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Can Children Be Wooed To Eat Bitter Vegetables? Wageningen University’s Gertrude Zeinstra Presents New Research: Is Repetition The Key To Healthy Habits? Insight Shared At The London Produce Show And Conference

Jim Prevor’s Perishable Pundit, June 6, 2017

There is near unanimity on the desirability of children eating more produce. But when one looks at the initiatives actually done, almost all wind up getting children to eat more sweet fruit and calling it a victory.

The problem, of course, is that many of the healthiest elements of produce consumption come about through consuming more vegetables. When we learned of a researcher focused on this area, we dove in and were thrilled when Gertrude Zeinstra  of Consumer Science agreed to share her research at The London Produce Show and Conference. We asked Pundit Pundit Investigator and Special Project Editor Mira Slott to find out more:

Gertrude Zeinstra
Consumer Science
Wageningen, The Netherlands

Q: Anecdotal evidence can sometimes distort or cloud the reality of a program’s success or failure in increasing children’s produce consumption. This is why we are so pleased you will be presenting your expert, scientifically based research at the London Produce Show to help attendees formulate and validate effective policies and strategies in this challenging arena.

Before we delve into a preview of your enlightening talk, could you share some background about the Consumer Science & Health group at Wageningen Food & Biobased Research, and your role?

A: I work as a consumer scientist and project leader at Wageningen Food & Biobased Research, The Netherlands, one of the research institutes at Wageningen University & Research. Our institute does research for industry, government and other customers.

The mission of our Consumer Science and Health group is to understand consumer’s food choices and eating behavior in order to facilitate healthy and sustainable food choices. I am educated as a nutrition and health researcher with a special interest in food choice and eating behavior of children.

Q: Do you collaborate with other universities and government/industry organizations on different programs. How important are these partnerships in reaching your goals?

A: Yes, we collaborate with various partners to share knowledge and to solve questions from customers in the area of consumer understanding. We are a contract-research organization. Collaboration via public private partnerships is also a requirement for receiving governmental funding.

Q: What kinds of projects will be most applicable to this London Produce Show seminar, Marketing Produce to Children?

A: Various projects involve trying to enhance children’s preferences for, and intake of, fruit and vegetables. For example, my PhD dissertation, Encouraging vegetable intake in children: the role of parental strategies, cognitive development and properties of food; and the EU project,Habeat: determining factors and critical periods in food habit formation and breaking in early childhood, a multidisciplinary approach.

I will highlight points in my dissertation because it’s kind of a starting point for doing much more research on vegetables. When I began this work, there was a lot of research on fruit but not so much on vegetables. My thesis aimed to develop new approaches to increase produce preferences and intake in 4- to 12-year-old children. I also will talk about the European Habeat project and a recent project we did in childcare.

Q: What are the key points you’ll be bringing out at the seminar?

A: What I think is important to point out is that for children, there is often a large difference between fruits and vegetables. Fruit in general is very sweet, and we have an inborn preference for sweet, so usually fruit is quite liked by children, whereas vegetables are often not so well liked. We have an inborn aversion to bitter tastes and sour tastes. Some vegetables are quite bitter, so that’s one of the reasons why children want to abstain from eating them. 

We also have an inborn tendency to like foods with a lot of energy, and vegetables do not contain a lot of energy. So, that’s also a reason why we probably don’t like vegetables that much right from the start. One thing I want to make clear… when it comes to vegetables, we really need to learn to eat them and it may take some effort.

Q: How do we go about tackling that challenge?

A: We know for children that taste is a very important determinant for intake.  In general, children won’t eat something they don’t like, even if it will have a positive impact, or relieve pain. For instance, if they have an earache and the medication is not-so-nice tasting, they will not take it, even if you explain it will be good for their ear and make the pain go away. It’s difficult. In our research, we looked at known strategies for increasing children’s likings of food, and we studied whether these strategies could be applied to vegetables as well.

First, we conducted a qualitative study with three age groups from primary school representing different cognitive development stages, and a parental survey study.

In this parental questionnaire, we wanted to see what Dutch parents were doing to increase their children’s fruit and vegetable intake. Here we also found a difference between fruits and vegetables. The atmosphere around vegetables was more negative than for fruit, mainly because there was more pressure put on the kids to eat the vegetables.  We know from the literature that using pressure to get kids to eat certain food items is often not a very beneficial strategy; it is usually counter-productive.  They might eat the food at the moment, but it’s the kind of message that conveys the food is not-so-nice or tasty.

These initial studies indicated that parents who give their children a choice or some kind of autonomy with eating fruits and vegetables have children who eat more fruit and vegetables. Texture is also an important factor for children. We know from literature that raw vegetables are often easier for children, and they don’t like very slippery or mushy vegetables, such as asparagus or mushrooms, but cucumber or raw carrots are usually quite ok for the children.  In our study, texture was more vital for 4 to 5-year-old children’s food preferences than for 11 to 12-year-olds.

We then conducted three intervention studies, focusing on vegetables only, since increasing vegetable intake seems more challenging than increasing fruit intake, and not much research had been done on vegetables in particular.

Q: Could you walk us through the methodology, how the studies worked, and what you learned, etc.?

A: We wanted to explore the effects of different preparation methods on children’s vegetable liking. We had carrots and French beans prepared using six different methods — mashed, boiled, steamed, grilled, stir-fried and deep-fried, to see which the children preferred most and least, in a ranked order. An adult panel scored the six preparations on their appearance, taste and texture characteristics.

We found that steamed and boiled were most appreciated by the kids, and boiled is also the most familiar method in the Netherlands. Familiarity is also important for children’s food choice; they are often reluctant to try unfamiliar foods.  We found when the appearance of the vegetable was uniform, it positively influenced their liking. This was also the case for a crunchy texture. Whereas, when there was brown coloring on the vegetables due to the preparation, it negatively influenced their liking. When the texture was granular (mashed with pieces inside), that was not really liked by the children either. So, the advice is if a child doesn’t like a certain preparation, try another. It’s important for parents to vary the methods of preparation.

Q: What other factors did you consider?

A: We wanted to test in an experiment whether choice and autonomy would influence child’s intake. If you offer children a choice of vegetables, will it increase their consumption? We conducted a study with 300 children. The child was invited with one of his/her parents to have a dinner in a restaurant. For each child, two target vegetables were selected that were similarly liked by the child. 100 children in the control group didn’t have a choice, and just got one vegetable. 100 had a choice between the two vegetables, and the other group had both of these vegetables on the plate, thus having choice and variation. We measured consumption and how much they liked the vegetables.

Q: Did the results meet your hypothesis?

A: Unfortunately, we did not see any difference in their intake, although we expected that this choice situation would influence results and be positive for children’s vegetable consumption. In addition, we found no difference in the liking of the vegetables.

Q: Wasn’t this counter to your initial parental survey study?

A: Yes, but we also asked the parents how reactant their children were beforehand via a questionnaire, so how much they were opposed to rules and so on. For the 100 kids who didn’t have a choice, there was a difference between high reactant children and low reactant children in their vegetable consumption. It seems there is something going on with choice.  We believe that going to a restaurant setting may have been overwhelming for these young children (aged 4-6 years) and may have influenced our finding. Sometimes there’s a positive effect of choice on children’s intake and sometimes, not, so more research is needed here.

Q: At one of our past New York Produce Shows, Gabriella Morini, a taste and food sciences professor and researcher at the University of Gastronomic Sciences in Pollenzo, Italy, presented research on the molecular aspects of taste, and argued the need to condition taste receptors, which are naturally opposed to bitter vegetables, starting at an early age, preferably when the child is in the womb.  And studies have found that children may require multiple repeated tastings of a vegetable before liking it, due to their developing and changing taste buds. Did this phenomenon play into your research?

A: This repeated tasting is one of the key findings, very consistent in the Habeat project. Most of these repeated exposure studies have been done in very young children when eating their first solid foods. When introducing new vegetable tastes, repeated exposure is very important to make the child familiar with the taste of this initially unfamiliar vegetable taste.

In the Habeat project, repeated exposure has been compared with other conditioning mechanisms. One of those mechanisms is pairing a new taste vegetable with a high energy food such as a sweet fruit, usually resulting in higher liking. It's an important question to ask if this works also for vegetables to use high energy as a bridge to liking. It’s an important question to ask if this works also for vegetables to use high energy as a bridge to liking. The other mechanism is to pair an unfamiliar taste with a liked sweet taste. Various studies have been done with these three mechanisms.

Q: Are these conditioning mechanisms effective when applied to vegetables?

A: What we actually saw in the Habeat project was that repeated exposure was the key.  Adding energy or adding sweetness was not really necessary. So, repeated exposure is very effective for unfamiliar/novel vegetable tastes.  But what can we do to increase consumption with more familiar vegetables? When children are three or four years old, they’ve already learned, ‘this is a vegetable I like, and this is one I don’t.’

Q: Have you weighed the influence of biological factors with environmental and social factors?

A: Yes, we often look into the whole picture; eating behavior is very complex. For example, role modeling is a very important influence.  In many studies, parental intake is a strong predictor of children’s intake.  For this Habeat project, we did a role-modeling study with child idols that took place at the schools. We played a movie where a role model was eating vegetables enthusiastically, and we did several exposures to the vegetables and this movie.  There was not an immediate effect of higher intake due to the intervention.

What we did find, though, was that the kids who were exposed to these role-model movies had a higher intake compared to the children in the control group over an extended period of time when we later measured intake of the children again. We were quite surprised that this effect was present in the longer term.

The study is challenging because there may be a delayed effect from the intervention. In future research, it is important to take into account direct and delayed effects. Especially with children that develop in steps, it is possible that delayed effects may occur.  

Q: Could you elaborate on the complications of studying delayed effects? Aren’t there many variables, which also differ from child to child? How can you adjust for all the impacts that could influence their consumption levels?

A: Definitely, it is impossible to control for all impacts that influence consumption. There is also a technical side.  It is costly to do a study where you follow children over time. Also, children vary a lot in their vegetable intake, so if you want to measure accurately, you have to measure at an individual level, what you give them, and the actual amount of what they consume at the different stages of the process. Therefore, longer term studies also require money.

And just as background, in the Netherlands vegetables are practically only eaten during dinner, so the recommended amount is hard to consume in one eating moment. In the Netherlands, we should try to increase the number of eating moments for these vegetables. Also for children, it’s quite a high amount to eat in one single session, and parents are eager for children to eat healthfully, so there will be pressure for them to eat all their vegetables. It would be better to spread out eating moments with snacks in between meals.

Q: Did you undertake a study to test this strategy?

A: We developed a project called Veggie Time in various daycare centers. In the Netherlands, you see that at the daycare centers they commonly have a time in the day set for a fruit snack. In the morning around 10:00 or 11:00, it’s fruit time, but there is no such moment for vegetables.  We thought it would be an ideal thing to do. We knew repeated exposure was such a strong mechanism; we thought it would be good to conduct an experiment in a natural setting. Most studies with repeated exposure are researcher-led or with only one vegetable. We wanted to see if the mechanism worked also in the daily practice of a daycare setting with various vegetable tastes. So, we chose three different unfamiliar vegetables for the experiment.

We had a control group and an intervention group, where we pre-measured their intake of these three vegetables. Then the intervention group had an exposure period of five months where they had these three vegetables offered repeatedly, in various product forms, where the control group did not have exposure to these three vegetables. After the five months, we went back and measured intake in the intervention and control group. We saw for two of the three vegetables, the children in the intervention group increased their intake quite nicely compared to the control group.

Q: What were the three vegetables, and why were just two out of three a hit with the children?

A: We used pumpkin, white radish and zucchini. Pumpkin and white radish showed an increase in intake. With the zucchini, we didn’t see the intake increase or decrease; it remained stable. This could be due to the blander taste of zucchini, or to the fact that – we also checked with the parents — zucchini was more often on the menu, so more familiar than expected beforehand.  

Q: Earlier in our discussion, you said that familiarity was an appealing trait for children. Yet, you found the increased intake with the less familiar items?  

A: Yes, in general, familiarity is a positive trait, but there is a difference between willingness to eat and increasing intake. Familiarity is a positive predictor of children’s intake; when a choice, most children will choose familiar foods. But when products are unfamiliar, repeated exposure is a very effective mechanism to increase liking and intake.

The daycare center also provided a good setting because of the peer involvement.  If other children are eating the vegetables, there is more incentive to join them. And also, if parents don’t like vegetables, they won’t serve them at home, so it’s another opportunity.

Q: Is availability and an appealing presentation half the battle?

A: Making the produce available is very important.  Usually children like fruit, and if you put it in front of them, they’ll eat it. If you put cookies in front of them, they’ll eat those as well. But additionally, the item should be accessible. If a young child eats his apple without the peel and cut in slices, then a complete apple is not accessible to this child. This is necessary to take into account.

Q: I was going to ask you what advice you have for attendees, so you’re answering this question…

A: Ready-to-eat is great for the children and easy for the parents to provide to the children. That’s indeed very important.

Also, it should not only be about health as a selling argument because young children don’t understand this term, it’s too abstract for them, and several studies show that children may associate healthy with distaste. So, it’s better to focus on fun and pleasant. That’s a key message to provide. And indeed, different preparation is important because children vary in their preferences and desired product textures.

Q: You’ve referenced several instances where you believe there is a need for future research. How difficult is the research you’re undertaking? Do you have interesting projects in the pipeline? What areas of research are you looking to explore next?

A: Especially with this research with children and vegetables, it is quite challenging.  I started this research in 2004, and there is no magic bullet. There are a lot of strategies one can do.  I’m now involved in a project at a primary school.  In the Netherlands, primary schools are between ages 4 and 12 years. There is a morning break, and there are some schools that have fruits and vegetables obliged, but not all. It is usually the case that parents provide the food for the break. 

We are trying to determine how fresh fruits and vegetables can become a more common habit; what’s the role of the parent, and what’s the role of the school?  If all children have a produce snack, it will be a nice moment, and more encouraging for them to eat it. Also, something I’d like to explore further in the future is the relation between texture variations and children’s liking.

Q: Is your research being tied into the Dutch National Action Plan for increasing fresh fruits and vegetables?

A: I have contact with those involved in the Action Plan, and I participated in an expert round. I know they are also going to focus on families with young children. It is not now directly linked, because the project I’m doing is already underway, and they’re in the phase of setting up these studies. I would be happy to be involved in these kinds of projects.

At our institution, we welcome a lot of applied research in real-life settings. The National Action Plan wants to focus on practical initiatives and on measuring effectiveness of these initiatives, which as scientists we can do very well.  Of course, they know about the work we’re doing, and I am confident we will find our way to work together.

Q: There’s one other issue I wanted to get your feedback on… In the United States, there are programs in schools, where children are offered free fresh fruits and vegetables. These programs often receive glowing feedback from teachers and administrators. Children are enthusiastic to take the free produce.  A teacher might describe how the students have more energy and are more alert in class and test scores are improved, but the evidence is usually anecdotal, since it is costly and challenging to scientifically measure the results.  What are your thoughts as a scientist here?  

A: There are also some of these kinds of programs in Europe. You usually see reports that they are very effective in increasing children’s intake. But we also see that as soon as the project stops, the effect is gone. When it’s free and delivered to the school, it is easy for parents. The concept works for the duration of the project. In the Netherlands parents usually provide the foods the kids eat at school, so they don’t have involvement if it’s offered for free at school.

Sometimes you see subscription programs where parents have to pay for it, and often you see that higher educated or higher income parents participate, whereas the lower income parents don’t participate, although their children are the ones who need it most. In general, lower socio-economic families have lower intake of fruits and vegetables. It is important to also find strategies to increase consumption in these lower income families. Offering free produce is nice because kids will eat it and it may diminish socio-economic differences concerning produce intake, but it’s often difficult to sustain. In this regard, it is not a sustainable strategy because of the large funding involved. 

I think it is important to make partnerships between the schools, the parents, and the produce sector/industry. Also, it’s important to check with the parents and the schools to understand what they want and what their needs are.   Programs should fit to these needs and wants in order to be effective.

I will continue research in this area to help increase produce consumption among children, because developing healthy eating habits early in life will be of benefit for one’s whole life.


This type of work is very important to the future of the produce industry. It has often been pointed out that poor diet impacts governments that pay for health care, private health insurers and others. So, logically, the industry could get important monies from governments, insurers, foundations etc., to help boost consumption, but these efforts have been stymied by a lack of good data showing efficacy.

It is a big job because we need to be able to prove two things:

First, that some effort or program can actually increase consumption of produce over some period of time — a week, a month, a year, a lifetime.

This itself is very difficult to do. For one thing it is not enough to study children in one setting. Imagine a child who comes home every day and whose mother gives him an apple as a  snack.  But this child never eats fruit in school. Now, imagine we launch a free snack fruit program and give out free apples to children at 1:00 PM. This child takes an apple every day and eats it in school. On the study, he will be a big success because he goes from zero to one on fruit consumption in school, but, after school, the child is bored with apples and his mother is satisfied that he ate an apple and so now allows him a brownie for a snack. The initiative has actually produced no net increase in fruit consumption and an increase in total calorie consumption.

But a cheap and easy study – limited to what happens in school — has just become a difficult and expensive study to accurately track not only total 24-hour fruit consumption but total calorie consumption.

Second, nobody outside the industry cares that much about produce consumption. They care about the health consequences of increased produce consumption.

So we not only have to track that we have increased produce consumption, but also that this has resulted in better wellness: Less disease, longer lifespans etc., and these studies can take decades.

For many trade associations and politicians, this has meant we couldn’t do these efforts but, what it really means is we need to begin well-designed studies now and support them for many years.

This type of research can give us a clue as to what kind of research we might ultimately conduct on a larger scale.

So come and talk about vegetables and the role they play in helping children get on healthy diets. Also talk about how good research can open the doors for broader funding of produce industry initiatives.

Register for The London Produce Show and Conference right here.

Let us know your hotel needs here.

And here is the website for the whole event.

We look forward to seeing you at The London Produce Show and Conference 2017! 

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