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Skin Integrity on Steam Pasteurized Almonds

CoolSteam Nut Pasteurization Eliminates Product Skin Lift

Precision Roaster Nut Pasteurizer CoolSteam


Prior to the adoption of the Almond Pasteurization Rule in 2007 by the Almond Board of California (ABC), various industry members, and other stakeholders, engaged in a vigorous debate regarding the potential impact of pasteurization on product quality. The concerns being voiced were significant and well-founded, ranging from the obvious possibility of degrading the flavor, appearance or texture of the product, to more subtle aspects including how pasteurization might impact further processing steps such as roasting. ABC took these concerns seriously and responded by engaging in multiple scientific studies which ultimately demonstrated that overall the various pasteurization systems available at that time, most notably hot steam based systems, did not alter the characteristics of the raw product to a degree that would be detectable by the average consumer. To further accommodate the concerns of stakeholders, such as large confectionary processors that use almonds as a key ingredient, ABC created the Direct Verifiable (DV) system whereby these processors were able to apply their own validated pasteurization step to maintain control of finished product characteristics and Food Safety.


After formal adoption of the rule and its publication in the Federal Register, Almond Handlers (processors) in California gradually moved forward with purchase of pasteurization systems. Most of these systems were “first generation” and in some cases utilized virtually saturated steam and high temperatures to achieve lethality. Generally, and as expected based on the testing performed by ABC, these systems were able to deliver an acceptable product to the market. Over time though, as more and more product was pasteurized some specific issues did arise. One in particular is “skin lift” damage being most prevalent on the popular Nonpareil variety. This loss of skin integrity, with the skin partially detaching from the kernel, was problematic for Handlers as it reduced the grade of the product, and consequently, its value.
It has since been learned that the root cause of skin lift is directly related to both how and when steam is applied to the product. With some of the first-generation systems, steam application is the very first step. Cool almonds, sometimes from cold storage, are exposed to hot, saturated steam. The benefit of this technique is that it inherently results in very rapid heat transfer and lethality. What happens is, as the steam comes in contact with the cool surface of the product, it immediately gives up its heat energy, (latent heat of vaporization), and the steam changes state from vapor to liquid water. Since the product was initially cool, it takes some time to equilibrate to the temperature of the steam, and during this time condensate formation continues. The end result is an excess of hot, liquid water on the surface of the product. This hot water is able to saturate the skin, and literally dissolves the natural “glue” that adheres the skin to the kernel. The result is as shown in the image below. It can be seen that just from 60 seconds in steam, the skin lifts and swells from the kernel. Then to make matters worse, some of the first-generation systems employed a process step to remove the free water by exposing the product to very high temperatures. Unfortunately, this application of high heat causes the water to boil, physically lifting the skin from the kernel and further damaging the product.

Laitram Nut Pasteurization Preheating


Fortunately, as the market opportunity for pasteurization equipment evolved, so did the technology. In 2016, Laitram Machinery recognized that pasteurization should be strictly a value adding process, without any unwanted side effects such as skin lift with its obvious negative impact to product chip and scratch grading criteria. To answer this need, Laitram introduced their next generation CoolSteam technology which incorporates a Pre Heating zone before the CoolSteam pasteurization step as shown in Figure 2. These two process steps work together as follows. First, within the Pre Heating zone, the surface of each kernel is elevated to a temperature slightly below that of the CoolSteam zone. Then, within the CoolSteam zone, lethal heating is applied by a uniform mixture of steam and air at temperatures always below the natural boiling point of 212 F (100 C), and in just the right proportions to provide essentially simultaneous condensational heating from the steam with enough air for virtually instantaneous drying of the product surface. The result is the small temperature difference on the product surface between the Pre Heating zone and CoolSteam naturally minimizes the amount of condensate formation and thereby avoids subjecting the product to the “wet” process conditions at the root of skin lift, as well as avoidance of exposure to excessive temperatures. This thoughtful thermal process design, combined with the highly precise temperature control and uniform forced convection in each process step position the Laitram CoolSteam technology as the ideal option for processors to obtain optimum quality and reliable 4 or 5 log Pasteurization.

Nut Pasteurization Line CoolSteam


To document the performance of the Laitram CoolSteam with Nonpareil variety, testing was conducted in August and November, 2022 at a customers facility utilizing their Laitram Model V3 2-1-2-2 Pasteurization System with stainless steel belt and automated quality analyzer system. The test methodology was very straightforward, as follows:


August test - Multiple product samples were collected at the inlet and discharge of the Laitram system during production operations. To ensure the processed and unprocessed samples roughly corresponded to the same bin and lot, sample collection times at the discharge were delayed by the total dwell time of the system. Samples were then analyzed by the automated Qcify system, and the results were tabulated. In addition, multiple samples of 100 kernels pre and post-process were also visually and audibly checked for any skin separation.

November test – Samples of raw kernels were collected during the November visit, segregated into lots of 5, and photographed. Each lot of 5 kernels were then filled into a small stainless steel mesh cage, embedded in the product bed at the inlet of the Laitram system, subjected to the full pasteurization and cooling process, then collected at the discharge. Processed product was then organized and photographed to create the images later in this report. In addition, to allow close up visualization of skin adherence of raw and pasteurized product, kernels were selected at random, cut, and photographed with specialized microphotography equipment to provide deep depth of field.

Pistachio Roasting Laitram


Test Results Pistachio Roasting



CoolSteam Nut Preheating results
Test Results Pistachio Roasting

Microphotographs of kernels before and after pasteurization. No separation of brown skin is evident.

CoolSteam Nut Preheating Test Results


Predicting the impact of thermal processing on agricultural products is always challenging due to the myriad of variables presented by the product itself, however, based on this and other studies, we are confident that Laitram CoolSteam Pasteurization Technology does not have a negative impact on almond skin integrity, and is demonstrably superior to other competing systems in the market place.

LAitram CoolSteam
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