demand, and leads to hesitancy of product developers to include English walnuts as an ingredient, thus limiting the market reach of the English walnut industry. Light-colored English walnut kernels receive a premium price, and prolonged storage of English walnuts can lead to darkening, in addition to off-flavor development and negative textural changes, decreasing the value of the product or causing it to be entirely unmarketable.
Off-flavor development is the result of the polyunsaturated fatty acids becoming rancid. Rancidity development in nuts occurs via oxidation and hydrolysis. Oxidative rancidity can be due to auto-oxidation, photo-oxidation, or enzymatic-oxidation, and hydrolytic rancidity is caused by the reaction of water with lipids in the presence of a catalyst (Shahidi and John, 2010). Oxidation of triacylglycerides results in the formation of hydroperoxides, an intermediary oxidation product that can then be broken down further into secondary and tertiary products, namely aldehydes, ketones, furans, acids, and hydrocarbons. Hexanal is a volatile aldehyde that is produced during the oxidative deterioration of linoleic acid hydroperoxides that is often related to “grassy” or “paint-like” flavors in food (Frankel, 1983). Hydrolysis of triacylgycerides yields free fatty acids. Quantifying the peroxide value, hexanal content, and free fatty acids provides insight into the amount of lipid degradation that has occurred (Gama et al., 2018).
English walnuts are harvested late summer to fall, hulled and dried before storage in the shell, and later cracked to yield kernels for further short-term storage and sale. In-shell English walnuts are also sold in limited quantities. Storage of shelled or in-shell English walnuts can range from weeks to 12 months. Although in-shell English walnuts are generally stored in large silos under ambient conditions, refrigerated storage is more frequently used for kernels, but relative humidity is generally not monitored or maintained, especially for bulk storage.
After hulling and before storage, whole English walnuts are dried to a moisture content of 8%. This moisture content reduces the growth of microorganisms in the English walnuts during storage (Gama et al., 2018; Labuza, 1980); however, depending on the relative humidity of the storage air, the moisture content and water activity of the English walnuts can increase. Water activity is a ratio of the vapor pressure of water in a food to the vapor pressure of pure water; the amount of unbound water in a substance that is available for metabolic activity (Labuza, 1980). Multiplying the water activity by 100 generally yields the equilibrium relative humidity (ERH), as a substance will equilibrate to the relative humidity of its surroundings. In storage, relative humidity greater than the ERH of the substance results in the material absorbing water from the atmosphere. General trends for water activity relating to shelf life, texture parameters, browning relationships, enzymatic activity, lipid oxidation, and microbial activity have been discussed for many foodstuffs (Labuza, 1980). Although low temperature storage of English walnuts has proven beneficial to quality retention, the effects of relative humidity in combination with temperature are not as well understood.
The first objective of this study was to assess the organoleptic profile of English walnut kernels after 6 and 12 months of storage under nine different storage conditions used to generate a range of physical and chemical quality factors that could be correlated to sensory traits. The second objective was to compare known and currently used objective quality indices with sensory attributes and changes in English walnut kernel quality over time in storage.
Materials and Methods
Storage of samples.
Chandler, Howard, Tulare, and Vina English walnut cultivars, harvested Sep. 2016, were procured from Diamond Nuts in Stockton, CA, USA, soon after harvest and commercial drying (average kernel moisture content of 5.3% to 6.7%), and stored at 0 °C until all cultivars arrived. Whole English walnuts in-shell were sorted by weight to remove undeveloped nuts and to evenly distribute sizes among treatments. Sorted English walnuts were divided into nine groups of ∼71 kg, each group containing an equal number of nuts from all four cultivars, and stored in-shell at 5, 15, or 25 °C in sealed stainless-steel bins (300 L) fitted with an inlet and outlet port. Each cultivar and replication of English walnuts was held together in an open mesh bag within each storage tank. Within each temperature, English walnuts were stored at 40%, 60%, or 80% relative humidity, with one tank for each temperature and relative humidity combination. Relative humidity was established by humidifying air using varying concentrations of glycerol and water, which then flowed through the bins through an inlet and outlet (Forney and Brandl, 1992). Flow rates were adjusted to achieve desired relative humidity. The bins were airtight and sealed with a nested lid with water in the trough. Relative humidity and temperature loggers (Onset Computer Corp., Bourne, MA, USA) were placed in each bin and the data were monitored continually. Water level of the glycerol-water solutions was monitored to replenish what was lost over time due to evaporation.
The initial quality of each English walnut cultivar was evaluated before placing the nuts in storage according to methods detailed in the following. After 6 and 12 months of storage, English walnuts were removed from storage and a quality and sensory analysis was completed.
Sample preparation for analysis.
English walnuts were hand cracked using a hammer and the shells were discarded. Every analysis going forward was completed on the shelled kernel pieces.
Moisture content and water activity.
For each treatment, water activity and MC measurements were completed in triplicate with 10 nuts per replicate. The mean and standard deviation were taken.
Kernel color.
Three replications of 10 English walnuts were used for both color and oil analysis. Kernel color was determined using a chromameter (Konica Minolta Sensing Americas, Inc., Ramsey, NJ, USA) with the CIELAB color space. Two readings were taken per nut (one on each kernel half). The L* value (representing the darkness to lightness of the sample) was used to determine kernel darkening. The same three groups of 10 nuts were then stored at −80 °C for later oil analysis.
Oil extraction.
At time of extraction, English walnuts were removed from the −80 °C freezer, and the three groups of 10 English walnuts were placed on plastic trays to thaw. Once thawed, English walnut oil was extracted in an unheated stainless-steel test-cylinder-outfit of a bench-top hydraulic laboratory press (model #3925; Carver Inc., Wabash, IN, USA). Ten English walnuts yielded 15 mL of oil that was then transferred to a 15-mL freezer-safe tube, flushed with nitrogen gas, then stored at −80 °C.
Peroxide and free fatty acid analysis.
Vials of stored English walnut oil were thawed, and 5 g of oil was weighed into 250-mL Erlenmeyer flasks (Fisher Scientific, Waltham, MA, USA). Oil was analyzed for free fatty acids according to the American Oil Chemists’ Society (AOCS) Official Method [Cd 3d-63 (Firestone, 1973)] by calculating the amount of KOH used to neutralize the oil. Peroxide value was measured by titrating with sodium thiosulfate according to the AOCS official method [Cd 8-53 (Firestone, 1997)].
Sensory analysis.
Eleven panelists were trained by completing nine 1-h training sessions before the 6-month sensory analysis. In training, a background on sensory analysis was provided, and a lexicon of English walnut descriptors and references for those descriptors were determined as a group. At the 12-month sensory analysis, a 1-h training course was hosted to refresh and re-familiarize the panel to tasting vocabulary and methods.
Nine sensory sessions (one for each of the storage treatments) were completed on separate days. Each session included the four cultivars. For each treatment, English walnut samples were hand-shelled by tapping with a hammer, cut into quarter pieces, mixed, and placed into cups with lids. For each of the four cultivars within a storage treatment, two biological samples were prepared for evaluation. Random three-digit codes were generated (Compusense Inc., Guelph, ON, Canada) for each sample. Samples were prepared the day before evaluation, stored overnight at 0 °C, and removed 1 h before tasting for kernels to warm and volatiles to equilibrate inside the cup.
The English walnut sensory lexicon, as decided on by the descriptive panel during their training, included 17 attributes related to aroma, texture, and taste (Table 1). These terms included aroma characteristics (intense, rancid, nutty, plant material, buttery, and earthy), texture characteristics (stiff, resilient, crunchy, chewy, oily mouthfeel, particulates, toothpacking, and astringent), and taste characteristics (bitter, sweet, salty). Definitions for the attributes, decided on with the panel, can be found in Table 1. Panelists were asked to evaluate the aroma first by shaking the cup to release volatiles, then uncapping and smelling it. Texture was analyzed second, and taste was analyzed last. The attributes were evaluated in the order listed in Table 1. Panelists were instructed to cleanse their palates between samples with unsalted crackers and sparkling water. The panelists were asked to rank the English walnuts on a 1 to 15 scale for each attribute.
Descriptions of attributes used for four cultivars of English walnuts descriptive sensory evaluations and their significance after 6 and 12 months of storage at 5, 15, or 25 °C with 40%, 60%, or 80% relative humidity.
References, previously agreed on collectively in training, were provided for each panelist and are outlined in Table 2. References were acquired from the following sources: wheat flour (Market Pantry Brand; Target Corp., Minneapolis, MN, USA), butter, banana chips, mixed dried fruit, and potato chips (Target Corp.); mixed nuts (Planters; Kraft, Glenview, IL, USA); whole oat cereal (Cheerios; General Mills, Minneapolis, MN, USA), whole grain wheat cereal (Wheaties, General Mills); and whole grain wheat crackers (Triscuit Crackers; Mondelez Intl., Chicago, IL, USA). English walnut powder was obtained from chopped pieces of English walnuts that had been baked in an oven. Oil-free English walnut patties were made by pressing the oil from 10 English walnuts and compressing it into a patty. Soil was mixed with a small amount of water before use. Mixed nuts and mixed dried fruit were each chopped and mixed before use as mixed nuts or dried fruit. All aroma references were capped and removed from the refrigerator 1 h before use. English walnuts were evaluated in individual booths for panelists with a red light to mask color differences.
References and scales agreed on during sensory panel training and used for evaluation of four cultivars of English walnuts following storage for 6 or 12 months at 5, 15, or 25 °C with 40%, 60%, or 80% relative humidity.
Gas chromatography detection of hexanal.
Hexanal was determined using headspace solid-phase microextraction and a gas chromatography system with a flame ionization detector (7890B; Agilent Technologies, Santa Clara, CA, USA). Methods were modified from Pastorelli et al. (2006). A 75-μm, polydimethylsiloxane fiber column (Carboxen; Sigma-Aldrich, St. Louis, MO, USA) and a 30-m × 250-µm × 0.25-µm, high-polarity, polyethylene glycol column (DB-WAX; Agilent Technologies) were used. Samples were agitated for 10 min at 60 °C, the fiber was conditioned for 7 min at 200 °C, and samples were extracted for 10 min. A blank vial with air was analyzed between batches, and the fiber was conditioned between each sample. The oven temperature increased from 50 to 240 °C with three ramps over a period of 28 min. Standards were made by diluting 7 µL of a stock solution (993 µL triacetin with 7 µL hexanal) into 993 µL triacetin. Fifteen microliters of standard solution was placed into clear 20-mL headspace screw top vials (Agilent Technologies). English walnuts were chopped and sieved (USA Standard testing sieves #12 and #25), and 1 g of sieved English walnut pieces were weighed into the same type of vials as indicated previously. Samples were capped and analyzed immediately after preparation.
Statistics.
Data were analyzed using Pearson product moment correlations, and principal component analysis using R and RStudio software (R Foundation for Statistical Computing, Vienna, Austria). Sensory data were analyzed using sensory analysis software (Compusense, Compusense Inc.). In addition to base statistical analysis, ggplot2, ggbiplot, and dplyr packages were used (R Foundation for Statistical Computing).
Results
Six months.
After 6 months of storage, aroma attributes that showed significant differences among the storage treatments included intense, plant material, and buttery; significant texture attributes included stiff, resilient, crunchy, particulates, toothpacking, and astringent; and significant taste attributes included bitter and sweet (Table 1). After 12 months of storage, significant aroma attributes were intense, rancid, and earthy; significant texture attributes included stiff, resilient, crunchy, and chewy; and significant taste attributes included bitter and sweet.
Principal component analysis included significant variables for objective quality indicators (water activity, moisture content, free fatty acids, peroxide value, hexanal content, and L* value) and the most significant sensory attributes. At 6 months, these included plant material, intense, stiff, resilient, crunchy, buttery, astringent, and sweet (Fig. 1), and explained 47.7% of the total variability.
Principal component analysis of objective quality parameters and sensory attributes after 6 months of storage of four cultivars of English walnuts. Storage conditions are shown in different colors and numbers according to the legend, with temperature, relative humidity (RH), and cultivar designation (C = Chandler, H = Howard, T = Tulare, V = Vina) as shown. Objective quality and sensory attributes are shown in dark red; H = hexanal, L* = L* value (kernel lightness value from light to dark, where 0 = black and 100 = white), A = astringent, MC = moisture content, Aw = water activity, PV = peroxide value, FFA = free fatty acids. Sensory data were analyzed using R and RStudio (R Foundation for Statistical Computing, Vienna, Austria) and sensory analysis software (Compusense; Compusense Inc., Guelph, ON, Canada) software. Water activity is a ratio of the vapor pressure of water in a food to the vapor pressure of pure water. Peroxide value is a measure of lipid oxidation.
Citation: Journal of the American Society for Horticultural Science 147, 6; 10.21273/JASHS05196-22
Samples in the biplot were clustered by their storage conditions (Fig. 1). The treatments at 25 °C were on the right side of the biplot, but the treatment with 40% relative humidity was near the top of the biplot and separated from those at 60% and 80% relative humidity, which were associated with indicators of rancidity (peroxide value, free fatty acids, hexanal) as well as the sensory attributes plant material, intense, and buttery. The sensory attribute plant material was related to hexanal, free fatty acids, and peroxide value, and the relationship was strongest with hexanal (Supplemental Table 1). Hexanal was strongly correlated with free fatty acids and peroxide value, and peroxide value and free fatty acids were correlated with each other (Supplemental Table 2).
Storage treatments at 15 °C with 40% and 60% relative humidity were clustered together near the top of the biplot, but the treatment at 15 °C with 80% relative humidity was separated at the bottom of the biplot close to water activity and moisture content. Storage treatments at 5 °C were clustered on the left side of the biplot near sweet. The treatment at 5 °C with 80% relative humidity was lower on the biplot than the 60% and 40% relative humidity treatments, closer to water activity, moisture content, and L* value (kernel color).
Textural attributes stiff and crunchy were clustered together in the bottom of the biplot near quality parameters water activity and moisture content and treatments at 15 and 25 °C with 80% relative humidity, indicating positive correlation (Fig. 1). The resilient attribute was associated with treatment at 25 °C with 80% relative humidity and correlated to peroxides, free fatty acids, and hexanal (Supplemental Table 1). Both water activity and moisture content were positively correlated with stiff, crunchy, and particulates. Many of the sensory attributes were also significantly related to each other (Supplemental Table 3). The stiff attribute was related to crunchy and toothpacking. The attribute resilient was related to particulates and toothpacking, and particulates was related to toothpacking.
Twelve months.
Representative mean values for objective quality parameters for ‘Chandler’ English walnuts stored 12 months under each temperature and relative humidity condition are shown in Table 3 as a point of reference to the overall quality values. There was a trend toward higher water activity and moisture content in English walnuts stored under higher relative humidity. The highest water activity and moisture content was recorded in English walnuts stored at 5 °C with 80% relative humidity. Kernel L* value trended higher in English walnuts stored at lower temperatures. Peroxide value and hexanal trended higher with storage at higher temperatures within each relative humidity. Free fatty acids trended higher with higher relative humidity and temperature storage.
Representative mean objective quality measures achieved by different storage conditions for ‘Chandler’ English walnuts stored for 12 months.
Among the aroma attributes, intense increased in significance, and rancid and earthy became significant between 6 and 12 months of storage. Positive aroma attributes, such as buttery and plant material, were significant at the 6-month analysis, but not at the 12-month analysis. Nuttiness was not significant at either storage time. The texture attributes stiff, resilient, and crunchy were highly significant at both 6 and 12 months. Chewy became significant at 12 months, and astringent, toothpacking, and particulates were significant at 6 months, but not at 12 months. Taste attributes bitter and sweet were significant at both storage times, but salty was not significant at either time point.
Principal component analysis for English walnut quality after 12 months of storage included significant variables for objective quality (water activity, moisture content, free fatty acids, peroxide value, L* value, and hexanal) and selected significant sensory attributes, including the aroma attributes intense and rancid; texture attributes stiff, resilient, crunchy, and chewy; and taste attributes bitter and sweet (Fig. 2). These parameters explained 59% of the total variability.
Principal component analysis of objective quality parameters and sensory attributes after 12 months of storage of four cultivars of English walnuts. Storage conditions are shown in different colors and numbers according to the legend, with temperature, relative humidity (RH), and cultivar designation (C = Chandler, H = Howard, T = Tulare, V = Vina) as shown. Treatment 6 was not evaluated after 12 months. Objective quality and sensory attributes are shown in dark red; H = hexanal, R = rancid, L* = L* value (kernel lightness value from light to dark, where 0 = black and 100 = white), MC = moisture content, Aw = water activity, PV = peroxide value, FFA = free fatty acids. Sensory data were analyzed using R and RStudio (R Foundation for Statistical Computing, Vienna, Austria) and sensory analysis software (Compusense; Compusense Inc., Guelph, ON, Canada) software. Water activity is a ratio of the vapor pressure of water in a food to the vapor pressure of pure water. Peroxide value is a measure of lipid oxidation.
Citation: Journal of the American Society for Horticultural Science 147, 6; 10.21273/JASHS05196-22
Peroxide value, hexanal, intense, rancid, bitter, and free fatty acids were all grouped on the right side of the plot (Fig. 2). Samples stored at 25 °C were clustered around these variables, indicating that rancidity as perceived by the trained panel (attributes of rancid, intense, and bitter) and by objective analysis of oil quality (peroxide value, hexanal, and free fatty acid content) were correlated. Free fatty acids content, an indicator of hydrolytic rancidity, was clustered near treatment with 80% relative humidity at 25 °C.
Although English walnuts stored at 25 °C were clustered near indicators of rancidity, English walnuts stored at 5 °C with 40% and 60% relative humidity, and to a lesser extent, English walnuts stored at 15 °C with 60% relative humidity, were clustered on the opposite side of the biplot near the attributes sweet and L* value (light color) (Fig. 2).
Treatment at 5 °C with 80% relative humidity was clustered with the texture attributes resilient and chewy, along with moisture content and water activity. Samples stored at 25 °C and 80% relative humidity were also close to this sector, and it seems likely that the treatment that was removed after 6 months of storage due to mold (15 °C and 80% relative humidity) would have been in this sector as well. Stiff and crunchy were found on the left side of the biplot between the 5 °C 60% and 80% treatments. Similar to the 6-month analysis, English walnuts stored at 5 °C with 40% or 60% relative humidity were similar in organoleptic profile (as indicated by their lack of separation in Fig. 2), whereas those stored at 80% relative humidity differed.
Although moisture content and water activity are related measurements (Supplemental Table 4), water activity was more strongly correlated to textural changes in English walnuts. Moisture content was correlated to chewy, but water activity demonstrated a stronger correlation to chewy, and was also correlated to crunchy and resilient, whereas moisture content was not (Table 4). Because water activity measures the amount of unbound water in the English walnut sample, it is better able to predict textural impacts than moisture content, which measures the total amount of water in the English walnut sample.
Correlations between sensory attributes and objective quality parameters of four English walnut cultivars after 12 months of storage at 5, 15, or 25 °C with 40%, 60%, or 80% relative humidity, and their level of significance.
The lightness of the English walnut kernel color was positively correlated with sweet and negatively correlated with bitter and rancid attributes, meaning darker kernels were less sweet and more bitter and rancid (Table 4). Sweet was also negatively correlated with bitter (Table 5). After 12 months of storage, English walnuts were darker than they had been at 6 months of storage, and the relationship between light-colored kernels and the sweet attribute was greater (indicated by longer vectors in Fig. 2) than at 6 months of storage when little kernel darkening had occurred and the relationship was statistically insignificant (Supplemental Table 1). L* value had a negative correlation with rancidity (Table 4) and with free fatty acids content and peroxide value (Supplemental Table 4), but it was not significantly correlated to hexanal (Supplemental Table 4). A higher L* value corresponds to a lighter English walnut, so as rancidity, peroxide value, and free fatty acid content increase, the L* value decreased, demonstrating that English walnuts that were high in oxidative products and perceived rancidity also had darker color.
Correlations between sensory attributes of four cultivars of English walnuts after 12 months of storage at 5, 15, or 25 °C with 40%, 60%, or 80% relative humidity, and their level of significance.
Rancid and earthy attributes were also correlated to hexanal (Table 4), but peroxide value had a higher and more significant correlation to the rancid attribute than did hexanal. In addition, peroxide value was positively correlated to intense and bitter and negatively correlated to sweet, whereas in terms of aroma and taste attributes, hexanal was only correlated to rancid and earthy. Although free fatty acids were not correlated to rancid, they were strongly and highly correlated to bitter. These relationships between off-flavor descriptive attributes and objective measures of oxidative and hydrolytic rancidity were stronger and more significant at 12 months of storage compared with 6 months.
Discussion
The results of this study confirm earlier findings of the importance of temperature during storage of English walnuts to the rate of quality degradation after harvest (Adkison et al., 2021). At low to moderate levels of relative humidity (40% to 60%), English walnut quality was similar following storage at 5 or 15 °C, but storage at 25 °C led to quality degradation, as indicated by increases in sensory attributes related to off-flavors (rancid, bitter, intense), and quality indicators related to oxidative and hydrolytic rancidity reactions (peroxide value, hexanal). These results are in accordance with oxidation occurring at faster rates at higher temperatures, as has been shown in peanuts [Arachis hypogaea (Evranuz, 1993)], almonds [Prunus dulcis (Lin et al., 2012; Pleasance et al., 2018)], and English walnuts (Adkison et al., 2021; Bakkalbaşı et al., 2012). Jensen et al. (2003) found that storage of English walnuts for 13 months at 21 °C compared with 11 °C resulted in 6-fold higher hexanal content, and panelists described the English walnuts stored at 21 °C as “inedible,” whereas those stored at 11 °C were described as “good.” In a study of eastern black walnuts (Juglans nigra), hexanal was identified as the volatile most associated with acrid and rancid flavors, and differed with the color of the kernels, in agreement with our data (Lee et al., 2011).
All storage temperatures with 40% relative humidity were clustered together on the biplot after 6 months of storage, close to sweet and L* value, but after 12 months, the storage treatment at 25 °C and 40% relative humidity was clustered with other 25 °C treatments and associated with rancidity indicators. It appears that by 12 months, the influence of rancidity was stronger than the association between treatments stored at 40% relative humidity. Storage of English walnuts at 40% vs. 60% relative humidity did not make a significant difference in organoleptic profile, but storage at 80% relative humidity significantly and negatively impacted English walnut quality. The English walnuts stored at 15 °C and 80% relative humidity developed mold between 6 and 12 months of storage, indicating favorable environmental conditions for mold growth. This treatment was associated with moisture content and water activity at 6 months. Mold did not develop at 25 °C or 5 °C with 80% relative humidity. It is interesting that water activity was more closely associated with textural changes, such as crunchy, resilient, and chewy than kernel moisture content.
In agreement with these findings, Lopez et al. (1995) analyzed English walnuts stored for 1 year at 40% or 60% relative humidity at temperatures of 3, 7, or 10 °C. They found no differences in consumer sensory quality related to temperature, but English walnuts stored at 10 °C were more highly favored when stored with 60% relative humidity than with 40% relative humidity. Maté et al. (1996) found more than a 9-fold increase in hexanal in English walnuts stored at 53% relative humidity compared with 21% relative humidity at 37 °C. Evranuz (1993) found that peanuts with high (3.9%) or low (1.4%) moisture content had faster rates of lipid oxidation than peanuts with moisture content of 2.2% or 2.9%, and found that crispness was lost when moisture content exceeded 3%.
Storage treatments at 5 or 15 °C with 80% relative humidity were well separated on the biplot from those stored at the same temperature with 40% or 60% relative humidity, indicating that the effect of relative humidity was more pronounced at lower temperatures, leading to an increase in hydrolytic rancidity and free fatty acids accumulation (and thus perceived bitter taste). This differed somewhat from our earlier findings, which looked at the effects of English walnuts storage conditions on objective quality indicators and found that relative humidity had a greater effect on quality at higher temperatures (Adkison et al., 2021). The earlier study was focused on objective rancidity indicators (peroxide value, hexanal, free fatty acids), which are more affected by temperature than relative humidity during English walnuts storage, whereas the descriptive sensory evaluation in the current study highlighted effects on kernel texture as well as flavor, including sweet, bitter, and rancid.
Jensen et al. (2003) found that water loss occurred in English walnuts stored at 21 °C compared with 11 °C, and as a result, the texture was affected, as English walnuts became drier. Our study showed that English walnuts stored at 5 °C and 80% relative humidity were the most moist and chewy, whereas other combinations of temperature and relative humidity yielded English walnuts that were not as moist. This may not necessarily be a benefit or detriment to English walnuts quality, as some of our sensory panelists volunteered that they preferred English walnuts that were crisp and dry, whereas others preferred oily and moist nuts, even though they were not evaluating sample preferences. Consumer sensory analysis would be needed to confirm this indication.
These studies, in addition to our own findings, indicate that an intermediate relative humidity during storage is best for English walnut quality. High temperatures were found to have the greatest influence on oil oxidation and rancidity development, but storage at high relative humidity, such as 80% (which sometimes occur if English walnuts are stored at ambient conditions), can cause negative textural effects and accelerate quality loss.
Kernel darkening during storage was accompanied by an increase in perceived bitterness and rancidity and a decrease in perceived sweetness of English walnuts. In a sensory study of black walnuts, darker kernels were found to have more negative flavor and taste attributes, including bitterness, than lighter kernels, although sweetness was unchanged among samples, in accordance with our data (Warmund et al., 2009). However, it should be noted that eastern black walnuts are very different from our English walnuts. It is not clear if any causal effect exists between kernel color and negative flavor attributes, or if they happen to be influenced by the same factors and are therefore only highly correlated.
The attribute intense was highly correlated with hexanal, free fatty acids, and peroxide value, indicating that the initial aroma of a sample was stronger (a higher intense value) when the sample was more oxidized. Sensory attributes related to rancidity (rancid, intense, bitter) were correlated to each other and had positive relationships to objective indicators of oil rancidity (peroxide value, hexanal, free fatty acids); however, peroxide value had higher and more significant correlations with sensory attributes related to rancidity than hexanal, which was only correlated with rancid and earthy attributes. Free fatty acids were not correlated with the rancid sensory attribute, but were strongly and highly correlated to bitter; more than any other variable. Additional work is needed to understand which sensory attributes influence consumer satisfaction most strongly.
There are no official industry-wide threshold values for indicators of rancidity, and acceptable limits of peroxide value and/or free fatty acids vary by processor. Some limits have been reported in the literature, but acceptable limits differ. Although the English walnuts used in this study had moderate values of rancidity-related byproducts, they scored relatively low on the “rancid” sensory attribute. Similar results were observed in a sensory analysis of English walnut oil, in which the extent of oil oxidation indicated by chemical means, was much higher than the characterization of rancid and off-flavors by trained panelists (Martínez et al., 2011). Although this method helped control for individual “bad nuts” and decreased variation in the data, this practice may have also limited the degree of rancidity that occurred as compared with commercial practices. L.C. Greve and J.M. Labavitch (unpublished data) found that the degree of rancidity in their laboratory studies of English walnuts was not near the degree of rancidity found in commercial storage facilities. It is also possible that the fluctuating temperatures and relative humidity that occur during commercial English walnut storage may promote faster English walnut deterioration as compared with the uniform temperatures and relative humidity under laboratory conditions. Sensory quality of English walnuts is complex and requires further study to establish thresholds for chemical indices of English walnut quality loss based on organoleptic perception. It is recommended that the English walnut industry conduct these studies and adopt threshold values for the most accurate indicators of sensory quality.
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Correlations between sensory attributes and objective quality parameters of four English walnut cultivars after 6 months of storage at 5, 15, or 25 °C with 40%, 60%, or 80% relative humidity, and their level of significance.
Correlations between objective quality parameters of four cultivars of English walnuts after 6 months of storage at 5, 15, or 25 °C with 40%, 60%, or 80% relative humidity, and their level of significance.
Correlations between sensory attributes of four English walnut cultivars after 6 months of storage at 5, 15, or 25 °C with 40%, 60%, or 80% relative humidity, and their level of significance.
Correlations between objective quality parameters of English walnuts after 12 months of storage at 5, 15, or 25 °C with 40%, 60%, or 80% relative humidity, and their level of significance.
