Brix Readings and Produce Quality

Does Brix Equal Quality?

Refractometer and kaleThis material was first presented as a poster: 

Lingyu Huang, Changzheng Wang and Michael Bomford. 2007. Effects of Plant Source, Age, and Foliar Molasses Application on Brix Readings of Kale Extracts. Amercian Society for Horticultural Science Annual Conference, 16-20 July, Scottsdale, AZ.

Abstract

Refractometers estimate the soluble solid content of juice by measuring the refraction of light passing through it, expressed in ºBrix. They are commonly used to assess fruit ripeness, but have also been proposed as instruments for measuring vegetable quality. In a series of three experiments we tested the precision and variation in Brix values of kale (Brassica oleracea L., Acephala group) extracts as a function of source, leaf age, leaf moisture content, and storage duration. We measured the ºBrix of filtered juice from fresh leaves purchased from three Kentucky producers and two supermarkets, then refrigerated in sealed plastic bags for 0, 7, and 18 days. A second experiment compared extracts of young and mature leaves over 2 weeks of refrigerated storage. A third experiment compared extracts of untreated leaves with those of leaves sprayed 2 weeks before harvest with a solution of black molasses, which is reputed to increase leaf soluble solid content. Despite the large variation between leaves, leaf source and moisture content affected mean Brix values. Leaves from one local producer and one supermarket had higher values than leaves from other sources. No consistent effect was found due to pre-harvest molasses treatment, post-harvest storage time, or leaf age at harvest. Leaf moisture content was inversely correlated with ºBrix, which increased rapidly in wilting leaves. A better understanding of any relationship between Brix values and vegetable quality parameters is needed before ºBrix value can be accepted as a vegetable quality indicator.


Introduction

Refractometers are commonly used to assess fruit ripeness, but they have also been proposed as instruments for measuring vegetable quality. An online publication (Harill 1994) provides charts relating brix values to vegetable quality, but the scientific basis for these charts is questionable. It is not clear whether Brix value reflects vegetable quality and how Brix may vary due to vegetable maturity and storage conditions.

Objective

Determine the variation in Brix values of kale (Brassica oleracea L., Acephala group) juice due to plant source, leaf age, leaf moisture content and storage conditions.

Materials and Methods

Experiment 1. Kale was harvested from the Kentucky State University research farm and purchased from two local farms and two supermarkets. Locally-sourced kale had been harvested within 24 hours of purchase; the harvest date and source of supermarket kale was unknown. All kale was refrigerated in plastic bags for 0, 7 and 18 days. On each sampling day, kale leaves were separated from stems and their juice was squeezed and filtered through nylon cloth before the Brix value was determined with a hand-held refractometer. Leaves were oven dried at 105ºC to constant weight to determine moisture content. Samples were ashed at 550ºC for 48 hours to determine ash content.

Experiment 2. Newly-unfolded (young) and mature kale leaves were harvested from the Kentucky State University research farm and stored in sealed plastic bags in a refrigerator for two weeks. Brix value and moisture content were determined as in experiment 1.

Experiment 3. Kale grown at the Kentucky State University research farm was sprayed with a 2.5% solution of black molasses in water, reputed to increase soluble solid content. Two weeks later, the leaves were collected and measured for Brix and moisture content.

Experiment 4. Kale harvested from the Kentucky State University research farms was stored in open or sealed plastic bags at room temperature or in a refrigerator. The Brix value and moisture content were determined as described above.

Results

Brix values of kale from different sources Moisture content of kale from different sources
Brix value of kale from different sources
stored in sealed plastic bags in a refrigerator
Moisture content of kale from different sources stored in sealed plastic bags in a refrigerator
Ash content by kale source Brix of kale from different sources
Ash content of kale from different sources
stored in sealed plastic bags in a refrigerator
Brix value of young and mature kale leaves stored in sealed plastic bags in a refrigerator
Moisture content of young and mature kale leaves Ash content of young and mature kale
Moisture content of young and mature kale
leaves stored in sealed plastic bags in a refrigerator
Ash content of young and mature kale leaves stored in sealed plastic bags in a refrigerator
Brix of kale sprayed with molasses Brix content of bagged and unbagged kale
Brix value of kale pre-treated with
or without molasses
Brix value of kale stored in open or sealed
plastic bags in a refrigerator
Relationship between brix value and moisture content of kale
Low brix kale high brix kale
Kale leaves stored in sealed (left) and unsealed (right) plastic bags. Leaves on the left have a
higher moisture content and lower brix value than the dehydrated leaves on the right.


Summary

  1. Brix value varied considerably between leaves (CV=27%);
  2. There is no significant difference in Brix value between young and mature leaves, but mature leaves had a higher ash content;
  3. Kale from a local vegetable farm, a supermarket, and the KSU farm had higher Brix values than kale from a different local farm and supermarket;
  4. Pre-harvest molasses treatment and post-harvest storage time did not affect Brix value of kale stored in a refrigerator;
  5. Storage in sealed bags prevented loss of moisture and changes in Brix value;
  6. There is a quadratic relationship between moisture content and Brix value;
  7. Brix value increases rapidly as leaves wilt.

Conclusion

A better understanding of any relationship between Brix values and vegetable quality parameters is needed before ºBrix value can be accepted as a quality indicator of vegetables like kale.


Last updated July 13, 2007

College of Agriculture, Food Science and Sustainable Systems