ABSTRACT
Canning techniques for specialty foods like fruit curds are currently highly requested by home-canners. In recommending a home-canning
process to consumers, the issue of primary concern to the Extension educator is to ensure a microbiologically safe, high-quality shelf-stable
product.
The objective was to experimentally calculate a boiling water thermal process and determine the effect of consumer procedural variation on
heat penetration patterns of lemon curd.
A standardized lemon curd formulation (equilibrium pH 3.7) was hot-filled into half-pint home canning jars. The cold spot was determined
with Ecklund Harrison copper-constantan thermocouples, inserted through lids, monitoring product temperatures at four potential cold spots
in sixteen canner loads. Sealed jars were placed in the canner and temperatures recorded using EllabTM software, through come-up, cool
down, and a processing time that heated all jars to a minimum of 2°C below canner temperature. Analyses of f(h) values (slope of the
straight line portion of a heating curve) located the cold spot at the geometric center of the jar. Cold-spot temperatures were then monitored
through confirmation canning processes that produced a minimum final temperature of 90.5°C, for both standard filling and low-initial
temperature filling variations. f(h) values were used to calculate the effect of consumer-induced procedural changes on the thermal process.
A boiling water process recommendation of 15 min was calculated for this product. Up to a 15 minute post-cook delay prior to filling jars did
not significantly change f(h) values when compared with the standard treatments.
Confidence in science-based thermal processing recommendations is essential for novel home-canned food products that are similar to
commercially available high-demand items. A 15 minute boiling water heat process for this lemon curd ensures a safe, shelf-stable product.
This study produced a research-based home-canning recommendation for a highly sought-after, distinct product category.
INTRODUCTION
Expanding food markets catering to eclectic tastes have made available a plethora of specialty food products, encompassing new forms of
local foods, as well as various ethnic and international items. Lemon curd, a mixture of eggs, butter, lemon juice and sugar that results in a
tart, sauce-like product is much coveted by epicures worldwide. Numerous and persistent requests for a safe USDA-recommended homecanning process for this category of foods prompted this research (Andress, 2001).
The objective of this project was to develop a high-quality product, determine an adequate thermal process for home-canned lemon curd,
and to study the effect of consumer procedural variation on the thermal process. Since this is an acid product, the primary concern was to
ensure the development of a safe, standardized product, to prevent spoilage from acid-resistant microorganisms during storage, and to
recommend a shelf-life for the developed lemon curd.
INGREDIENTS AND PREPARATION PROCESS
Lemon Curd:
Ingredients: 2½ cups superfine sugar*, ½ cup lemon zest (freshly zested), optional, 1 cup bottled lemon juice**,
¾ cup unsalted butter, chilled, cut into approximately ¾" pieces, 7 large egg yolks, 4 large whole eggs
Yield: About 3-4 half-pint jars.
* If superfine sugar is not available, run granulated sugar through a grinder or food processor for 1 minute, let settle, and use in place of
superfine sugar. Do not use powdered sugar.
** Bottled lemon juice is used to standardize acidity. Fresh lemon juice can vary in acidity and is not recommended.
Procedure:
1. Wash 4 half-pint canning jars with warm, soapy water. Rinse well; keep hot until ready to fill. Prepare canning lids according to
manufacturer's directions. Fill boiling water canner with enough water to cover the filled jars by 1-2 inches. Use a thermometer to preheat
the water to 180°F. Caution: Do not heat the water in the canner to more than 180°F before jars are added. If the water in the canner is
too hot, the process time will not be sufficiently long. The time it takes for the canner to reach boiling after the jars are added is expected
to be 25 to 30 minutes for this product. Process time starts after the water in the canner comes to a full boil over the tops of the jars.
2. Combine sugar and lemon zest in a small bowl, stir to mix, set aside about 30 minutes. Pre-measure lemon juice and prepare the chilled
butter pieces. Heat water in the bottom pan of the double boiler until it boils gently. The water should not boil vigorously or touch the
bottom of the top double boiler pan or bowl in which the curd is to be cooked. Steam produced will be sufficient for the cooking process to
occur.
3. In the top of the double boiler, whisk the egg yolks and whole eggs together thoroughly. Slowly whisk in the sugar and zest, blending until
smooth. Blend in the lemon juice and add butter pieces to the mixture. Place the top of the double boiler over boiling water in the bottom
pan. Stir gently but continuously with a silicone spatula or cooking spoon, to prevent the mixture from sticking to the bottom of the pan.
Continue cooking until the mixture reaches a temperature of 170°F. Use a food thermometer to monitor the temperature.
4. Remove the double boiler pan from the stove and place on a dish cloth or towel on the counter top. Continue to stir gently until the curd
thickens (about 5 minutes). Strain curd through a mesh strainer into a glass or stainless steel bowl; discard zest. Fill hot strained curd into
the clean, hot half-pint jars, leaving ½-inch headspace. Remove air bubbles and adjust headspace if needed. Wipe rims of jars with a
dampened, clean paper towel; apply two-piece metal canning lids.
5. Process in the prepared boiling water canner for 15 minutes (altitudes up to 1000 ft). Cool, undisturbed, for 12-24 hours & check for seals.
PREPARATION NOTES
Shelf- life: For best quality, store in a cool, dark place, away from light. Use canned lemon curd within 3 -4 months. Browning and/or
separation may occur with longer storage; discard any time these changes are observed.
Variation: For Lime Curd, use the same recipe but substitute 1 cup bottled lime juice and ¼ cup fresh lime zest for the lemon juice and zest.
THERMAL PROCESS DEVELOPMENT
Determination of the cold spot for this product and jar combination was made using data collected for heat penetration curves at
4 potential cold spot locations in the jars in 16 canner loads (see Table 1).
Procedural variation based on a pre-fill lag time was used in testing for process calculations. Temperature profiles were
compared for fill temperatures (direct-fill, and after a 15 minute wait), which had means of 55.80° and 46.66°C, respectively.
Process calculation was accomplished by using thermocouples in each of six jars in different canner loads of each of the two fill
methods (standard, and low initial temperature). These jars were processed to 90.5°C plus an additional 5 minutes.
Processing was done in a boiling water canner using the stovetop burners of a household gas range (Frigidaire Gallery Model
ES III). Data was recorded using an Ellab E-ValTM Monitoring System and Software, and Ecklund needle Type T copperconstantan thermocouples. Analysis of variance was used to determine if significant (p<.001) differences existed between the
treatments using the General Linear Model procedure in SAS 9.1 (2002-2003).
RESULTS
COLD SPOT LOCATION
· The cold spot for this product and jar (half-pint) combination was located at the geometric center of the jar (Table 1).
· The f(h) value is the number of minutes it takes the straight line portion of the heat penetration plot to pass through one
logarithmic cycle.
· A larger f(h) represents a slower rate of heat penetration, and is indicative of the location of the cold spot in that jar.
TABLE1: COLD SPOT DETERMINATION OF LEMON CURD IN HALF- PINT JARS
Thermocouple height in
half-pint jar
Average f(h) value
n=16
Range
Standard Deviation
Center
39.521
34.25-45.20
2.88
½ “ Below Center
38.50
34.06-43.78
2.93
1” Below Center
36.32
31.77-40.47
2.36
1-½ “ Below Center
35.22
31.79-39.62
2.60
1Location
of cold spot, as determined by largest individual f(h) value (worst-case scenario)
THERMAL CHARACTERISTICS OF JARS PROCESSED BY TWO
PROCEDURES
• The initial canner temperature was consistently maintained at 80.35-82.31C prior to the loading of filled jars (Table 2).
• The initial temperature for this product as prepared and filled into jars by usual home canning practices ranged from 54.3656.88°C in the standard series and 45.23° - 47.65°C in the LIT (low initial temperature series).
• Thus there was slightly greater variability among initial temperatures in the standard series, but this did not affect the
interpretation of findings or the ultimate process recommendation.
TABLE 2: THERMAL CHARACTERISTICS OF JARS PROCESSED BY TWO METHODS
Standard
n=12
Total Fill Weight
Procedures
Low Fill Temperature
n=24
235.2 g (mean)
237.7 g (mean)
Canner Initial Temperature
°C
81.33 ± 1.38
°C
81.77 ± 0.44
Jar Initial Temperature1
Jar Temperature at Start of Boiling
Mean temperature change during come-up time
Jar Temperature at the end of experimental process2
55.74 ± 0.73
68.85 ± 1.93
+13.11
92.93 ± 0.44
46.66 ± 0.62
67.60 ± 2.97
+20.94
92.73 ± 0.70
Maximum temperature change during process
+ 37.19
+ 46.07
1
Heat penetration data for 6 jars each were collected from different canner loads
2 Heat penetration data were collected by allowing the slowest-heating jar to reach 90.5°C plus an additional 5 minutes heating time
This project was partially funded through a grant from the National Integrated food Safety Initiative (Grant No. 0051110-9762) of the Cooperative State Research, Education, and Extension Service,
U.S. Department of Agriculture.
DETERMINATION OF THE CALCULATED THERMAL PROCESS
• Pflug(1998) outlines guidelines for thermal process calculations, based on the equilibrium pH of the product. This is then
correlated to a F200°F in minutes, based on product pH. Since the equilibrium pH of the lemon curd product is 3.7, according to
process development guidelines a minimum F200°F of 0.1 minutes is enough to ensure an appropriately canned product.
• The F200°F of 0.1 minutes for the lemon curd is achieved within 10-11 minutes from the start of process time (i.e. 10-11 minutes
after come–up time).
• Thus, a 15 minute process time was determined for the product, this time would be sufficient to achieve the desired lethality, as
well as ensure a proper vacuum seal for the jar lid and sterilization of the glass jar (Table 3).
• The shorter the come-up time, the longer it takes for the F200°F of 0.1 minutes to be reached, regardless of f(h). Hence, it is
essential for this particular process, to specify in process instructions, that the initial canner temperature should be 82°C. This
allows the requisite come-up time to be achieved, and the accompanying F200°F of 0.1 minutes to be achieved well within the
recommended process time for this product.
TABLE 3: RECOMMENDED PROCESS TIME FOR LEMON CURD IN A BOILING-WATER CANNER.
Style of Pack:
Jar Size:
Hot
Half-Pints
Altitude:
Processing Time:
0-1,000 ft
15 min
1,001-6,000 ft
20 min
Above 6,000 ft
25 min
EFFECT OF FILL WEIGHT AND INITIAL JAR TEMPERATURE
• A 15 minute pre-fill cooling time (which resulted in a mean 9°C temperature difference) had no significant effect on f(h) values
and thus the thermal process, for the lemon curd product in half-pint jars.
• The procedural variation of lowered initial jar temperature had no significant effect on the final product temperature at the end
of the process.
• A 9C decrease in fill temperature did not significantly change the number of minutes at boiling for the cold spot to reach
90.5C (Table 4).
TABLE 4: EFFECT OF FILL TEMPERATURE ON HEAT PENETRATION OF LEMON CURD IN HALF-PINT JARS
Procedures
Standard
n=12
Low Fill Temperature
n=24
235 g
237 g
Jar Initial Temperature (°C)
55.74 ± 0.73
46.66 ± 0.62
Mean f(h)
42.35 ± 1.83
42.76 ± 1.71
26.41 ± 1.24
25.87 ± 2.45
Total Fill Weight
1
Average Minutes to Reach 90.5°C at boiling
1
Time after water in canner returned to boiling. This comparison of averages is for statistical purposes; in practice, the process
time would be determined by the slowest heating individual jar (Garner, 2002).
SUMMARY AND CONCLUSIONS
• A pre-fill lag period of up to 15 minutes did not change the heat penetration rate (fh) or processing time for this product.
• Canning instructions should be specific for the product composition, jar dimensions, and fill weight of jars.
• With this style product, initial canner temperature is a critical influence on the time taken to achieve process lethality.
REFERENCES
Andress, E.L. 2001. A national survey of current home canning practices in the U.S. Athens, GA: National Center for Home Food Preservation,
Department of Foods and Nutrition, The University of Georgia. Unpublished data.
Garner, H. H. and Andress, E.L. 2002. Effect of fill weight and initial temperature on processing time for a home pickled jicama relish. Poster presented at
IFT Annual Meeting, Anaheim, CA.
Pflug, I. J. 1998. Microbial Control Processes F200°F for Acid Foods. In Microbiology and Engineering Processes. Environmental Sterilization Laboratory,
Minneapolis, MN
Statistical Analysis Software, SAS 9.1, 2002-2003. Cary, NC: SAS Institute Inc.