Study Notes: Early Experiences and Sweet/Sour Taste Preferences in Childhood

Study Overview and Research Background

• Study Title: Sweet and Sour Preferences During Childhood: Role of Early Experiences.
• Authors: Djin Gie Liem and Julie A. Mennella from the Monell Chemical Senses Center, Philadelphia, PA.
• Core Objective: To investigate how early dietary experiences, specifically the type of infant formula consumed, influence later preferences for sweet and sour tastes in children aged $4$ to $7$ years.
• Historical Foundation:
  • Charles Darwin ($1877$) observed that children inhabit different sensory worlds compared to adults.
  • Scientific inquiry into human taste ontogeny began approximately one century after Darwin's observations.
  • Research consistently shows a strong acceptance of sweet-tasting sugars during infancy and childhood across diverse global populations (e.g., Brazil, France, Iraq, Israel, Mexico, Netherlands, and North America).
  • Sweet preferences typically remain high throughout childhood and decrease to adult-like levels during late adolescence.
• Mechanisms of Variation: Individual differences in taste are driven by:
  • Genetic Variation: Factors such as sensitivity to $6\text{-n-propylthiouracil}$ (PROP).
  • Early Experience: For instance, infants fed sweetened water in the first months of life show higher preferences for it at $2$ years of age compared to those without such experience.
  • Dietary Context: Children learn through experience what foods "should" or "should not" be sweet.

The Ontogeny of Sour Taste Preference

• Innate Response: Unlike sweet taste, which is innately preferred, newborns typically reject sour tastes (citric acid), demonstrated by facial grimacing and reduced intake.
• Hypothesis on Experience: Previous research by Mennella and Beauchamp ($2002$) suggested that infancy experiences impact childhood sour preferences.
• The Model System (Infant Formulas):
  • Milk-based Formulas: Described as slightly sweet with "sour and cereal-type" aromas.
  • Protein Hydrolysate Formulas: (e.g., Nutramigen, Alimentum). These have an extremely unpalatable, offensive taste and an "off" odor, characterized by high levels of sourness and bitterness due to their amino acid content.
• Prior Findings: Children ($4$ to $5$ years old) fed protein hydrolysates during their first year were significantly more likely to prefer sour-flavored ($0.040\,M$ citric acid) apple juice and showed fewer negative facial expressions than those fed milk-based formulas.

Subject Demographics and Methodology

• Participants ($N = 83$):
  • Divided into four groups based on age ($4\text{--}5$ years vs. $6\text{--}7$ years) and formula history (Milk-based vs. Hydrolysate).
  • Sex distribution: $42$ girls, $41$ boys.
  • Thirteen additional children were excluded for failing to understand the task.
• Formula Feeding History:
  • Children in the hydrolysate group typically started with milk or soy formula for the first $1.9 \pm 0.4$ months before switching to hydrolysate (usually per pediatrician recommendation).
  • Hydrolysate feeding lasted until an average age of $11.8 \pm 0.9$ months.
  • $83\%$ of the hydrolysate group began this formula within the first $3$ months of life.
• Environmental Controls:
  • Testing occurred at the Monell Center in a dedicated, high-ventilation sensory room.
  • Day $1$ and Day $2$ of testing were separated by $3.0 \pm 0.5$ days.
  • Mothers sat $2\,ft$ behind children and remained silent during testing.
• Testing Procedures:
  • Task Type: An age-appropriate, gamelike, forced-choice sip-and-swallow procedure.
  • Stimuli Presentation: Pairs of solutions ($10\,ml$ each) were served in identically colored opaque tumbler cups with slits to obscure the liquid color.
  • Intervals: $10\text{-sec}$ between pairs; $5\text{-min}$ break after the first $8$ pairs (with crackers and water for palate cleansing).

Concentrations and Measures

• Sourness Testing (Citric Acid): Six solutions of citric acid in apple juice:
  • $0$, $0.007$, $0.012$, $0.022$, $0.039$, and $0.070\,M$.
  • Preference was calculated based on how often children chose the two most sour concentrations ($0.040$ and $0.070\,M$).
• Sweetness Testing (Sucrose): Six solutions varying in sucrose concentration:
  • Diluted milk-based stock ($0.16$ and $0.22\,M$).
  • Stock apple juice ($0.34\,M$).
  • Sucrose added to stock ($0.47$, $0.66$, and $0.93\,M$).
• Questionnaires:
  • Mothers: Measured food neophobia ($10$ items), general neophobia ($8$ items), and child temperament ($25$ items).
  • Subjective Preferences: Mothers and children separately identified "favorite" cereals and candies. Agreement was $65\%$ for cereal and $54\%$ for candy.

Statistical Results: Demographic and Temperamental Data

• Ethnicity: A significant difference existed between groups ($\chi^2(2) = 6.53, p < 0.04$).
  • Hydrolysate group: $77\%$ Caucasian, $17\%$ African-American, $6\%$ Other.
  • Milk group: $44\%$ Caucasian, $52\%$ African-American, $4\%$ Other.
• Temperament Interactions ($p < 0.05$ for all):
  • $4\text{--}5$ year-old hydrolysate children were perceived as less social, less active, and more shy than $6\text{--}7$ year-old hydrolysate children.
  • Younger milk-fed children were perceived as more social than older milk-fed children.
  • Temperament did not show a main effect on the actual taste preference outcome variables.
• Variety Seeking: No significant differences were found in mothers' variety-seeking, food neophobia, or the general habit of adding sugar across age/formula groups.

Results: Taste Preference Findings

• Sour Preference (Figure 1):
  • A significant interaction occurred between age group and formula history ($F(1, 79) = 4.91, p < 0.05$).
  • Younger children ($4\text{--}5$ years) fed hydrolysate formula preferred extreme sour juices significantly more than older children ($6\text{--}7$ years) fed the same formula ($F(1, 33) = 5.81, p < 0.05$).
  • No such age-related decline in sour preference occurred for the milk-based formula group ($F(1, 46) = 0.54, p = 0.50$).
• Sweet Preference (Figure 2):
  • No significant interaction between age and formula history ($F(1, 79) = 0.10, p = 0.76$).
  • Nearly all children liked juice with added sugar ($p < 0.001$).
  • Average sugar content of favorite cereals: $30.6 \pm 1.5\,g/100\,g$.
• Maternal Influence (Table 2):
  • Children of mothers who "frequently" added sugar were more likely to prefer sweeter juice ($F(1, 78) = 4.68, p < 0.05$) and sweeter cereals (32.9 \pm 1.7\,g/100\,g vs. 25.2 \pm 2.8\,g/100\,g in the "never" group).
  • Mothers who added sugar were more likely to be African American ($\chi^2(2) = 17.06, p < 0.0001$) and older ($F(1, 78) = 11.03, p < 0.001$).

Discussion and Practical Implications

• Formula Impact Duration: Early experience with protein hydrolysates influences sour (but not sweet) preferences for years after exposure, but the effect was only detected in the $4\text{--}5$ year age group.
• Explaining the Age Shift:
  • The authors hypothesize that older children ($6\text{--}7$ years) have expanded food and flavor experiences, which likely lead them to learn the "appropriate" level and context of sourness in specific foods (e.g., juice).
  • With continued exposure, preferences may persist; for example, PKU patients fed charcoal-treated protein hydrolysate throughout adolescence often prefer lemon-flavored additives.
• Alternative Explanations Dismissed:
  • The heightened sour preference in younger children was not due to poor sensory testing ability, as sweetness testing was identical and showed no age-related performance difference.
  • The sour preference was not a byproduct of reduced sweet preference.
• Maternal Perception (The Vulnerability Concept): Research by Forsyth and Canny ($1991$) notes that mothers who switch formulas due to feeding problems in infancy may perceive their children as more "vulnerable" years later, potentially impacting mother-infant interactions and behavioral reporting.
• Sweet Taste Experience: This study confirms that maternal habits (adding sugar) and early exposure to sweetened water correlate with a lifelong preference for higher intensity sweetness in daily foods like cereals.
• Final Conclusion: Flavor preferences in childhood are influenced by a wide variety of experiential factors, including infancy formula types and current maternal dietary habits, emphasizing the need to assess children's preferences directly rather than relying solely on parental reports.