Tomato Flavor and Plant Nutrition: A Brief Review By R.L. Mikkelsen

Why don’t most tomatoes purchased in grocery stores have that same great flavor of vine-ripened tomatoes straight from the backyard garden? The answer is not simple. Here’s a summary of information that helps explain the situation.

uch concern has been expressed flavor associated with high soluble solids regarding the perceived low qual- and soluble solids/titrateable acidity ratios. Mity of tomatoes available in the High sugar and high acid contents gener- consumer market. Some of the blame has ally have a favorable effect on taste. been laid on modern tomato varieties, but Differences between the flavors of the degree of ripeness also has an effect. varieties…and the weaker flavor of green- Packing houses commonly expose toma- house-grown or artificially ripened toes to supplemental ethylene gas (a natu- tomatoes…are explained by the different ral hormone produced by many types of quantitative proportions of the volatile fruit) in order to accelerate the ripening substances. Of the environmental factors, process. Commercial tomatoes are fre- light has the most profound effect on the quently selected for disease and pest resis- fruit sugar concentration. Generally, more tance or growing season restrictions that sunlight reaching the fruit results in higher are best served by a particular hybrid. Cul- sugar content. As a consequence, green- tural practices such as picking fruit before house tomatoes grown during the winter it is vine ripened also can have negative months contain substantially less sugar effects on taste and quality. than field-grown tomatoes in the summer. For the home vegetable grower, taste The characteristic tomato flavor is in- is probably the most important concern. fluenced by many volatile substances, Commercial growers have many other con- many of which develop during ripening. cerns involved with successfully producing Kadar (1977) reported that tomatoes and marketing their crop, in addition to picked at under-ripe stages were less sweet, taste. The effects of phosphorus (P), po- more sour, less “tomato like”, and had more tassium (K), and other nutrients are gen- off-flavor than those picked at the table- erally positive on fruit quality, although ripe state. The development of long-chain some benefits have not been carefully stud- carbonyls and terpene esters that occurs ied. during the ripening stage is essential for the typical tomato aroma. Tomato Flavor and Degree of Ripeness Assessing tomato flavor is not a simple Effects of P and K on Tomato Quality matter. The intensity of flavor properties Several studies have directly or indi- of tomato fruits is determined largely by rectly examined the effect of plant nutri- the amount of sugar (primarily fructose tion on tomatoes, which are briefly re- and glucose), the organic acid content viewed here. Of the mineral nutrients, (primarily citric, malic, and total acidity), K…by influencing the free acid and the volatile compound composition. content…and P…due to its buffering Typically, human taste panels find best capacity…directly affect tomato quality.

14 Better Crops/Vol. 89 (2005, No. 2) Tomatoes receiving standard nutrition current soil K recommendations be (100%) were compared with enhanced nu- adjusted upwards for maximum fruit yield, trition (150%). The enhanced nutrition and that optimizing fruit color uniformity treatment was found to have a significant may require a greater soil K supply than positive effect on tomato quality, color, and needed for maximum fruit yield. acceptability (Kimball and Mitchell, Summary 1981). Other studies have also shown that There are many complex factors that K and P nutrition has a positive effect on determine the flavor and quality of tomato fruit sugar and acid content (Lacatus et al., fruit. Commercial fresh tomato production 1994). Of the nutrition factors, the soil K is not always geared to produce the most content most affects the total acid content flavorful fruit, since other economic con- in the fruit. Davies and Winsor (1967) cerns must also be considered. In addition found a positive logarithmic correlation to primary factors (such as tomato vari- between the K level in the soil and the acid ety selection, degree of ripeness during content of the fruit. However, Wright and picking, and growing conditions), proper Harris (1985) reported that increased ni- plant nutrition will also positively contrib- trogen (N) and K fertilization had a detri- ute to tomato flavor and appearance. BC mental effect on tomato flavor, as scored Dr. Mikkelsen is PPI West Region Director, Davis, by a taste panel (although increased acid- California; e-mail: [email protected]. ity and soluble solid content resulted from increasing fertilization). Selected References The development of red color in to- Davies, J.N., and G.W. Winsor. 1967. Effect of nitro- mato fruit during ripening is mainly due gen, phosphorus, potassium, magnesium and to the synthesis of various carotenoid pig- liming on the composition of tomato fruit. ments, particularly lycopene. The lack of J. Sci Food Agric. 18:459-466. uniform coloration is a common ripening Hartz, T.K., E.M. Miyao, R.J. Mullen, and M.D. Cahn. disorder, often referred to as “blotchy rip- 2001. Potassium fertilization effects on process- ing tomato yield and fruit quality. Acta Hort ening” and “yellow shoulder.” Trudel and 542:127-133. Ozbun (1971) used sand culture to grow Kadar, A.A., M.A. Stevens, M. Albright-Holton, L. tomatoes with various K concentrations in Morris, and M. Algazi. 1977. Effect of fruit the nutrient solution. They found that the ripemess when picked on flavor and composition K content of both fruit and petiole in- in fresh market tomatoes. J. Amer. Soc. Hort. creased with increasing concentration of Sci. 102:724-731.

K in the sand culture. Total carotenoids in Kimball, B.A., and S.T. Mitchell. 1981. Effects of CO2 the fruit generally increased with increas- enrichment, ventilation, and nutrient concentra- ing amounts of K and the lycopene con- tion on the flavor and vitamin content of to- tent rose as the K level increased. mato fruit. HortScience 16:665-666. Lacatus, V., C. Botez, M. Chelu, R. Mirghis, and V. Processing tomatoes have a high K re- Voican. 1994. The influence of organic and min- quirement. Crop uptake can exceed 350 lb eral fertilizers on tomato quality for processing. K/A, the majority of which is removed in Acta Hort 276:329-332. the fruit. Uniform fruit color is important Lachover, D. 1972. The effect of potassium on a for this industry. Lachover (1972) reported ‘Roma’ variety of processing tomato with spe- that K fertilization increased fruit yield cial reference to potassium uptake, yield and and solid content, even in soils with high quality. Qual. Plant Mater. Veg 21 (3):165-177. K availability. The incidence of both in- Petro-Turza, M. 1987. Flavor of tomato and tomato ternal and external blotchy ripening was products. Food Review International 2:309-351. generally decreased with increased K sup- Trudel, M.J. and J.L. Ozbun. 1971. Influence of po- tassium on carotenoid content of tomato fruit. ply. Hartz et al. (2001) conducted a field J. Amer. Soc. Hort. Sci. 96:763-765. survey looking at yield and color under a Wright, D.H., and N.D. Harris. 1985. Effect of nitro- variety of K fertilization practices for pro- gen and potassium fertilization on tomato fla- cessing tomatoes. Results suggested that vor. J Agric. Food Chem. 33:355-358.

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