Pasteurization
The Purpose of Pasteurization is two-fold: Denature enzymes and kill organisms. This makes your sake flavor less likely to change over time.
Enzymes are specialized proteins which reduce other proteins, fats, and starches into smaller molecules like amino acids, fatty acids, and glucose. Our sensory organs can more more easily process these and the more present they are, the more the flavor changes. Ex: starch doesn’t taste like much, but when enzymes turn it into sugar, it becomes incredibly sweet. Enzymes function like a lock-and-key, which only work if they maintain their original “natural” shape. Therefore by chemically deforming (de-naturing) their shapes, we prevent them from carrying out this function, essentially pausing any flavor and texture changes.
In addition to Enzymes, many organisms are present in solutions like sake that contain readily available nutrients such as nitrogen, glucose, fatty acids, etc. These organisms, such as yeast, bacteria, or even harmful pathogens will do serious damage to your products. By raising the temperature to certain levels and holding that temp long enough, these organisms will eventually die. This is called their “lethal temp” and it is what you need to seek out in order to ensure a safe product to sell to the general public.
Many factors effect the lethality of “heat” and in some cases no heat is even required. Have you even eaten raw fish that was soaked in lime juice or vinegar? PH can have an incredible effect on enzymes and organisms. When extremes are reached in nature, it causes a failure of basic protective systems and organisms succumb to their environment.
Give me the answer already!
Ultimately you probably came here looking for the magic numbers: Time and Temp
You’re really not going to like this… but in sake, there is honestly no straight-forward “answer”. Sake can be very delicate and as such, can be hugely effected by high temperatures. In addition to that, pH range can actually invalidate your traditional assumptions about heat-death, ultimately increasing an organisms’ resistance by an incredible amount. Actually, it makes more sense to probably say it the other way around. A lower pH, say below 4.1, creates a much less survivable environment for pathogens, so you really don’t need as much heat or time to inactivate what remains.
What we can do is list out the various method, explain why and how they are used, and hopefully you can use that to decide how you want to proceed. Ultimately, you need to understand this:
Pasteurization is not a forever effect. An open bottle is susceptible to contamination from the environment the same way it was during fermentation. Safe manufacturing practice requires a fundamental knowledge of food science. Developing low-alcoholic or non-alcohol beverages is extremely challenging and dangerous. You must take care to ensure you don’t endanger yourself or your customers.
Concepts
Temperature and holding time are the main “drivers” of lethality. Everything else just modifies levels of lethality.
Time and Temp are cumulative. Even while you are warming up or cooling down, it technically counts, but its logarithmic, so it not doing much until you get to a certain temperature.
Just because your thermometer says you are at temp, there are “cold spots” to do poor fluid homogenization and other issues like specific heat capacity to consider. For instance, cloudy/nigori sake requires more time because rice solids have a higher heat capacity and will still absorb heat from its environment until the system has neutralized.
Different Organisms have different “lethal” ranges. Yeast die around 50-55°C, but many bacteria can live at 80°C and for some time.
Alcohol has a massive additive effect on the lethality of heat. Mainly because the alcohol has already killed organisms before the heat even matters.
PH has a similar effect to alcohol on many organisms, ultimately disrupting the ionic balance of a cell wall, preventing it from being able to perform core functions like excretion and consumption, leading to cell-death. Think of it like really strong magnets repelling each other. Probably too loose of a metaphor, but its essentially what it happening.
Along with pH is Osmotic Stress caused by high sugar or salt content. If a cell remains in a high-gravity environment for too long, it can cause similar problems to pH imbalance. You might notice your yeast for example expresses SO2, or sulfur dioxide. This is a stress response that can be reversed by lowering the “stress” (aka: dilute with water)
Spores are incredibly resilient. Far more than mature organisms. This means even if nothing is happening after you pasteurize, there is a chance if you didn’t achieve a certain level of lethality that your beverage is a time-bomb that will come back to life when its sitting on a room temperature shelf in a store 6 months from now.
The Reference Temp
Lethality is based around the following formula: PU = t⋅10^(T−60)/Z
Time multiplied by 10, raised to temperature - 60°C, divided by the coefficient of thermal resistance (6.94°C)
Temperatures below the reference temperature (60°C) have very little effect, but as you increase “T” it has a substantial change in the resulting lethality. (source) “Every ~6.94°C below 60°C = 10× less PU per minute.”
So, if you use 1 Minute at 60°C as your starting point, you calculate “one PU” or “Pasteurization Unit”. This is the basis for describing the heat resistance of various organisms.
If you’ve ever used a crock-pot, you might know that you really shouldn’t even leave it below 55°C for any length of time. 55°C is coincidentally the temperature we use to make Amazake and you can generally leave it at this temp without causing any major health issues because the lethality out-paces the growth or pathogens and bacteria. Here are some comparisons to see the impact of time and temp:
28 min at 50°C = 1 PU
5 min at 55°C = 1 PU
1 min at 60°C = 1 PU
5 min at 60°C = 5 PUs
5 min at 65°C = 26.3 PUs
In the Tamonin Nikki (Diary of Tamonin, 1570), a method of pasteurization is described in which sake is heated to 63-65°C to kill harmful bacteria and inactivate remaining enzymes. This was 300 years before the discovery of Pasteurization by Louis Pasteur of France. [Reference: 変わり行く日本酒]
This ancient process has evolved but many breweries still pasteurize sake for 20min at 65°C. That’s 105 PUs!! Even milk doesn’t have time and temps that high.
With a consistent system, you should be able to achieve something akin to ~5 PUs, which should be effective to kill anything in sake that is above 12% alcohol. That could be 65°C for 1 min or 72°C for 7 seconds.
Once again… keep in mind that this is NOT a simple equation and you need to consider the composition of your beverage. Rice solids, alcohol content, etc.
Process Authorities
If you venture into the corporate beverage space or attempt to work with any co-packers who will be involved in your production process, they will almost certainly want you to submit a Process Authority Letter, which is something you would request from a lab. This lab will request all kinds of information about your formulation, production process, and pasteurization process. They can even help you out if you don’t know what to do, but in the end you are going to end up with a Time and Temp association with your product and they will legally attest to the safety of those numbers from a food safety perspective. Oddly enough… you don’t need this to submit your product to a distributor, but eventually you’ll encounter someone that wants this from you, so be aware.
Pasteurization Methodologies
Conceptually, as we learned above, we need to expose our product to heat for a particular amount of time for there to be the desired effect on enzymes and organisms.
A) Traditional methods often use timeline that is referred to as LTLT or “Low Temp, Long Time”. The practice was developed at first by noticing that sake heated for to a particular temp for some amount of time would maintain a high quality longer, even during summer months. This started to be standardized by heating sake to 65°C, holding that temperature for 20 minutes and than cooling it down. (again… this is 105 PUs) This practice is very common when using in-bottle pasteurization, but it is extremely over kill.
B) While some/many traditional brewers still practice the 20 minute hold time, it has been shown that “hiochi bacteria were killed at 70°C for 30 seconds in tests using sake diluted to 13% alcohol”. (source) Keep in mind that alcohol does a lot of work in that sentence. Ultimately, that’s ~14 PUs, so we could do 72°C for 15 seconds to achieve the equivalent PU amount.
This “High Temp Short Time”, or HTST, is a modern way to achieve high flow rates on production lines. But, keep in mind that you need a fairly high-powered heat-generation system in order to keep up with the energy demand to bring your product up to temp, even with a plate heat exchanger.
Both of these options will achieve the goal of “stabilizing the flavor of sake”. Since there are no active enzymes, nothing can cleave sugars, proteins, or fats into smaller chains or individual molecules. This will prevent increases in acidity or sugar levels. If there is no active yeast, the sugar level will not decrease and the alcohol level will not increase.
Strategies
Depending on the intended flavor and aroma of the sake, decisions need to be made on how to pasteurize or even to pasteurize at all. Typically these are categorized into the following:
Do Not Pasteurize (Namazake - 生酒)
Let the natural, fresh, bright flavor shine through. It needs to be kept cold (<5°C) to avoid changes in flavor, color, aroma, etc. See: Namahine.
Pasteurizing before storage, but not before shipping. (NamaZume - 生詰め)
This is done because you are not going to bottle for a while. Honestly, there are a bunch of practical reasons you might do this. Sake doesn’t always ferment the way you want. Not enough time to get into that here, but suffice it to say, sometimes you want to mature that and are not sure you are committing to bottling yet. Heck, you may want to blend it with other tanks but need more time before they finish fermenting. So Pasteurizing “in-line”, usually with a plate heat exchanger or “jakan” stainless coil , will warm up the sake an either gradually cool down or you can use another heat exchanger to cool it down for storage.
This also means you don’t pasteurize the bottles. This could be to control the pasteurization more and use HTST methods to reduce the impact on flavor. It also means you’re going to want to make sure you clean those bottles realllllly well. Because anything that is in the bottle becomes a potential contaminant. Especially hiochi bacteria. You honestly might just know this product is going to be cold-chain storage only and aren’t worried about warm (20°C) environments. Lucky you.. you must live in Japan.
Store unpasteurized, but pasteurized before shipping. (NamaChozo - 生貯蔵)
You might decide you want the sake to change. Maybe you didn’t like it and you want to see how quick you can alter the flavor. You might also just like the flavor when it sits for a bit and oxidizes. Honestly, sake brewing can be weird.
When you bottle, you are going to put it through a tunnel pasteurizer or do a process called “hot fill”, which is when you put hot product over 63°C into the bottle and then cool it down after the product AND the bottle have been able to reach lethality.
Pasteurize before storage and before shipping (Twice Pasteurized - 二回火入れ)
Doing both just combines the concepts. There are soooo many reasons you might want to choose this and to be honest, if you ever had a scare with contamination, you might just do in-tank pasteurization to ensure a safe maturation period. In-bottle pasteurization is also just a great way to ensure stability in the market.
We could go much more in-depth on this topic and describe the equipment used, but for now, you can consider how it will effect your product and how you want to approach this.
Flavor and Aroma Effects of Pasteurization
Aroma
Increase: aldehyde, wood, spices, Caramel-like (honey, dried fruit, molasses, soy sauce), Sulfide-like (polysulfide)
Decrease: Ginjo scent, fruit-like, esters, alcohol (ethanol)
Flavor
Increase: bitter taste, Pungent taste (smooth)
Decrease: astringency, stimulating taste (coarseness, roughness)